The growth potential generated in citrus fruit under water stress and its relevant mechanisms

A mild water stress was imposed upon potted tangerine trees (Citrus sinensi s Blanco, cv. Zhuju) by water withholding during early juice sac expansion stage. While fruit growth was inhibited by water stress a growth potential was built up inside the fruit, which was not expressed until rewatering. Th e more powerful water uptake force of the stressed fruit was caused by its more negative fruit water potential. The mechanisms involved were both pass ive and active in nature: more water loss from fruit to transpiring leaves during water stress and some active adaptive physiological responses of fru it to water stress. The physiological responses involved both osmotic adjus tment and cell wall adjustment. The former was reflected in higher soluble solute contents in both fruit juice and fruit skin (on a dry weight basis) resulting in the drop of osmotic potential (psi(s)). The latter was reflect ed in the cell wall loosening of fruit skin in response to water stress cau sing a further fruit turgor (psi(s)) drop. These two responses further redu ced fruit water potential, which promoted post-stress fruit expansion growt h. (C) 2000 Elsevier Science B.V. All rights reserved.