Water and carbon balance in developing citrus fruit: Water retention in juice sacs and sugar conversions under cyclic water stress

Water and carbon budgets were examined in developing fruit of grapefruit (Citrus paradisi Macf.) and calamondin orange (Citrus madurensis Lour.), followed by analyses of water movement, water relations, and sugar conversions under conditions of varied water availability.; Fruit water was conserved markedly by grapefruit (but not calamondin) after initial expansion. Maximal carbon accumulation occurred only after virtual cessation of cell expansion in both fruits. Total respiratory losses during development reduced imported carbon by 36% and 23% in grapefruit and calamondin orange, respectively. Growth of 1 grapefruit was estimated to cost ca 126 g sucrose and 1.3 kg water (1.6 g sucrose and 16 g water g{dollar}sp{lcub}-1{rcub}{dollar} DW).; Water in juice sacs was found to be markedly, but not completely, isolated from the transpiration stream. The halt-time for tritiated water loss from juice sacs ranged from 10 to more than 30 days. Tritiated water exited peel 6-fold more rapidly than juice tissue.; Total water losses via xylem back-flow and transpiration were similar in the system studied here. During a 4-day period, xylem back-flow alone accounted for a 5.4% loss of peel water and a water potential drop of 0.6 MPa. Respective values for juice sacs were only 1% and 0.1 MPa.; Water and sugar parameters appeared to stabilize in fruit after approximately 9 days of water stress. Pre-stress osmotic status was regained 5 days after reirrigation, but lost after only 3 days of additional stress. "Sweeter" fruits produced under mild stress appeared to result from a decrease in fruit size, but sugar conversion occurred.; The most striking observations in alternating stress experiments were dramatic sucrose-hexose conversions in fruit apparently associated with osmotic adjustment. Total reducing sugar content per fruit increased 41% under stress. Sucrose content dropped accordingly, but the conversion increased osmolality by 15% (commercial juice is valued in this manner). The process was partially but rapidly reversed by reirrigation.