Department of Horticulture
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Browsing Department of Horticulture by browse.metadata.advisor "Barry, G. H."
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- ItemCritical 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.
- ItemEcophysiological responses of citrus trees and sugar accumulation of fruit in response to altered plant water relations(Stellenbosch : University of Stellenbosch, 2007-03) Prinsloo, Johan Andries; Barry, G. H.; Rabe, E.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.This study was undertaken to quantify some of the effects of daily fertigation on ecophysiological responses in citrus trees (Citrus spp.). Initial research was conducted to optimise and standardise the sampling procedure to quantify stem water potential (ψstem) in citrus trees. To reliably determine the plant water status of citrus trees, the following conditions are required to minimise unwanted variation in ψstem measurements. Bagging of leaves with black polyethylene envelopes covered with aluminium foil 3 to 4 hours prior to measuring ψstem allows the plant water status in those leaves to equilibrate with whole-tree plant water status, thereby providing a realistic measurement of the current water status. The use of aluminium foil to cover the bagged leaves, reduces unwanted heat stress by reflecting sunlight, and dramatically reduced variation in ψstem. The time of day at which ψstem measurements are made is important to ensure consistency in comparisons among treatments and interpretation of irrigation treatment effects. “Physiological midday” is the preferred time of day to measure ψstem, i.e. 1100 HR. Transpiring leaves with open stomata would be in sun-exposed positions on the east side of trees and should be used for making ψstem measurements. Under similar experimental conditions as those used here, only three leaves per replicate are required to detect a difference of 0.05 MPa in ψstem between treatment means. Plant water status categories were developed which may have useful practical applications, i.e. >-1.0 MPa = no water deficit; -1.0 to -1.2 MPa = low water deficit; -1.2 to -1.4 MPa = moderate water deficit; <-1.4 to -1.6 MPa = high water deficit; and <-1.6 MPa = severe water deficit. Attempts are being made to develop systems that improve crop management and enhance citrus fruit production through efficient and timeous application of water and mineral nutrients which has led to the use of daily drip fertigation or the open hydroponics system (OHS). However, the perceived benefits are not necessarily supported by facts. Fruit size and yield are apparently enhanced, but possible negative aspects of the system have not been quantified. Fruit produced on trees grown under daily drip fertigation generally have a lower total soluble solids concentration than on trees under micro-sprinkler irrigation. This is mainly due to a dilution effect that is caused by the greater availability of water and the uptake thereof. Sugar accumulation can be optimised by controlling the amount of water that the plant receives at different developmental stages. Therefore, it is essential to quantify the ecophysiological responses and benefits of OHS/daily fertigation, as well as the effects of this technology on fruit quality. ‘Nules Clementine’ mandarin (C. reticulata Blanco) trees in two commercial orchards in Simondium, Western Cape province, South Africa, received differential irrigation treatments. The treatments were applied at the end of stage I (± mid December) of fruit development. Stem water potential, fruit size and internal fruit quality were determined. Water-deficit stress enhanced sugar accumulation of ‘Nules Clementine’ mandarin by 0.3 to 0.6 °Brix under certain conditions. These conditions require that the difference in ψstem should be of a sufficient intensity of between 0.16 and 0.3 MPa, and this difference should be maintained for a sufficient duration of between 4 and 6 weeks. Furthermore, deficit irrigation should be applied relatively early in fruit development, namely during the sugar accumulation stage which starts within 4 weeks of the end of the fruit drop period and continues until harvest.
- ItemFactors affecting post-storage quality of ‘Nules Clementine’ mandarin fruit with special reference to rind breakdown(Stellenbosch : University of Stellenbosch, 2006-12) Khumalo, Ngcebo Parton; Barry, G. H.; Huysamer, M.; De Kock, A. V.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.Rind breakdown of ‘Nules Clementine’ mandarin (Citrus reticulata Blanco) is a physiological rind disorder that develops during storage. The disorder appears following leakage of essential oil from oil glands in the flavedo, which then leaks into and oxidises the albedo. Oxidised tissue appears as brown spots in the rind. Occurrence of this disorder over the years has caused high financial losses to ‘Clementine’ mandarin producers and exporting companies. Therefore, research aimed at solving this problem was identified as a priority by the citrus industry. Several factors have been reported to be associated with rind breakdown of ‘Clementine’ mandarin fruit, and include environmental factors, fruit maturity at harvest, ethylene gas degreening, storage temperature and storage duration, canopy position where fruit are borne, plant growth regulators, and differences in susceptibility among selections. Practical information has been generated on rind breakdown, but the basic physiology of the disorder is still unresolved. The objective of this study was, therefore, to quantify the effects of various factors on the development of rind breakdown of ‘Nules Clementine’ mandarin fruit, as well as to establish an association between rind pigments and rind antioxidant capacity on the development of this disorder. In this study a series of five experiments was conducted, and included quantifying the differences in susceptibility to rind breakdown between ‘Nules’ and ‘Oroval Clementine’ mandarin fruit, investigating the effects of fruit canopy position, harvest date, ethylene gas degreening, storage temperature and duration on the development of rind breakdown. The effect of these factors on rind pigments and antioxidant capacity was also reported. Generally, ‘Nules’ and ‘Oroval Clementine’ mandarin fruit exhibited similar characteristics at harvest, in terms of maturity and antioxidant capacity. After storage, ‘Nules Clementine’ mandarin developed higher levels of rind breakdown than ‘Oroval Clementine’ mandarin. However, the difference in susceptibility to rind breakdown of ‘Nules’ and ‘Oroval Clementine’ mandarin fruit could not be associated with the antioxidant capacity measured at harvest...
- ItemFactors affecting rind oil content of lemon (Citrus limon (L.) Burm. f.)(Stellenbosch : Stellenbosch University, 2005-12) Van der Merwe, Hester E. (Hester Elizabeth); Barry, G. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: Essential oils are derived from volatile natural oils in plants and have been used by mankind for millennia. Citrus essential oils are widely used in various applications and lemon [Citrus limon (L.) Burm. f.] rind oil is the most important citrus oil in commerce. Rind oil glands are located in the exocarp, or flavedo, of the fruit and are formed schizogenously. The purpose of this study was to quantify the factors affecting rind oil content of lemons. The factors studied were light exposure and canopy position, growing region in South Africa, genotype, i.e. scion and rootstock, as well as the relationship between seedless clones of cultivars and the cultivars from which the seedless clones were derived, and various plant growth regulators were screened to determine whether they influenced rind oil content. Following the sampling of fruit from different positions in the tree's canopy, light exposure was found to affect rind oil content of 'Eureka' lemon fruit. Fruit borne on the outside of trees, higher in the tree, north-facing or not within the hedgerow had the highest rind oil content. Photosynthetically active radiation data supports the hypothesis that rind oil content is correlated with light exposure. To optimise rind oil content of lemons, trees should not be too dense or too high as to overshadow the lower parts of adjacent trees. South Africa has a diverse climate, and rind oil content from fruit produced in different growing regions was compared. 'Eureka' lemon fruit from Upington had the highest rind oil content in all seasons sampled. Fruit from Malelane and Marble Hall ranked second to Upington and rind oil content for fruit from Karino was intermediate. Rind oil content for fruit from Vaalharts was the lowest at each sampling time. When rind oil content was regressed against cumulative heat units there was a positive linear relationship in 2003, but in 2004 the relationship was weak. However, III general, rind oil content increased with increasing heat unit accumulation. A large variation exists among citrus cultivars and rootstocks and their effect on fruit growth and quality. 'Lirnoneira 8A', followed by 'Cicily', 'Lisbon' and 'Genoa' had the highest rind oil content. 'Villafranca', 'Messina' and 'Yen Ben Lisbon' had the lowest rind oil content. Rind oil content from 'Eureka' lemon fruit was disappointingly low. Seedless cultivars, 'Eureka SL' and 'Lisbon SL', had ~18.0% higher rind oil content than the seeded cultivars from which they were derived. With regards to rootstock, fruit from lemon trees budded on non-invigorating rootstocks, e.g. X639 [e. reshni Hort. ex Tan. x P. trifoliata (L.) Raf.], had the highest rind oil content, whereas rind oil content was low on invigorating rootstocks such as rough lemon (e. jambhiri Lush.). Synthetic gibberellins, cytokinins, ethylene and auxins were applied on lemon trees at different times and concentrations to screen their ability to enhance rind oil content. Of all the gibberellins and cytokinins applied, Promalin®, a combination of gibberellic acid 4/7 and benzyl adenine-phosphate, a cytokinin, had a small, but nonsignificant effect on rind oil content. Ethephon, which induces ethylene synthesis, affected rind oil content in 2004, when applied 8 weeks before harvest. However, ethephon and aminoethoxyvinylglycine (AVG, an ethylene biosynthesis inhibitor) had an inconsistent effect on lemon rind oil content. Auxins did not affect rind oil content. Further experiments should be conducted, especially on the timing and concentration of applied gibberellins, e.g. Promalin®, and ethephon.
- ItemPreharvest manipulation of rind pigments of Citrus spp.(Stellenbosch : University of Stellenbosch, 2006-12) Le Roux, Smit; Barry, G. H.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.Rind colour is one of the main cosmetic preferences consumers use when purchasing citrus (Citrus spp.) fruit. To enhance the cosmetic quality of citrus fruit, attempts were made to stimulate preharvest chlorophyll degradation and carotenoid biosynthesis to obtain a deeper, more uniform, orange rind colour in early-maturing citrus cultivars. As part of a larger study to stimulate rind colour enhancement, an initial study was conducted on ‘Eureka’ lemon [C. limon (L.) Burm. f.] nursery trees to determine the concentration of various gibberellin biosynthesis inhibitors required to obtain a biological response in citrus trees, as measured by vegetative growth. Thereafter, different concentrations of prohexadione-calcium (ProCa; Regalis®) were applied at various stages of fruit development on early-maturing citrus cultivars to establish the concentration and timing of ProCa required to improve rind colour by enhancing chlorophyll degradation and carotenoid biosynthesis. In addition, a search to enhance rind colour development of early-maturing citrus cultivars was conducted by screening various nutritional, hormonal and possible physiological stress-inducer products and some combination treatments thereof. Multiple applications of gibberellin biosynthesis inhibitors on ‘Eureka’ lemon nursery trees significantly reduced internode length and hence vegetative growth. Regalis® applied at 4 to 8 g·L-1 and Sunny® (uniconazole) applied at 10 to 20 mL·L-1 had the greatest effect in reducing internode length, and were therefore identified as potential candidates for further field studies to test their effect on rind colour enhancement of citrus fruit. The late, double applications (6 plus 3 weeks before anticipated harvest) of ProCa applied at 400 mg·L-1 consistently improved rind colour of all Citrus spp. tested. However, these effects were more pronounced after harvest, as ethylene degreening and cold-storage stimulated additional chlorophyll degradation, unmasking the carotenoids, resulting in overall better coloured fruit. In most instances in this study, ProCa stimulated chlorophyll degradation allowing the underlying carotenoids to be expressed. Therefore, the improvement of rind colour of citrus fruit following the application of a gibberellin biosynthesis inhibitor (400 mg·L-1 ProCa applied 6 plus 3 weeks before harvest) supports the hypothesis that there may be a relationship between vegetative vigour and rind colour development of citrus fruit. Preharvest applications of boric acid, Thiovit® (elemental sulphur), ammonium thiosulphate (ATS) and half the recommended rate of Ethrel® (48% ethephon) in combination with Thiovit® and ATS stimulated chlorophyll degradation in both orange- and yellow-rinded fruit, and ColourUp® (neutralised calcium carbonate) and Figaron® (ethyclozate) stimulated chlorophyll degradation only in orange-rinded fruit. Boric acid and the Thiovit®-ATSEthrel ® combination treatment stimulated carotenoid biosynthesis in orange-rinded fruit, thereby improving the carotenoid to chlorophyll ratio. The screening of chemical products which stimulate chlorophyll degradation in combination with chemical products which stimulate carotenoid biosynthesis warrants further evaluation. Worldwide, research on rind colour improvement has received attention for several decades, particularly during the 1980s. Yet, rind colour still remains a problem at the beginning of certain seasons. In the present study, the approach to improving rind colour was to manipulate rind pigments through the reduction of vegetative vigour, which was hypothesised to be an antagonist of chloro-chromoplast transformation. To this end, the preharvest application of prohexadione-calcium stimulated chlorophyll degradation and carotenoid biosynthesis in citrus fruit rinds. Furthermore, preharvest applications of various chemical products provides a novel approach to stimulate chlorophyll degradation and carotenoid biosynthesis. Together, the results of this study provide potential commercial treatments that will result in deeper, more uniform orange rind colour, thereby meeting consumer needs.
- ItemTime-temperature interaction on postharvest rind colour development of Citrus(Stellenbosch : University of Stellenbosch, 2004-12) Van Wyk, Angelique A. (Angelique Ann); Barry, G. H.; Huysamer, M.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Rind colour is one of the most important external quality characteristics of citrus fruit and plays an important role in purchasing decisions by consumers. Consumers perceive brightlycoloured fruit to be sweet and mature, whereas citrus with a green rind is perceived to be sour and immature. However, there is a poor correlation between rind colour and internal quality, contradicting what is generally assumed by the fruit-buying public. In general, a bright orange rind colour improves consumer acceptance. Thus, it is important to ensure that the rind of citrus fruit is well-coloured on arrival at the market. Various pre-harvest cultural practices and postharvest techniques can be applied to improve rind colour. Degreening with ethylene gas is the most commonly used postharvest technology to improve rind colour, but has various negative side-effects. Degreened fruit are more prone to decay, have rinds which appear dull and flaccid, have been reported to develop off-flavours and have a shorter shelf-life period. Therefore, it is necessary to find alternatives to ethylene degreening and to extend shelf-life of citrus fruit. Under normal orchard conditions, rind colour development is associated with low night temperatures, usually experienced during autumn or following the passing of a cold front. To simulate cold front conditions, a hydrocooler and cold room were used to rapidly drop fruit temperature to 4 ºC for 6 hours, and then fruit were incubated at 20 to 22 ºC for 72 hours. This “cold shock” treatment of ‘Nules Clementine’ mandarin improved rind colour to a level similar to that of degreened fruit in the 2002 season due to a decrease in chlorophyll content and increase in carotenoid content. However, this result could not be repeated. Storage temperature is one of the most important postharvest factors affecting rind colour. Citrus fruit shipped to export markets requiring low temperatures (-0.6 ºC) for pest disinfestations purposes have been reported to arrive with poor rind colour. Shipping under low temperatures results in poor rind colour of fruit on arrival in the market. To comply with the USA’s phytosanitary requirement for imported citrus, fruit is held at -0.6 ºC for a minimum of 22 days. The effect of shipping at various temperatures (-0.6 ºC or 4.5 ºC), durations and the influence of initial rind colour, “orange” or “yellow”, on fruit colour upon arrival in the market was evaluated. Fruit shipped at a higher temperature (4.5 ºC) had a marginally better rind colour than fruit shipped at -0.6 ºC. The perceived loss of rind colour following shipping at sub-zero temperatures is probably due to carotenoid degradation. Therefore, initial rind colour plays a critical role in final product quality. Depending on market destination and shipping temperature, pale-coloured fruit should not be packed for markets sensitive to rind colour. Holding temperature after shipping can be effectively used to improve the rind colour of fruit arriving in the market with undesirable rind colour. An intermediate holding temperature of between 11 and 15 ºC resulted in the greatest improvement in rind colour after 2 weeks. A high holding temperature (20 ºC) caused colour degradation, whereas a low holding temperature (4.5 ºC) resulted in the maintenance of rind colour. By selecting the correct holding temperature, even after shipping at sub-zero temperatures, final colour can be improved.