Browsing by Author "Prins, Michael du Toit"

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    The impact of shade netting on the microclimate of a citrus orchard and the tree's physiology
    (Stellenbosch : Stellenbosch University, 2018-12) Prins, Michael du Toit; Cronje, P. J. R.; Graham, G. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: This study aimed to quantify the change in microclimate under shade netting and its effect on the leaf physiology of ‘Nadorcott’ mandarin (Citrus reticulata Blanco) in a winter rainfall production area. The first experiment quantified the change in orchard microclimate. The reduction in solar radiation affected the ambient, soil temperature and the number of hours for specific physiological and phenological temperature ranges under the shade netting. A reduced wind speed under the netting potentially led to less removal of moisture from the air and slightly increased relative humidity leading to a decrease in vapour pressure deficit (VPD) of the air. Therefore, the atmospheric evaporative demand was reduced and increased the volumetric soil water content. The second experiment focused on how the changes in microclimate affected a citrus leaf’s physiology. A reduction in VPD, especially in summer, led to increased stomatal conductance and resulted in increased CO2 assimilation rates of leaves under the shade netting. Therefore, the shade netting did not influence the leaf physiology negatively. Thirdly the total carbohydrate assimilation and distribution of different carbohydrate components, i.e., reducing sugars, polysaccharides, and starch, in leaves and roots were investigated as glucose equivalents. The most notable change under the shade netting in carbohydrate levels was in leaves, with an increased starch content especially noted after harvest. Root carbohydrates showed some differences between treatments, however, these could not only be attributed to the shade net treatment. The fourth experiment investigated how the change in microclimate under shade netting influenced the tree water status or tree water potential as well as fruit and canopy growth. This was accomplished by conducting pre-dawn pressure chamber readings of the main treatments as well as additional irrigation treatments. An increased tree water potential for trees under the shade netting was recorded as well as increased tree canopy volume and final fruit size. It can, therefore, be concluded that 20% white shade netting altered the orchard microclimate without negatively affecting the leaf physiology associated with photosynthesis in ‘Nadorcott’ mandarin trees. Leaves under the shade netting favoured the production of storage carbohydrates and the reduction in solar radiation and increased soil water content that led to a less water stressed environment, increasing the tree water potential. Further research should include how the microclimate can be altered in citrus production areas with different climatic conditions in South Africa.