Diurnal and Seasonal Effects of Environment on Plant Water Relations and Fruit Diameter of Citrus1

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Abstract
Leaf water potential of ‘Valencia’ orange trees was influenced by soil water availability, soil temp, and vapor pressure deficit (VPD) of the atmosphere. Low soil water availability prevented full nighttime recovery from water stress. Soil temp below 15°C decreased root permeability for water absorption and contributed to low daytime water potentials (high water stress), but at night when transpiration was minimal, complete recovery occurred. VPD effects on leaf water potential were also most noticeable during the day. Stage II fruits exhibited a recurring diurnal size change during daylight hr. This reversible shrinkage was correlated with simultaneous changes in fruit and leaf water potential and fruit diffusion resistance. However, the irreversible nighttime growth rate could not be correlated with day or night water potentials or air temp, or with night VPD. While differences between fruit and leaf water potentials occurred, no evidence presented here or elsewhere permits an unequivocal statement that transpiring fruits can behave as a midday reservoir of water for leaves.

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