Seasonal and Diurnal Citrus Leaf and Fruit Water Relations

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Abstract
Water relations of sun and shade leaves and fruit of early-season navel and late-season ''Valencia'' oranges [Citrus sinensis] were compared at various times throughout the growing season. Sun leaves had lower leaf water potentials (.psi.l) than shade leaves and seasonal .psi.l were similar for both cultivars. Midday .psi.l of navel were below those of fruit (.psi.f) during the spring and fall. ''Valencia'' .psi.l were below .psi.f during the fall. Leafwater potential apparently is largely separate from .psi.f since, during the fall, mature navel fruit have higher water contents than immature ''Valencia'' fruit. Leaf osmotic potentials in the fall diurnally decreased a maximum of 0.6 megapascal (MPa) by dehydration but osmotic potentials of turgid leaves changed little through the season. This lack of true osmotic adjustment indicates that variations in leaf turgor reflect variations in .psi.l and that citrus .psi.l is a good indication of plant water status. During fruit expansion, water loss from fruit decreased as cuticle thickness increased and stomatal density decreased.