Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Subject "6-Benzyladenine"

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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.