High Temperature Effects on Citrus Orange Leaf Gas Exchange, Flowering, Fruit Quality and Yield
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High midday temperatures in subtropical citrus growing regions can lead to excessively high leaf temperature (Tlf) and large leaf-to-air vapor pressure difference (D) in sun exposed leaves. This heat stress reduced net CO 2 assimilation (AC), growth, fruit yield and quality. 50% shading reduced average mid-day Tlf and increased AC, stomatal conductance (gs) and leaf water use efficiency, but decreased the internal concentration of CO 2 (Ci) in the mesophyll compared with sun-exposed trees throughout the season. However, shade had little effect on leaf transpiration. Heat stress increased non-stomatal limitations to AC in the mesophyll of sunlit leaves that were greater than stomatal limitations. Shade treatment reduced leaf carbohydrates but did not affect tree canopy volume or fruit size. Shading in the late fall until harvest increased orange yield and juice content more than compensated for the lower total soluble sugars (TSS) and resulted in net increases in TSS per tree. Shaded 'Navel' orange fruit developed better external color but lower TSS than sunexposed fruit. Shade did not affect orange fruit yields. In warm citrus producing regions, shading could improve photosynthesis and fruit quality especially in young trees where most of the leaves are exposed to direct sunlight. Improved fruit color, yield and/or TSS per hectare of high-value varieties could offset the costs of particle film sprays or shading in commercial orchards.
