Doctoral Degrees (Horticulture)

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Alternative postharvest technologies for the maintenance of ‘Parthenon’ broccoli quality during export and domestic marketing.
(Stellenbosch : Stellenbosch University, 2022-04) De Beer, Tarryn; Crouch, E. M.; Crouch, Ian; Theron, K. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Broccoli is prized for its nutritional quality. The green colour and a compact head symbolise freshness. Export and domestic marketing of broccoli is limited by its high perishability. The aim of this study was to investigate the use of two 1-methylcyclopropene (1-MCP) application methods in combination with packing to maximise broccoli storage life under different temperature regimes typically found in the export and domestic handling chains. Under simulation of domestic handling chains, optimal temperature storage (0 °C) of broccoli (cv. Parthenon) in combination with 1-MCP and packaging [Xtend™ modified atmosphere packaging (MAP) or macro-perforated high-density polyethylene (HDPE) liners] resulted in a maximum storage duration of 30 days. In a second season, optimal temperature storage with MAP (RipeLock™) allowed for 42 days storage. Storage at suboptimal temperatures (viz. 7.5 °C or 12 °C), in mixed loads with ethylene is, however, common during domestic handling. Under such environments 1-MCP prevented broccoli (cv. Parthenon) yellowing that occurred with increasing storage temperature from 0 °C-7.5 °C and 7.5 °C-12 °C. Furthermore, 1-MCP alone or in the presence of ethylene extended shelf-life of broccoli (cv. Parthenon) compared to controls, at both temperatures. This research showed how an in-box treatment system of 1-MCP can be used as a viable alternative to the traditional, bulk, in-room application method, having a similar effect in reducing yellowing compared to controls. Sensory quality of broccoli was maintained in MAP (RipeLock™) at 0 °C and 7.5 °C. At 12 °C, where sensory quality was the poorest, 1-MCP reduced the perception of ‘sulphur odour’ compared to untreated and ethylene exposed broccoli. Dimethyl trisulphide (DMTS), an off-odour related volatile, was the most abundant volatile measured at all storage temperatures. The oxidative state of broccoli increased with an increase in storage temperature from 0 °C to 7.5 °C to 12 °C. Ascorbic acid and glutathione were oxidized with an increase in storge temperature. Glutathione, however, had a consistently higher percentage oxidation compared to ascorbic acid. Under simulated export conditions, broccoli was stored at 0 °C in various MAP types with or without 1-MCP. 1-MCP was either applied in-room before packaging or in-box during packaging. Results showed that extended storage without packaging is not recommended due to excessive weight loss. Across two seasons variations in shelf-life were noted. In the first season, a maximum storage duration of 25 days was realised with broccoli stored in Xtend™, or a generic HDPE MAP film with 1-MCP. In the second season, the maximum storage duration was 35 days, irrespective of MAP type or 1-MCP treatment. Here, the limiting factor for shelf-life was sensory quality. At optimal temperature volatiles related to off-odour were negligible, indicating that sensory deterioration was not related to fermentation in the MAP environment. In the second season it was reported that if good quality broccoli is stored at 0 °C, RipeLock™ or Xtend™ MAP in combination with 1-MCP should yield no benefit over untreated broccoli in non-specific, generic liners bags (15 μm HDPE).
Aroma profiles and non-destructive determination of quality parameters of Japanese plums (Prunus salicina Lindl.)
(Stellenbosch : University of Stellenbosch, 2011-03) Louw, Esme Denise; Theron, K. I.; University of Stellenbosch. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Plums with good taste, aroma and eating quality lead to repeat purchases and sustained demand. Taste includes non-volatile compounds, e.g. sugars and acids, and has been well researched to meet the consumers’ preferences. Plum aroma, however, has not enjoyed the same attention. Limited literature is available on the aroma of Japanese plums and none could be found on the effects of relatively long cold storage on the profiles. The main aim of this study is to investigate the changes in aroma compounds of Japanese plums throughout maturation and ripening and the effects of commercial cold storage regimes. Near infra-red (NIR) spectroscopy was also evaluated as a non-destructive method to determine plum quality parameters aimed at minimising sample variability. In Paper 1, NIR spectroscopy was used to develop prediction models for total soluble solid (TSS), total acidity (TA), sugar-to-acid ratio, firmness and weight in three cultivars (Pioneer, Laetitia and Angeleno) and a multi-cultivar model. Samples were collected for seven consecutive weeks and repeated over two seasons. TSS results showed excellent predictability (R2 = 0.817-0.955; RMSEP= 0.453-0.610 % Brix) but the TA models did not perform well. The sugar-to-acid ratio models had results comparable to that of TSS. Both the firmness and weight models had acceptable results. The models of ‘Pioneer’ and ‘Laetitia’ had a better predictability capacity than the ‘Angeleno’ model. Although the multi-cultivar models outperformed the single cultivar models on R2 values it had higher prediction errors. The robustness of all the TSS, TA and firmness models is high in terms of seasonality, range and cultivar. Papers 2 and 3, the main focus of the study, are concerned with the aroma profile dynamics of Japanese plums. HS-SPME was used in both papers to extract the aroma compounds followed by GC-TOFMS for separation and identification. In Paper 2, the aroma volatile compounds of three cultivars (Pioneer, Laetitia and Angeleno) were determined for a seven week period including samples from three maturity stages (immature, harvest and tree-ripe). A total of 35 compounds were identified of which ten were generic. Each cultivar had five unique compounds resulting in different aroma profiles for each of the maturity stages and distinct separation patterns using discriminant analysis. The study was extended in Paper 3 where the aroma volatile compounds of six cultivars (Pioneer, Sapphire, Laetitia, Songold, Larry Anne and Angeleno) and one plumcot (Flavor King) were determined at three functional stages (commercial harvest, tree-ripe fruit and cold stored fruit). A total of 62 compounds were identified and classified into three groups (‘unique’ (31), ‘generic’ (11) and ‘frequent’ (20)) based on their frequency of occurrence. The aroma profiles of ‘Larry Anne’ and ‘Flavor King’ are the most affected by cold storage conditions and ‘Pioneer’ appears to be the least affected. All the cultivars have significantly different aroma profiles at all three of the functional stages with ‘Sapphire’, ‘Larry Anne’ and ‘Flavor King’ showing the largest differences. ‘Flavor King’, a plumcot, presented a ripe aroma profile that was much diverged from that of the true plums.
Browning and watercore disorders in 'Fuji' apples explored by means of X-ray computed tomography (CT)
(Stellenbosch : Stellenbosch University, 2020-12) Chigwaya, Kenias; Crouch, E. M.; Crouch, Ian; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: The Fuji apple cultivar occupies 9% of land under apple production in South Africa. To ensure all year-round fruit availability, ‘Fuji’ apples are stored for extended durations. However, ‘Fuji’ apples are prone to internal browning (IB) during storage. IB is characterized by patches of brown flesh in the apple tissue, which makes fruit unmarketable and causes financial losses. Browning symptoms that have been identified in apple cultivars include radial browning, diffuse browning, combination browning, CO2 damage and core-flush. Techniques such as X-ray computed tomography (CT) that can evaluate IB disorders non-destructively are important. This study aimed to explore IB types and watercore in ‘Fuji’ apples quantitatively and qualitatively using X-ray CT. Exposure of fruit to high CO2 conditions for 3 days after harvest at 21 °C induced IB in the core region of fruit. The construction of porosity maps for intact fruit enabled characterization of tissue structure before and after disorder development. Porosity distribution of ‘Fuji’ was higher in the cortex region compared to the core region. High-resolution X-ray CT scans performed on IB affected and unaffected fruit tissue showed differences in microstructural properties such as porosity, pore size distribution and pore connectivity. Fruit size had a significant effect on the susceptibility of ‘Fuji’ apples to CO2 stress-induced IB. Radial porosity profiles did not differ significantly between fruit that developed IB and fruit that did not develop IB. However, porosity along the axial profile was generally higher for fruit that did not develop IB, particularly in the region between the calyx end and the core region. This was the first X-ray CT study carried out on South African ‘Fuji’ apples to evaluate how fruit microstructural properties relate to the IB types identified under different storage conditions. A further study was done to determine microstructural properties of watercore affected fruit tissue and the effects on storability of ‘Fuji’ apples. X-ray CT scans showed that fruit tissue with watercore had a significantly low porosity and connectivity of pores. This may have a negative impact on respiratory gas diffusion in the fruit and could increase susceptibility to IB during storage. Furthermore, watercore affected tissue had significantly smaller cells due to plasmolysis. For long-term storage experiments, it was found that regular atmosphere (RA) stored fruit had a significantly higher incidence of core-flush compared to fruit from controlled atmosphere (CA) and delayed controlled atmosphere (delayed CA) storage. Although CA and delayed CA were effective in reducing core-flush incidence, they both resulted in a significantly higher incidence of radial browning. Fruit with CO2 damage and cavities were also evaluated in this study. CO2 damage was associated with cell damage and increased pore sphericity. All IB types evaluated resulted in an increased tissue porosity and altering of pore sphericity, anisotropy and pore size distribution. IB after short-term exposure to CO2 stress occurred only in the core region while IB types observed after long-term storage occurred in all fruit tissue regions. This study provided unique insights into the microstructural properties of different IB types occurring in ‘Fuji’ apples.
Cell wall compositional differences between mealy and non-mealy ‘Forelle’ pear (Pyrus communis L.)
(Stellenbosch : University of Stellenbosch, 2011-03) Crouch, Elke Monika; Huysamer, Marius; Holcroft, Deirdre; University of Stellenbosch. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Mealiness, a soft, dry textural disorder of ‘Forelle’ pear (Pyrus communis L.), is a problem for the South African fruit export industry. Soft, dry textural disorders seem to be related to changes in cell wall breakdown. The aim of this work was, therefore, to investigate the occurrence of mealiness‐associated changes in the cell wall and elucidate the mechanism by which mealiness occurs in ‘Forelle’ pear, as well as to characterise cell wall changes occurring during normal ripening. Mealy ‘Forelle’ tissues had significantly lower total galacturonic acids associated with the middle lamella (water‐ and CDTA‐soluble fractions). The water‐soluble pectin of mealy tissues was depolymerised at an earlier stage of ripening. The widespread disintegration of cell‐to‐cell adhesion in mealy cell walls only, suggests that the middle lamella and the plasmodesmata are more broken down. In mealy ‘Forelle’ tissues there was no indication of less broken down high molecular weight polyuronides in the CDTA fraction, normally associated with these dry, soft textures. The pectins from mealy tissues were more broken down and both mealy and non‐mealy tissue polyuronides depolymerised. Furthermore, there was a lack of light toluidine staining in the larger air spaces, which would indicate such water‐insoluble pectins. These data suggest that the formation of high molecular weight pectate gels is unlikely in mealy ‘Forelle’ pear. The slight increase in the galactose content in mealy tissues in CDTA‐ and Na2CO3‐soluble fractions and slight decrease in the 1 M KOH glycan fraction during later stages of ripening (6+11, 9+7, 9+11; weeks at ‐0.5°C plus days at 15°C) may indicate that galactose loosely interlinked into the glycan fraction broke down sooner for mealy tissues. This didn’t increase molecular size profiles in the CDTA fraction. Arabinose content was slightly higher in the 4 M KOH fraction and slightly lower in mealy tissues of water‐ and CDTA fractions. This did not influence the molecular weight of the glycans compared to those in the nonmealy tissues. ‘Forelle’ data therefore seem to be more congruent with a decrease in intercellular adhesion as the mechanism by which mealiness occurs, rather than the formation of high molecular weight pectins taking up the cellular fluid. ‘Forelle’ pear water‐soluble pectin content increases with increased ripening. High amounts of watersoluble pectin and low amounts of Na2CO3‐soluble pectin suggests that solubilisation of rhamnogalacturonan‐I pectins must have taken place during early ripening (at a fruit firmness of > 4.7 kg (7.9mm tip). Galactose and glucose in the pectin fraction dramatically decreased after fruit ripened to a firmness of 4.5 kg, whereafter they remained unchanged. This was also the period in which fruit softened the most and the biggest increase in pectin water‐solubility occurred. It is not known whether these events are coincidental, or linked causally. Rhamnose and arabinose extractability increased in the water fraction and xylose, fucose and mannose increased in glycan fractions with ripening. The biggest changes in polyuronide solubilisation and depolymerisation occurred in water‐ and CDTA fractions between storage and ripening durations of 3+7 (4.7 kg) and 6+4 (2.7 kg).
Colour dynamics in Leucadendron
(Stellenbosch : University of Stellenbosch, 2010-12) Schmeisser, Michael; Steyn, Willem J.; Jacobs, G.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticultural Science.
ENGLISH ABSTRACT: The bright colouration of involucral leaves in Leucadendron is unfortunately transient in nature. Undesirable colour changes render this cut flower unmarketable, resulting in a considerable loss of profit. A deeper understanding of the mechanism leading to colour change is needed to form the framework on which future manipulation strategies can be built. Yellow Leucadendron possess the ability to degreen and regreen naturally, a phenomenon linked to the controlled degradation of chlorophyll and the lesser degradation of carotenoids, which then impart the yellow colour. This colour change is directly linked to the development of the inflorescence. Involucral leaves degreen towards anthesis and are entirely yellow at full bloom. They begin to regreen again when the last florets on the cone have wilted. Deconing before flowering completely inhibits the colour change. Deconing at full bloom, results in leaves regreening sooner. Therefore the inflorescence appears to be the origin of the cue for colour change. Any factor that expedites the death of the florets, results in sooner regreening of involucral leaves. Ultra-structurally, the degreening and regreening resulted from a transdifferentiation of mature chloroplasts to gerontoplast-like plastids, which upon regreening completely redifferentiated into fully functional chloroplasts. In the red Leucadendron cultivar Safari Sunset, the photosynthetic pigment degradation pattern is identical to that of yellow cultivars. However, colour expression is complexed by the presence of anthocyanins. Anthocyanin concentration was shown to be directly related to the opening of the flower head rather than to the phenological development of the inflorescence. With opening, the previously shaded inner involucral leaf surfaces are exposed to higher levels of irradiance and respond by turning red, presumably for photoprotection. Similar to yellow cultivars, any factor leading to the death of the florets before flowering, not only prevents the degreening of involucral leaves, but also prevents the opening of the flower head and therefore the associated change in anthocyanin levels. The ecological significance of regreening was also investigated. What does a female Leucadendron plant stand to gain by regreening rather than discarding the involucral leaves? Regreened involucral leaves were shown not to play a significant role in providing photosynthates for the developing cone. Although the presence of regreened involucral leaves were shown to provide protection against high irradiance and radiant heating of the cone, they were not essential to ensure survival of the cone. The small floral bracts were shown to be very capable of adaptation. The most plausible reason for regreening is therefore assumed to be based on a cost-benefit relationship. As most Leucadendron are adapted to grow on very nutrient poor soils, the question should maybe be rephrased. Why waste valuable resources? Sclerophyllous leaves, like the involucral leaves, are costly to make and therefore reusing, rather than discarding them does seem a sensible strategy for survival.
Critical factors concomitant to the physiological development of alternate bearing in citrus (Citrus spp.)
(Stellenbosch : Stellenbosch University, 2018-12) Stander, Ockert Petrus Jacobus; Cronje, P. J. R.; Barry, G. H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: The significance of carbohydrates, mineral nutrients and phyto-hormones was investigated in relation to their possible roles in selected phenological events in alternate bearing ‘Nadorcott’ mandarin (C. reticultata Blanco) trees. Crop load in ‘Nadorcott’ mandarin trees was influenced by flowering intensity. The most important determinants of flowering intensity were the amount of new vegetative shoot growth and resulting number of new potential floral buds that developed during summer, and the influence of fruit on floral bud development during winter. The lack of development of summer vegetative shoots in “on” trees was not related to leaf carbohydrate concentration. In “off” trees, root sugar concentration peaked during full bloom and high root growth activity was observed prior to the vegetative shoot flush in summer. In “on” trees, fruit were the major carbohydrate sinks and probably disturbed the balance between vegetative shoot development and root growth. Sugar concentration in roots in “on” trees was 3-fold lower, root growth was absent, and shoot growth was halved. The concentration of mineral nutrients in leaves was a response to fruit load and not related to parameters of flowering or vegetative shoot growth. Measurements of phyto-hormones in leaves and roots confirmed that the inhibition of summer vegetative shoots was related to a high concentration of 1 H-indole-3-acetic acid (IAA) in leaves. High concentrations of dihydrophaseic acid and the abscisic acid (ABA) glucose ester suggested that IAA might have acted synergistically with ABA to create a growth inhibition in fruiting shoots. As a result, cytokinins did not contribute to the development of new summer vegetative shoots. High gibberellin concentration in leaves in May and June contributed to limited flowering in “on” trees. Consistent with this interpretation, treatment of “off” trees with 40 mg·L-1 gibberellic acid inhibited flowering, whereas soil and foliar treatments of “on” trees with 1000 mg·L-1 paclobutrazol or uniconazole, gibberellin biosynthesis inhibitors, increased flowering and resulted in fruit development from buds of “on” shoots.
Development of value-added dried pomegranate arils and juice powder: Effects of cultivar, harvest maturity and storage duration of whole fruit
(Stellenbosch : Stellenbosch University, 2020-12) Adetoro, Adegoke Olusesan; Fawole, Olaniyi; Opara, Umezuruike Linus; Manley, Marena; Caleb, Oluwafemi; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: In South Africa, the increasing rate of pomegranate fruit production, processing and research has witnessed tremendous growth due to consumer interest in its high concentration of bioactive compounds. However, the edible part of the fruit (arils) has a short shelf-life of five to seven days. Agro-processing through drying reduces postharvest losses, improves the shelf-life and increases the storability of the product. Before drying, several pre-treatments, are carried out to preserve the quality attributes of the product, however, little is known about the effects of blanch-assisted drying of pomegranate arils and the prospects of developing high quality pomegranate juice powder (PJP) for formulation and/or fortification to promote product diversification within the agro-processing industry. Therefore, the overall aim of this study was to develop value-added and shelf-stable dried products from pomegranate aril with potential for multiple applications and to provide science-based tools for processing and preservation of the nutritional components. This thesis is divided into seven papers that is organised into four themes. Theme A comprised of the general introduction and comprehensive literature review. Theme B (Papers 1, 2 and 3) investigated dried arils of three pomegranate cultivars (Acco, Herskawitz and Wonderful) for optimum quality attributes. Paper 1 shows that ‘Wonderful’ had 8.1% and 22.4% higher total soluble solids (TSS) than ‘Herskawitz’ and ‘Acco’, respectively. In Paper 2, dried arils of fruit at commercial and late harvest had significantly (p<0.05) higher TSS than at early harvest. Paper 3 studied the effects of cold storage of whole fruit (cv. Wonderful) harvested at commercial maturity. This paper showed that after 12 weeks of cold storage, arils dried in hot-air had better colour retention based on total colour difference, (TCD = 3.02 vs 23.6) and retained 46% higher TSS compared with freeze-dried arils. Theme C provided information on the process optimization of dried arils. In Paper 4, blanched samples of ‘Wonderful’ (7 h) ‘Acco’ (7 h) and ‘Herskawitz’ (8 h), had shorter drying times than unblanched samples (11, 15 and 20 h), respectively. The results from Paper 5 showed that blanching at 90°C, 30s and 100°C, 60s reduced enzyme activity by 76% and 68%, respectively, compared to unblanched samples; this also indicates less browning of blanched arils. Development of value-added juice powder with multiple applications in the food industry and extended shelf-life (Theme D, Paper 6), showed that juice powder made with maltodextrin appeared 44% redder (a*) than with gum arabic. Similarly, total anthocyanin retention was 54% higher in maltodextrin than waxy starch. In Paper 7, results showed that at the end of 12 weeks storage, PJP packed in aluminium foil laminated pouches (AFLP) had lower moisture content (6.1%) and water activity (0.54) than other packaging materials, indicating better storability of PJP packed in AFLP. In conclusion, the quality of dried arils from ‘Wonderful’ at commercial harvest maturity was retained better than other cultivars and harvest maturities. Also, blanching at 90°C, 30s retained the quality of dried arils and was thus recommended as a viable aril pre-treatment condition. Finally, maltodextrin produced PJP had better quality retention compared to gum arabic and waxy starch.
Effect of biological amendments on soil microbial properties and performance of pome fruit trees
(Stellenbosch : Stellenbosch University, 2009-12) Van Schoor, Louise; Stassen, P. J. C.; Botha, Alfred; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: The global movement in agriculture is towards more environmentally friendly, sustainable production practices, since the role of soil microbial functions in ensuring crop production and soil fertility has become more evident in agricultural systems. Furthermore, with the impeding phase-out of methyl bromide, apple replant disease (ARD) is becoming an increasingly important problem and biological management practises are needed. Since microbial activity is generally carbon-limited in agricultural soil, it is widely accepted that management practices providing a range of organic compounds on a regular basis will tend to maintain an active and diverse microbial population. It was hypothesised that the application of various biological amendments can affect soil microbial numbers and function, thereby having a positive effect on fruit tree growth and yield. The effect of continued applications of organic material, various microbial inoculants and biostimulants on tree performance were evaluated in conventional management systems. Field trials were established in a conventional pear orchard, potential apple replant disease sites, as well as an optimally managed, high density apple orchard under controlled fertigation. The use of compost, compost extracts, a Bacillus inoculant and humates were investigated intensively. Furthermore, to improve our understanding of soil biological systems a combination of simple, practical methods were used to evaluate the effect of biological amendments on soil microbial properties and effects were related to tree performance. Regular application of compost extract in combination with compost showed the most significant effect in improving tree performance in commercial pome fruit orchards under various conditions. In the pear orchard, cumulative yield over the first two seasons was improved by more than 50% compared to controls, while in the fertigated orchard yield was improved by 22%. Biological amendments also showed improved growth in orchards suffering from stunted growth symptoms typical of ARD. However, in severe ARD cases methyl bromide fumigation showed the most consistent effects. Other biological amendments which showed positive effects on yield were application of Bacillus inoculants (Biostart®) in combination with a labile C source and a low dosage humate product, as well as a combination of compost and humates. It was clear that a combination of labile organic matter and a diverse group of microorganisms showed most promise. Although for some specific treatments increased microbial numbers and activity may have resulted in improved tree performance, in general, changes in culture-based plate counts, soil enzyme activity and carbon utilisation profiles could not be used as an indicator of yield. It was suggested that improved synchronisation of nutrient release and plant uptake, as well as microbial phytohormone production, may play an important role in improving tree performance with application of biological amendments. More research is needed on the exact mechanisms through which compost extracts improve yield and studies on root growth proliferation, as well as effects in the rhizosphere are recommended.
Effect of heat, ultraviolet-B and photosynthetic active radiation stress on apple peel photosystems
(Stellenbosch : Stellenbosch University, 2015-03) Hengari, Simeon Ngaitungue; Steyn, Willem J.; Theron, K. I.; Midgley, S. J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH SUMMARY: The study was undertaken to analyse the response of apple fruit peel photosystems of different cultivars to ultraviolet-B (UV-B) radiation, photosynthetic active radiation (PAR) and heat stresses under laboratory conditions. UV-B, PAR and heat are claimed to be the main fruit sunburn inducing stress factors. The aim was to identify biochemical, physiological and fruit peel anatomical characteristics that provide photoprotection against sunburn inducing factors and to determine stress threshold levels for photodamage. Previously sun-exposed peels of apple fruits were resistant to photodamage under high UV-B dosage throughout fruit development. However, the shaded peels of mature fruits incurred photodamage under UV B stress. Furthermore, fruit photosystems at all development stages were equally sensitive to heat stress combined with moderate PAR (500 µmol m-2 s 1). Photodamage induced by heat and PAR stress during fruit development was not well correlated to fruit pigments, phenolic levels or fruit peel anatomical characteristics. In addition, repeated heat and PAR stress up to 9 hours did not induce any fruit sunburn symptoms. The photosystems of the less sunburn susceptible ‘Golden Delicious’ and more susceptible ‘Granny Smith’ appeared to be equaly sensitive to heat and PAR stress. The possible involvement of the xanthophyll cycle in fruit sunburn susceptibility needs further investigation as a variation in the dependancy of different cultivars on this cycle for photoprotection under heat and PAR stress was observed. Heat stress alone appears to cause the highest damage to fruit photosystems, while the presence of UV-B and PAR enhances this effect. The results presented in this document suggest that sensitivity to sunburn browning may not only be related to the heat, PAR and UV-B stress sensitivity of fruit peel photosystems. General non-photoprotective biochemical responses to the experienced stress may also play a role in sunburn symptom development.
Effect of rest-breaking and fruit thinning treatments on reproductive development in apple
(Stellenbosch : Stellenbosch University, 2008-12) Sagredo, Karen X.; Theron, K. I.; Cook, Nigel C.
Lack of winter chilling is a major problem in producing temperate-zone fruit in warm climates. Delayed foliation and protracted bud burst and flowering are the main problems necessitating artificial means to break dormancy. In South Africa (SA), most apple production areas receive insufficient winter chilling, and an annual application of rest breaking (RB) agents is included as standard practice. The most used RB agent in SA was dinitro-o-cresol (DNOC) but its use was discontinued. Hydrogen cyanamide (HC) became the replacement. It has been effective in apple, but variable effects on fruit set, blossom, yield and fruit quality have been reported. Thidiazuron (TDZ) has also shown the ability to break dormancy in apples. Another important practice in apple production is chemical thinning (CT). However, results are highly influenced by the type of chemical, weather conditions, cultivar and blossom pattern. With the increasing efficacy of RB and by identifying its effects on vegetative and reproductive development, it will be possible to determine more effective chemical thinning treatments. The objective of this study was to determine appropriate RB treatments for apple trees in a warm winter climate, identifying their effect on vegetative and reproductive development and the influence on CT efficacy. The research was performed in the Elgin area (34°S, 300 m) SA, over a period of three years, on ‘Golden Delicious’ and ‘Royal Gala’. In evaluating the effect of different HC concentrations and oil, no synergistic or antagonistic effects were observed on budburst and yield. Mineral oil at 4% plus 1 to 2% Dormex® combined were sufficient to break dormancy. Dormex® at 4% (2.08% HC) reduced fruit set and yield. In general, the rest breaking treatments (DNOC, HC and TDZ) enhanced the final vegetative bud burst compared to the control, while reproductive bud burst in 2002 and 2003 was not significantly influenced. The treatments compressed and advanced flowering periods, but this effect was not always evident when the spring was warm. The treatments synchronised flowering on the tree and between the two cultivars. The mixture of 0.245% HC and 4% oil was less effective in terms of increasing bud burst in ‘Royal Gala’ compared to other rest-breaking treatments. The mixture of 0.49% HC and 4% oil effectively compressed and synchronised flowering in ‘Golden Delicious’. TDZ-oil used at the lower rates also increased bud burst and concentrated flowering. However, it appears that after a cooler winter, higher rates could result in an exacerbated bud burst effect with excessive vegetative growth. The rate and timing of TDZ-oil application influenced the reproductive development of apples and therefore fruit quality. In ‘Golden Delicious’ increased fruit set, number of seeds, and reduced fruit russeting appear as beneficial results of TDZ-oil, whereas fruit set and russeting was not affected in ‘Granny Smith’. TDZ-oil, when applied late and at increasing rates, led to an increase in the malformation of calyx cavities, especially when chemical thinning was performed using the cytokinin-like compound benzyladenine. The effect seemed to be cultivar specific, with ‘Golden Delicious’ being the most severely affected. Increased return bloom in response to late TDZ application in ‘Golden Delicious’ and ‘Royal Gala’ appeared to be beneficial.
Environmental and endogenous influences on carbohydrate assimilation and allocation of apple trees (Malus domestica Borkh)
(Stellenbosch : Stellenbosch University, 2006-04) Pretorius, Jeremia Jesaja Bierman; Midgley, S. J. E.; Theron, K. I.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture
ENGLISH ABSTRACT: Market preferences for larger fruit have forced producers to adopt cultural practices that will ensure bigger fruit even if this comes at the expense of reduced total yields. In order to obtain acceptable fruit size there must be an adequate supply of photosynthetic carbon products especially during the cell division stage of fruit growth. Competition between fruits and between fruit and vegetative growth, as well as adverse climatic conditions, may limit the carbon supply to the fruits at this critical period and thus limit the final fruit size. Growers are showing renewed interest in the use of girdling or scoring in combination with the usual fruit thinning program to achieve growth control and increase fruit size. A new and milder chemical growth retardant, prohexadione-calcium (ProCa), is now also available for vegetative growth control of apple trees. [n this study, the effects of, and potential interaction between scoring, ProCa and fruit thinning were investigated, with respect to shoot and fruit growth, yield and photosynthetic capacity, in 'Royal Gala" 'Fuji' and 'Cripps' Pink' apple trees. ProCa decreased final extension shoot length in all three cultivars. Extension shoots were generally more sensitive than bourse shoots to scoring and ProCa. ProCa seems to be a more effective way of controlling shoot growth than scoring, with 'Fuji' and 'Cripps' Pink' being more sensitive than 'Royal Gala' to the application of ProCa. Scoring led to increased fruit growth rates during the first 40 days after full bloom (DAFB), and culminated in better fruit size at harvest. Scoring improved the total soluble solids concentration (TSS) of 'Fuji' and 'Cripps' Pink' compared to control and ProCa treatments. ProCa inhibited shoot growth effectively, but no evidence was found for improved carbon allocation to fruits and reproductive buds. Yield efficiencies of scored trees were significantly improved in all cultivars during the second season, due to better reproductive bud development after the first year of scoring. On 'Royal Gala' and 'Cripps' Pink', the efficiency of scoring to stimulate reproductive bud development on old and new spurs declined after 4 weeks after full bloom (WAFB). In 'Fuji', scoring later than I WAFB led to a decreasing positive response on old and new spurs. In contrast, reproductive bud development on long shoots increased with later scoring (6 and 8 W AFB) on all cultivars. It seems that the most beneficial time of scoring is 2-4 W AFB, as early as possible during the cell division stage offruit growth, but not before natural drop has occurred. The combination of early-season scoring and application of ProCa seems to hold potential for increasing carbon allocation to the fruit and improving fruit size and quality attributes. Scoring early in the season reduced photosynthetic capacity, and this reduction in carbon availability led to earlier cessation of shoot growth as well as shorter shoots. Later in the season, reduced fruit numbers led to a decrease in photosynthesis. The optimum temperature range for photosynthesis was found to adjust according to seasonal temperature variations. The harvest-induced reduction in sink strength changed stomatal sensitivity to higher temperature. Due to a reduced demand for carbohydrates by the plant, the maximum rate of photosynthesis (Amax) was reduced post-harvest. Following this reduction in sink strength and Am .. , stomata became more sensitive to high leaf temperatures, thus restricting water loss. Pre-harvest there was a strong demand for carbohydrates, therefore stomata were kept open at higher temperatures to ensure a high rate of C02 incorporation, but at a cost with regard to water use efficiency. The sharp increase in dark respiration (Rd) in leaves and fruit with an increase in temperature would mean that significant carbohydrate shortages could occur in trees during source limited periods early in the season, especially under warm weather conditions commonly experienced in the Western Cape region.
Flower initiation and development of Protea cv. Carnival
(Stellenbosch : Stellenbosch University, 2006-12) Hoffman, Eleanor Wilhelmina; Jacobs, G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Advancement of the flowering time of Protea cv. Carnival by approximately three months, without compromising the product quality, was achieved by the application of 6- benzyladenine-containing plant growth regulators to three-flush shoots in autumn. This earlier flowering time coincides favourably with the prime European marketing period (November-January). The percentage three-flush shoots initiating an inflorescence following the brush application of the 6-benzyladenine (BA)-containing regulators, ABG- 3062 (active ingredient: BA 2% w/w) and Accel® (active ingredients: BA 1.8% w/w; gibberellins A4A7 0.18% w/w) on dormant terminal buds, increased with later application dates and flowering percentages as high as 90% was achieved. No inflorescences were initiated on flushes induced by Promalin® (active ingredients: BA 1.8% w/w; gibberellins A4A7 1.8% w/w). Phenological phase progression of green point, flush expansion and inflorescence development of 'Carnival' shoots as induced by BA was calculated to have base temperatures of 8°C, 6°C and 1°C respectively. The days required from application of the BA-containing growth regulator until green point stage increased progressively over the six consecutive treatment dates in autumn (14 March - 22 May 2003). In contrast, the days required to complete inflorescence development decreased with each successive treatment date. The days required between the respective stages were mostly negatively correlated with temperature, except for the phase 'green point to flush expansion', where the relationship was unclear. For three-flush shoots of eight-year old plants, between 13-57, 39-65 and 121-177 days were required to reach green point, to achieve full flush expansion following green point and to complete inflorescence after flush expansion respectively. BA application enhanced budbreak in most dormant shoots, irrespective of plant age, BA concentration, decreasing temperature over time or shoot characteristics. However, twoflush shoots treated in late May had low budbreak and hence low flowering percentages. Shoots varied considerably in their responsiveness to BA treatments. BA application (500mg·L-1) as MaxCelTM (active ingredients: BA 1.9% w/w) to terminal buds alone of mature three-flush shoots from less vigorous growing plants resulted in the highest flowering percentages. Applications were most effective when applied to the terminal bud in the dormant state or up to the ‘green point’ stage. Shoot characteristics such as flush length, leaf area, shoot dry mass, number and proximity of the leaves to the terminal bud were all positively correlated with the propensity of shoots to initiate inflorescence under BA induction. Terminal flush intercalation shoot diameter (>7mm) was identified as the most important variable influencing the likeliness of flowering and can effectively serve as a nondestructive estimation of a shoot's propensity to flower. The presence of developing inflorescences or possible floral inhibiting factors derived from the previous flowering season is suggested to be inhibitory to inflorescence initiation following BA application. Synchronisation of shoot growth by pruning plants in late winter appears to be an essential step to ensure high percentages inflorescence initiation with BA treatment the following autumn. The use of BA as a management tool to control flowering times in Protea for better market opportunities is shown to hold considerable commercial potential.
Growth manipulations of Leucospermum cut flowers with the application of plant growth regulators
(Stellenbosch : Stellenbosch University, 2020-04) Louw, Eugenie-Lien (nee Bezuidenhout); Hoffman, E. W. ; Jacobs, Gerard; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Leucospermum, an indigenous South African cut flower genus, is cultivated worldwide for its striking and boldly-coloured inflorescences. However, the over-production of short (linked to disbudding or production in warm areas) and curved (associated with certain cultivars’ spreading growth habits) shoots negatively affects the Cape flora brand and profitability. This study therefore evaluated the efficacy of selected plant growth regulator (PGR) treatments to overcome the growth and developmental limitations that lead to short and curved shoots in selected Leucospermum cultivars. When Leucospermum shoot growth was monitored to determine the timing of appendage formation relative to internode extension, the full complement of true leaves was found to be differentiated early during shoot extension. This period proved to be the optimum period for PGR application to promote shoot length and straightness. Seven PGR treatments consisting of 6-benzyladenine (6-BA) and/or gibberellin (GA3, GA4 and GA7) were applied to single ‘Succession II’ and ‘Soleil’ shoots in 2013–14. In 2014– 15 the following treatments were applied to ‘Succession II’ and ‘Soleil’ at the environmentally contrasting Arnelia and Pomona farms, as well as to ‘Jelena’ at Pomona: 3×500 mg L−¹ GA3, 1×250 mg L−¹ 6-BA followed by 2×500 mg L−¹ GA3, and 3×100 mg L−¹ 6-BA and GA4+7. In 2015–16, ‘Soleil’ at Arnelia and ‘Jelena’ at Pomona received applications of 3×500 mg L−¹ GA3 or 3×100 mg L−¹ 6-BA and GA4+7. The PGR treatments elongated shoots by as much as 20 cm, producing more, but smaller, lighter leaves and higher stem dry weights cm−¹, while inflorescence quality was generally unaffected. Plants treated with PGRs displayed more upright growth, which resulted in straighter stems, thereby increasing the stem marketability by 40%. This adjusted plant morphology may be linked to higher gibberellin levels, smaller leaves, and a more even distribution of xylem in the vascular bundles. Based on trials on ‘Soleil’ plants at Arnelia in 2016–17, the lowest GA3 application to significantly affect shoot length was 100 mg L−¹. Three applications of combined 100 mg L−¹ GA3 and 100 mg L−¹ 6-BA and GA4+7 at 7–14 day intervals, when developing shoots are generally shorter than 10 cm, is therefore recommended for commercial use. Despite significant shoot variability, PGR treatment successfully improved yields of shoots in the 60–70 cm length categories. To explore the curvature-inducing effects of gravity and wind on Leucospermum shoots, ‘Succession II’ and ‘Soleil’ plants, with their contrasting growth habits, were compared for their response to gravity as well as rate of lignification. ‘Succession II’ displayed a lower modulus of elasticity and produced bends with a large radius of curvature. Results suggested that the desirable, upright growth habit of ‘Succession II’, despite exposure to constant early summer winds, may be due to its weak reaction to gravity, as was observed when stems were placed at an angle. The commercial impact of this study is significant and the PGR protocol to extend shoot length and increase shoot straightness may be expanded to other problematic cultivars.
Impacts of preharvest and postharvest handling and processing on bioactive compounds and functional properties of pomegranate fruit fractions and by-products
(Stellenbosch : Stellenbosch University, 2016-03) Mphahlele, Rebogile Ramaesele; Opara, Umezuruike Linus; Fawole, Amos Olaniyi; Stander, Marietjie; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Pomegranate fruit (Punica granatum L. Punicaceae) is highly valued owing to its high concentration of bioactive compounds found in the arils and peel. In fact, evidence from literature indicates that pomegranate fruit consumption has been associated with reduced risk of life threatening non-communicable diseases such as cancers and cardiovascular disorders. Although substantial amount of research has been reported on the effects of preharvest factors on phytochemical and functional properties of pomegranate, including cultivar and micro-climatic differences, little is known about the effects of postharvest and processing techniques on individual phenolic concentrations of fruit fractions such as arils and peel. The aim of this study was therefore to examine the impacts of preharvest and postharvest handling factors and processing methods on bioactive components and functional properties of pomegranate fruit and by-products. Drying characteristics and a thin-layer drying model for pomegranate peel over a wide temperature range were included in this study given the importance of drying as a commonly applied processing method in the processing of high-moisture products such as fruit. The results showed that concentrations of total phenolic and total tannin as well as radical scavenging activity (RSA) by DPPH assay declined as fruit maturity advanced, while ferric reducing antioxidant power (FRAP), total anthocyanin, total flavonoid and vitamin C concentration increased significantly (P<0.01). Principal component analysis (PCA) demonstrated that fruit grown in areas with lower altitude were associated with higher bioactive compounds at the full ripe stage. The study also showed significant (P<0.05) interaction effect between fruit maturity and altitude of the growing location on the phenolic compounds concentration. Studies on the effect of different extraction methods on phenolic compounds and antioxidant properties of pomegranate juice did not show significant influence (P>0.05) on fructose and total soluble solid concentration of pomegranate juice. Juice obtained from arils plus seed had the lowest citric acid concentration (18.96 g/L) and high juice colour saturation (2.69). Juice obtained by pressing fruit cut in half along the longitudinal axis (halved fruit) had significantly higher total phenolics, total tannins, radical scavenging activity and ferric reducing antioxidant power, which highlights the impact of extraction method on the quality of pomegranate juice. The influence of packaging and long term cold storage of whole pomegranates on phenolic compounds and antioxidant properties of fruit fractions and by-products thereof was also investigated. The result showed that total phenolics in pomegranate juice and peel decreased significantly (P<0.05) with prolonged storage duration regardless of package type. Catechin increased by 65.43% under modified atmosphere package (MAP) while rutin increased by 139.39% in individual shrink wrap package after 4 months of cold storage. Rutin was the predominant flavonoid in peel (3446.24 mg/kg dry matter), and its concentration decreased by 65% in fruit peel stored in MAP at the end of the storage (4 months). The study showed that punicic acid constituted 68.09% of total fatty acids in the seed oil and the concentration did not change significantly after 4 months under MAP and individual shrink wrap packaging, respectively. Fruit peel of whole pomegranates stored in individual shrink wrap package showed poor inhibitory activity against Gram negative bacteria (Klebsiella pneumonia), with minimum inhibitory concentration (MIC) of 1.56 mg/mL while seed oil showed better activity against diphenolase with inhibitory concentration (IC50) of 0.49 μg/mL after 4 months of storage. The effects of drying on the phenolic concentration, antioxidant, antibacterial and anti-tyrosinase properties were also studied. Freeze dried peel extracts had the highest total phenolic, tannin and flavonoid concentration compared to oven dried peel at the temperatures studied (40°C, 50°C and 60°C). Pomegranate peel extracts dried at 50°C showed the highest inhibitory activity with MIC value of 0.10 mg/mL against Gram positive bacteria (Staphylococcus aureus and Bacillus subtili) and monophenolase (22.95 mg/mL). Drying behaviour of pomegranate peels showed that drying time decreased as the oven drying temperature increased. The effective moisture diffusivity of pomegranate peel ranged from 4.05 x 10-10 to 8.10 x 10-10 m2/s over the temperature range investigated, with mean activation energy (Ea) of 22.25 kJ/mol. Empirical models were successfully applied to describe drying kinetics of pomegranate peel and these models could be used as analytical tools for future drying performance assessment.
Improving resilience in rainfed and irrigated agriculture under future climate in the winter rainfall region of the Western Cape: lessons from the 2015-2018 drought
(Stellenbosch : Stellenbosch University, 2022-04) Theron, Simone; Archer, Emma; Midgley, S. J. E.; Walker, J; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Drought has been identified as a key vulnerability for agriculture under climate change - most notably in southern Africa. Recently, the Western Cape Province of South Africa experienced a multi-year severe drought from 2015-2018, which negatively impacted the rural and urban economy. Studies have shown that short-term emergency responses can guide long-term planning. Thus, lessons from the drought can inform responses to climate change and improve resilience in the agricultural sector. The overall aim of this study was to capture the lessons learned from the 2015-2018 Western Cape Drought and how these lessons can be used to build resilience to drought for crop production under climate change in the Western Cape. Through this aim the study intended to shed light on various aspects of climate change response, including adaptation, resilience, adaptive capacity, and barriers to adaptation. The study focused on commercial wheat and apple farmers to represent rainfed and irrigated agriculture, respectively. Four production regions in the Western Cape were studied: the Swartland, the Rûens, Ceres and Elgin-Grabouw-Vyeboom-Villiersdorp. The study used observed climate datasets and production data to understand the impacts of the drought. Drought indices were computed to assess the drought severity, as well as its spatio-temporal extent. The impacts of climate change on the production regions were gauged using a CMIP5 ensemble up to 2065. In-depth interviews with farmers and actors within the agricultural sector were conducted and supplemented with an online questionnaire which was analysed using RandomForests. The study also used the Sustainable Livelihoods Framework, which allowed for the assessment of financial, human, and social capital, which was not captured in the climate analysis. It was found that the Western Cape experiences recurrent drought and that the 2015-2018 drought was the most severe in the past 30 years. This may be an early indication of the effects of climate change on the region. In response to the drought, most farmers acknowledged they had learnt several lessons and have since changed some of their farming practices. It was found that farmers who make use of weather forecasts were more likely to feel that their farm’s response to the drought was effective. It was also found that human and social capital were instrumental in reducing the impacts of the drought. The study found that drought indices can improve the skill of seasonal drought forecasts in the winter rainfall region. The results suggest that climate change is likely to impact all crop production regions in the province, albeit to varying extents. Increases in minimum and maximum temperature, as well as drought intensity, were found to be significant, with significant change likely to occur between 2040-2050. From the results, five key lessons were identified: drought is a reoccurring phenomenon in this region; forecasts are an essential tool for building resilience; drought indices can be a valuable component of seasonal drought prediction; farmers have high autonomous adaptative capacity; and improving multiple capitals available to farmers can improve resilience. It is envisaged that the results of this study can guide policymakers and government in understanding the risks faced by commercial crop farmers in the Western Cape and other water-stressed regions. Furthermore, it is hoped this study can add to the discourse surrounding agricultural adaptation and sustainability under climate change.
Inflorescence initiation and development, and the manipulation therof [sic], in selected cultivars of the genus Protea
(Stellenbosch : Stellenbosch University, 2000-03) Gerber, Audrey I. (Audrey Inga); Jacobs, G.; Theron, K. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
ENGLISH ABSTRACT: Little is understood regarding flowering in the genus Protea. The information available on inflorescence initiation and development in the family Proteaceae was reviewed and discussed. A number of experiments were conducted to investigate inflorescence initiation and development, and their manipulation for commercial production, in selected Protea cultivars, in the Western Cape, South Africa (33°S, Protea species can be allocated into groups according to similar times of flower initiation and of harvest. The stages occurring during flower initiation, and their synchrony relative to shoot growth were investigated for three cultivars, viz. Protea cv. Carnival (P. compacta x P. neriifolia), Protea cv. Lady Di (P. compacta x P. magnifica) and Protea cv. Sylvia (P. eximia x P. susannae), when flower initiation occurred on the spring growth flush. For all three cultivars the spring flush was preformed and enclosed in the apical bud before spring budbreak. During elongation of the spring flush the apical meristem produced floral primordia which differentiated into involucral bracts. After completion of the spring flush meristematic activity continued, to produce floral bracts with florets in their axils. The three cultivars showed differences and similarities in the time of budbreak, and the rates of shoot growth, appendage formation and flower development. The presence of mature leaves on an over-wintering shoot is essential for inflorescence initiation on the spring growth flush of 'Carnival'. Inflorescence initiation in 'Carnival' started at spring budbreak, and production of involucral bracts occurred concurrently with spring flush elongation. Shoots were defoliated at different degrees of severity at intervals from pre- to post- spring budbreak. Total defoliation applied earlier than 6-7 weeks before spring budbreak prevented flowering. Defoliation closer to spring budbreak affected characteristics of the spring flush and the inflorescence subtended by the spring flush. Effects were most marked following total defoliation and diminished with less severe treatments imposed by partial defoliation. Total defoliation applied before spring budbreak resulted in slower inflorescence development and lead to later anthesis. Defoliation treatments applied after completion of spring flush elongation had no effect on either vegetative or reproductive spring growth. The requirement for mature overwintering leaves to effect inflorescence initiation in 'Carnival' suggests that environmental factors, such as low temperature and daylength may play an inductive role. Shoots were in the induced state and committed to flowering 6-7 weeks before spring budbreak. A change in source size and position subsequent to different severalties of defoliation in 'Carnival' lead to reduced dry mass accumulation and altered partitioning. Mature leaves on the overwintering shoot supported growth of the spring flush and the early stages of inflorescence development. When these leaves were removed by total defoliation dry mass accumulation in the spring flush was reduced. A hierarchy of priorities between competing sinks was revealed by defoliation during growth of the spring flush and concomitant inflorescence development: formation of involucral bracts> leaf growth> stem elongation. Dry mass accumulation of the inflorescence subtended by the spring flush was supported by the spring flush leaves and was only indirectly affected by defoliation. Treatments which resulted in the production of a weaker spring flush lead to a reduction in dry mass accumulation of the inflorescence. Different severalties of partial defoliation, whereby either upper or lower leaves were removed from a shoot, indicated that the position of leaves relative to the active sink is more important, with respect to source availability, than the number of leaves on the shoot. Mature overwintering leaves are essential in 'Lady Di' for shoots to achieve the induced state for flowering; and are also crucial to the early stages of inflorescence initiation. Defoliation applied before formation of involucral bracts was complete prevented flowering. Defoliated shoots either remained vegetative or produced inflorescences which aborted. Reserve carbohydrates in the stem and leaves of overwintering shoots were low, and early growth and development of both the spring flush and inflorescence were, therefore, supported by current photosynthates from the overwintering leaves. Likewise, reserve carbohydrates available in the flowering shoot were insufficient to account for the dry mass increase during the major portion of growth of the spring flush and inflorescence. This rapid increase in dry mass occurred after elongation of the spring flush was complete and was supported by current photosynthates from the leaves of the spring flush. Defoliation treatments that did not prevent inflorescence initiation, had no effect on inflorescence development, and flowering time of 'Lady Di' was not delayed by defoliation. 'Sylvia' has an open window for inflorescence initiation and can initiate flowers throughout the year. Despite the 'open window' inflorescences are initiated more readily on the spring flush, when it is subtended by one or more overwintering shoots. This may be the expression of a facultative response to inductive conditions for which 'Carnival' and 'Lady Di' have an obligate requirement. The date of pruning affected flowering time of 'Sylvia' by influencing on which flush inflorescence initiation occurred, and the harvest could be manipulated to fall within the optimum marketing period for export to Europe. Flowers initiated on the spring flush reach anthesis in January and February; on the first summer flush predominantly in April and May; on the second summer flush in July and August; and on the autumn flush in November and December. Thus, shoots harvested within the optimum marketing period (September to February) initiated inflorescences on the autumn and spring flushes. Due to the readiness of shoots to initiate inflorescences on the spring flush many shoots harvested in January and February (following initiation in the previous spring) were short and were rendered unmarketable. For commercial production pruning in July is recommended. Long flowering stems will be harvested in October to November of the following year. Since the vegetative and reproductive cycles necessary to produce inflorescences on long stems span more than a year, a biennial cropping system is recommended.
The influence of cell wall bound calcium, cell number and size on the development of mealiness in 'Forelle' pear. Evaluation of X-ray CT and NIR as non-destructive techniques for mealiness detection
(Stellenbosch : Stellenbosch University, 2016-03) Muziri, Tavagwisa; Crouch, E. M.; Theron, K. I.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
ENGLISH ABSTRACT: ‘Forelle’ pear (Pyrus communis L.) is the second most planted pear and the second highest generator of foreign exchange for pears in South Africa. It is favoured for its red blush, melting texture, sweet taste and pear flavour. However, ‘Forelle’ develops mealiness, a floury, soft and dry texture with low extractable juice. Consumers dislike mealy fruit. ‘Forelle’ mealiness has been characterized by a loss of cell to cell binding during ripening in a previous study. This study aimed to further understand the role of cell wall bound and free Ca2+, as well as the cell size and cell number in the development of mealiness in ‘Forelle’. In addition, two non-destructive methods for the detection of mealiness in intact pears were examined. It was found that free Ca2+ constituted about 49-73% of the total cell Ca2+. Depending on farm origin, mealy fruit contained a lower free Ca2+ concentration compared to non-mealy pears. Plant growth regulators and selective blossom thinning that caused larger cells had a higher mealiness percentage. Scanning electron microscopy revealed larger intercellular spaces for treatments with a higher mealiness incidence. Macro X-ray computed tomography (X-ray CT) showed a higher percentage of defects in the neck of fruit that would become mealy after storage, and after softening. To our knowledge this is a first such finding. Micro (X-ray CT) found that cells of mealy fruit were larger and ellipsoidal in shape while non-mealy cells were smaller and more rounded. Mealiness was also associated with high fruit porosity. A further study described physicochemical measurements which relate to mealiness. Mealy fruit were mostly larger with a higher total soluble solids (TSS), TSS:TA ratio and lower juice area and juice weight obtained by a confined compression method. Fourier transform near-infrared absorbance spectroscopy (FT-NIR) was employed to determine if spectra could be used to distinguish between mealy and non-mealy fruit using sensory and TSS based schemes. Classification was done using orthogonal partial least squares discriminant analysis (OPLS-DA). This study showed that FT-NIR spectra can indeed be used to discriminate between mealy and non-mealy ‘Forelle’ pears. Two–class (mealy and non-mealy) discriminant analysis produced models with accuracies ranging from 51% to 95%. Mealiness caused an increase in transmittance in specific regions of the spectra. FT-NIR was then evaluated for the quantification of TSS using partial least squares (PLS) regression. Validated models had root mean squared error of prediction (RMSEP) = 0.76-0.94 and relative prediction deviation (RPD) = 1.53-2.17, with the equator blush consistently giving better performance for three farms making the model ideal for hand held FT-NIR applications. External validation results of farm location showed reduced model robustness. The decrease in prediction performance was attributed to the differing TSS ranges in locations and possibly seasons. It is recommended that future studies on FT-NIRs calibration models for ‘Forelle’ use fruit from wide origins with wide TSS ranges over various seasons.
Influence of crop based water and nutrient strategies on physiological aspects of apple trees ‘Brookfield Gala’
(Stellenbosch : Stellenbosch University, 2008-12) Lebese, Thabiso C.; Stassen, P. J. C.; Midgley, S. J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
It is a common practise in the Western Cape to use micro sprinklers as the standard irrigation system for apple trees. Over the past forty years much effort has been put into the optimisation of the tree canopy. Less attention has been given to root proliferation, and the question as to whether root stimulation and proliferation, through intensive water and nutrient management, can contribute towards improved tree efficiency and more efficient water use. This is addressed in this study. ‘Brookfield Gala’ apple trees were studied in the Genadendal area near Greyton, in the Western Cape Province, South Africa. The trees were planted in Dundee soil (well aerated sandy loam soil) during winter 2003. Both horticultural aspects (tree growth, shoot growth, fruit yield and quality, trunk circumference and root growth) and gas exchange were studied from 2004/5 until 2007/8 under three different water application strategies, namely micro sprinkler irrigation, daily drip and pulsing drip irrigation and using two different rootstocks: M793 and M7. Irrigation under micro sprinkler irrigation was applied once to three times weekly, daily drip irrigation once daily/twice daily, and pulsing drip irrigation one to six times daily. Water use for bearing apple trees was calculated using long-term evaporation data (for Villiersdorp and Caledon) and existing crop factors for apples. Annual nutrient requirements were adapted from literature and divided percentage-wise into the requirements for five different phenological stages. Soil sensors were used to keep plant available soil water between 100% and 50%. A computer software program was used to incorporate all the above mentioned information and calculate the exact amounts of water and nutrients, and the application times. In general, drip irrigation systems used ±26% less water than micro sprinkler irrigation system. Significantly higher fruit yields were obtained with trees under daily or pulsing drip irrigation than those under micro irrigation during 2005/6 and 2007/8. During 2006/7 the crop load was low due to unfavourable weather conditions during flowering, resulting in poor fruit set and no differences in yield. There was a significantly higher number of thin plus medium roots (3mm and less in diameter) in the 0─400mm rooting zone and total root mass at 0─800mm rooting zone under drip irrigation systems than under micro sprinkler irrigation. Brookfield Gala’ apple trees grown under daily drip irrigation and pulsing drip irrigation performed better compared to those grown under micro sprinkler irrigation with respect to CO2 assimilation rate (A), stomatal conductance (gs), water use efficiency (WUE) and leaf water potential. None of the three irrigation systems affected the biochemical efficiency of the leaf significantly, except on a few occasions during the pre-harvest period. This implied that the changes in leaf biochemical efficiency were as a result of both stomatal and non-stomatal effects (temperature and vapour pressure deficit). The removal of fruit at harvest had a great influence on leaf photosynthetic capacity under micro irrigation but less so under drip irrigation systems. Higher chlorophyll a and chlorophyll b concentrations were observed under drip irrigation systems than under micro sprinkler irrigation, implying efficient biochemical efficiency under these systems compared to micro sprinkler irrigation during the post-harvest period. Use of daily drip irrigation and pulsing drip irrigation delayed the process of leaf ageing. This study demonstrated the benefits of more intensive water and nutrient application for apple trees. Improved root proliferation, increased fruit yield and photosynthetic efficiency have been found under drip irrigation system than under micro sprinkler irrigation.
Leaf analysis as a means of assessing the nutrient status of deciduous fruit trees and vines in the Western Cape Province
(Stellenbosch : Stellenbosch University, 1958-12) Beyers, Ewald; De V. Malherbe, I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
OBJECTIVE. High economic production has ever been the aim and aspiration of the agriculturist and no less that of the fruit farmer. In striving towards this aim the latter has for a long time been at a disadvantage with regard to control of his nutritional programme. Even on naturally fertile soil, the question continually arises as to what the correct fertilizer treatment should be to maintain high productivity and how such a decision can be arrived at. A satisfactory answer to these questions could have been obtained from fertilizer trials if it was not such a difficult matter, in view of the extensive and long-term nature of such trials with fruit trees, to establish a sufficient number for each fruit species on different soil types and under different climatic conditions. Efforts to find a new approach to the problem have turned attention to the plant itself and its chemical make-up as affording the best index of its nutritional requirements. Intensive work in this direction has resulted in the evolution of a new tool in agriculture, the technique of diagnostic leaf analysis or 1Toliar diagnosis" as originally proposed by Lagatu and Maume in France and Thomas in u.s.A. A review of the literature is presented indicating the prodigous amount of research which has been applied to studies of the relationship between plant response and nutrient supply in terms of plant composition. Agriculturists have been quick to recognize the potentialities of leaf analysis as a practical guide in nutritional problems and advisory services based on foliar analysis have already been established for certain crops overseas. The experimental basis for formulating such a scheme for deciduous fruit in the Western Cape Province is provided by the factual evidence presented in this thesis.THE TECHNIQUE. The technique of diagnostic leaf analysis comprises sampling of leaves, preparation of sample for analysis and the analysis itself followed by interpretation of the analytical results by comparison with previously determined nutritional standards. Numerous factors were found to influence the final composition of the leaf sample as determined by analysis, such that strict adherence to a standardized procedure through all phases of sampling and preparation of leaf samples for analysis is required to eliminate or reduce errors likely to cause misleading interpretations. Experimental data are presented suggesting how the leaf sample should be selected on a tree and how it should be handled, cleaned, dried, ground and stored to reduce sampling and other errors. The final procedure as adopted eliminates most of the potential sources of experimental error but two unavoidable sources of e~ror remain to be accounted for, that due to tree variation and seasonal effect. The variation in leaf composition from tree to tree was found to be very considerable, so that aampling from a large enough group of trees (6 to 10) to reduce the error involved is essential in order to obtain leaf data which correctly reflects the nutrient status of the portion of the orchard concerned. Secondly, on the grounds of marked consistency found in different fruit species as to seasonal and year to year variation in mineral nutrient concentration, correction factors have been formulated and are suggested as a means of overcoming these sources of error. THEORETICAL BASIS. A diagnosis of the nutrient status in terms of the analytical results as finally determined is obtained by comparison of the data with previously established leaf composition standards of reference and by correct interpretation of the deviations from these standards. The theoretical basis for setting up these index values is discussed. The criterion used is based on the concept of Optimum Values which aaequately integrates the known relationships between plant response and nutrient supply in terms of internal nutrient concentration. A modification of this concept is proposed to the effect that for maximum growth and yield there exists an optimum range of nutrient concentrations with upper and lower limits for each of the functional elements, and that within this range the interrelationship between the individual nutrient elements is also optimal. Since no local fertilize~ trials with deciduous fruit trees are available and only one for grapes, data from highly productive plants in commercial orchards and vineyards were used to determine the upper and lower limits of the "optimum range", on the following premise. If leaf analysis data are available from a sufficient number of high performance orchards in different localities representing a wide range of nutrient supply and environment, the highest and lowest values obtained may be considered to represent a close approximation of the limits of the range required for optimum performance. It is contended that index values obtained in this way must be of practical value in assessing the nutrient status of fruit trees. It is further postulated that the lower limits for the micro-nutrients and even for magnesium may be justifiably adjusted according to the concentration levels associated with symptom expression. INDEX VALUES. The necessary data for determining standards of leaf composition were obtained from leaf analysis surveys of orchards and vineyards and from a grape fertilizer experiment in the Western Cape Province. Visual symptoms of prevailing nutritional disorders are described (supplemented by photographic illustrations) and their relation to leaf composition indicated. Tentative index values have been determined on the basis indicated for each fruit species, apple, pear, peach, apricot, plum, prune and grapes. These nutritional levels comprise upper and lower limits for the nutrients N, P, K, Ca, Mg 1 Mn 1 Fe and Cu, as well as the upper limits for B and Na. DIAGNOSTIC INTERPRETATIONS. Assessment of the nutrient status in terms of these index values suggests that many orchards and vineyards in the Western Cape Province, particularly prune, apricot and grapes, are suffering from malnutrition in some form and are likely to show a marked response to nutritional treatment as suggested by foliar diagnosis. The use of diagnostic leaf analysis constitutes an important advance in dealing with orohard problems in that an immediate decision is possible regarding nutrient status and related aspects such as selection of suitable sites for fertilizer trials and adjustment of the fertilizer programme.
Leaf blackening of proteas
(Stellenbosch : Stellenbosch University, 2003-04) Stephens, Iain Andrew; Jacobs, G.; Holcroft, D. M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticultural Science .
ENGLISH ABSTRACT: Leaf blackening is a particular problem limiting vase life and marketability of Protea cut flowers. This research investigated suppression of Protea leaf blackening with a specific focus on Protea cv. Sylvia (P. eximia x P. susannae) cut flowers. Leaf blackening decreased significantly with decreasing storage temperatures m 'Sylvia' proteas and this was attributed to lower respiration rate and conservation of carbohydrate. Low storage temperatures were beneficial in short term handling procedures encountered during airfreight. However, use of low temperatures alone during the longer sea freight period was unsatisfactory in either maintaining or extending 'Sylvia' protea vase life. Cooling of 'Sylvia' proteas under vacuum significantly suppressed leaf blackening and was of greater benefit than forced air cooling. Although removal of the uppermost leaves delayed leaf blackening in short term storage no significant benefit was found for longer storage periods. Girdling directly beneath the 'Sylvia' protea flowerhead significantly reduced leaf blackening and in combination with low storage temperatures (O°C) enabled a significant extension in both storage and vase life of 'Sylvia' proteas. 'Sylvia' proteas did not exhibit a climacteric respiration peak during 96 h storage at O°C. Exposure to ethylene did not increase Protea leaf blackening or have a detrimental effect on vase life of either proteas or pincushions evaluated. No beneficial response to sucrose supplementation was found in 'Sylvia' proteas. Analysis of the sugar content of both flowerhead and leaves indicated that glucose supplementation might be of benefit and was investigated. Holding solutions of 2.5 % glucose significantly extended vase life due to a significant reduction in leaf blackening. Vase life was terminated due to flowerhead collapse instead of leaf blackening for the first time in 'Sylvia' protea cut flowers. Vase life was significantly extended by 2:3% glucose pulse solutions and leaf blackening significantly suppressed with increasing glucose pulse concentration. Solution uptake was facilitated by use of high intensity PAR lights in the early morning and was attributed to increased stomata opening and a consequent increase in both transpiration and glucose solution uptake. The faster uptake of glucose solutions in shoots harvested in the afternoon was attributed to higher shoot temperatures and consequent transpiration rate to those harvested in the morning. There was a significant reduction in uptake time with increasing pulse temperature, which enabled vacuum cooling to be performed earlier further benefiting storage and vase life extension. Enclosure of 'Sylvia' proteas in polyethylene (PE) lined cartons did suppress leaf blackening in non-pulsed shoots. However, this had no practical significance on useful vase life, which was terminated at this point due to excessive leaf blackening. Water loss appears to have a minimal influence on 'Sylvia' protea leaf blackening. Shading at four and three weeks prior to harvest coincided with a period of significant flowerhead dry mass increase. It is thought that shading at this point, concurrent with an increased carbohydrate demand by the developing flower head resulted in a temporary limitation in carbohydrate supply resulting in the appearance of preharvest leaf blackening. It would appear that proteas do not store large quantities of carbohydrate. Although accentuating winter light conditions by shading did result in a decrease in carbohydrate content the fact that carbohydrate content was already low precluded shading from having a significant impact on postharvest leaf blackening. The finding that glucose was beneficial in extension of both storage and vase life of 'Sylvia' proteas directed research into its use for other Protea and Leucospermum cut flowers. Significant differences in the response to glucose supplementation were found in both Protea and Leucospermum (pincushions). The significant difference in sensitivity to glucose concentration in 'Pink Ice' proteas (phytotoxic at 2:4%) and 'Susara' proteas (no apparent toxicity), in conjunction with a lack of response in 'Cardinal' proteas, a hybrid from the same parents as 'Sylvia' indicates the need to direct future research to individual cultivars. Glucose supplementation had no beneficial effect on vase life of 'Scarlet Ribbon' and 'Tango' pincushions, whilst significantly extending vase life of 'Cordi', 'Gold Dust', 'High Gold' and 'Succession' pincushions.

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