Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by browse.metadata.advisor "Hoffman, E. W."

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    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.
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    Physiological and molecular mechanisms underlying shoot manipulation strategies with the plant growth regulator ethephon for the improvement of flowering in litchi (Litchi chinensis Sonn.) cv. ‘Mauritius’
    (Stellenbosch : Stellenbosch University, 2023-03) Cronje, Regina Barbara; Hoffman, E. W. ; Theron, K. I.; Huang, X. M.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Erratic flowering is a major challenge in most litchi (Litchi chinensis Sonn.) producing countries, including South Africa. In recent decades, above-average temperatures in autumn and early winter have increased the incidence of vegetative shoot growth prior to floral induction, further exacerbating irregular flowering and rendering the conventional ethephon shoot control measures less effective. Therefore, this study aimed to investigate the effect of ethephon applied to mature terminal shoots for its potential to prevent shoot growth prior to floral induction, to delay panicle emergence to a period with consistently low temperature and to promote carbohydrate accumulation for improved flowering and yield compared with conventional spot spray applications to emerging, immature vegetative shoots. In a four-year orchard-based study, ethephon was applied to ‘Mauritius’ trees, either as single or double whole-canopy spray at 500, 750 and 1000 mg⋅L⁻¹, each at three different dates between autumn and early winter, or as a combination of whole-canopy spray at first signs of shoot growth and spot sprays on developing shoots, in two climatically different production areas of South Africa. All whole-canopy treatments were compared with the conventional spot spray applications at 1000 mg⋅L⁻¹ and untreated trees. Results revealed that whole-canopy ethephon applications to mature terminal shoots successfully inhibited pre-induction vegetative shoot growth in a dose-dependent manner, subsequently delaying panicle emergence to a cooler period, which promoted carbohydrate accumulation, improved inflorescence quality, flowering rate and yield without delaying fruit maturity, compared with spot-sprayed and untreated trees. The date of application played an important role in the efficacy of ethephon applications, with higher ethephon concentrations being more beneficial under warmer conditions, such as in early autumn or generally in warm areas, while lower concentrations were sufficient for applications later in autumn and early winter. In a second study, ethylene evolution as well as phenological, physiological and molecular changes underlying ethephon application and its associations with bud dormancy and flowering were investigated, by comparing a single whole-canopy ethephon application at 1000 mg⋅L⁻¹ to mature terminal shoots with untreated trees. Ethylene evolution peaked in leaves on the day of application with a rapid decline thereafter, but persisted in buds for seven days before gradually declining. Ethephon application significantly increased relative expression of LcEIN3 and LcFLC in terminal buds one day after application, while LcFT2 expression in leaves and LcAP1 expression in terminal buds were significantly increased at the bud break stage. The ethephon treatment also significantly increased soluble sugar concentrations in leaves and shoots at the bud break or floral initiation stage. The current study provided evidence that ethephon application plays an important role in the physiological and molecular regulation of bud dormancy and subsequent floral regulation of litchi. By preventing pre-flowering shoot growth and influencing the time of bud break, ethephon application to mature terminal shoots proved to be a more effective than conventional spot spray applications and can be a powerful tool to manage panicle emergence under less inductive conditions. Moreover, the modified use of ethephon contributes to the sustainability and expansion of the South African litchi industry.