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sur le développement des zones arides et semi-arides

Titre : Quantifying the contribution of windbreaks on fruit quality, yield and tree performance of bearing citrus cultivars in the Western Cape

Auteur : Geldenhuys, Heinrich

Université de soutenance : Stellenbosch University

Grade : Master of Science (MScAgric) 2022

Résumé partiel
Strong winds in the Western Cape have a direct and indirect effect on the performance of citrus trees. Tree-based windbreaks are used extensively in South Africa to protect citrus orchards from wind scar damage, thereby avoiding major export quality losses. The study was conducted over two consecutive seasons (2019/20 and 2020/21). In the first trial, the microclimate (wind speed and air temperature), tree performance (yield, fruit size and shoot growth) and fruit quality (external and internal) were quantified at different distances from established Beefwood and Poplar windbreaks on two ‘Tango’ mandarin orchards (Rust en Vrede and Babylonstoren) in the Western Cape, South Africa. The prevailing wind direction was from the southeast and north during summer and winter, respectively. Considerably higher (33%) wind speeds were recorded during summer compared to winter. During summer, the wind speed was consistently higher (17%) at 16H (140 m from windbreak) than 3H (30 m), however, no clear changes in air temperatures were observed between distance treatments (H = mean height of the windbreak tree). Leaf temperatures were significantly higher at the most wind-protected treatments. The summer stomatal conductance (gs) was twice as high compared to winter. The results for gs and stem water potential (SWPMD ) were inconsistent between distance treatments. Fruit at 2H and 3H had significantly lower titratable acidity (TA) compared to 4H (trial 2), and 10H and 16H (trial 1) respectively, yet export standards for late mandarins were met. The highest packout (Class 1 fruit) was observed at 3H and decreased at 10H (90 m) and 16H. During the first 12 weeks after fruit set, 87% of severe wind scar damage occurred. During winter, up to 18% damage (light and severe) occurred on mature fruit after periods of strong wind gusts from the north. No significant differences were recorded between distance treatments for tree volume, trunk circumference, fractional light interception (FLI), fruit size and shoot length during both seasons, at either site. In trial 2, the efficiency of newly established multi-row windbreaks (MRW) was also recorded by quantifying the effect of wind on ‘Nadorcott’ mandarin and ‘Ruby Star’ plum trees at Welgevallen experimental farm. The highest number of hours above the threshold value for wind scar damage (> 4.5 m.s -1 ) was recorded in December (2019/20) and January (2020/21).

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