EFFECTS OF WATER-STRESS CYCLES ON TURGOR MAINTENANCE PROCESSES IN PEAR LEAVES (PYRUS-COMMUNIS)

Effects of water deficits on leaf turgor maintenance processes were an alyzed for pear trees (Pyrus communis L. cv. ''Barlett'') grown in 120 -liter containers. Four irrigation treatments were applied: a well-wat ered control treatment, a spring water stress cycle (Sp), a summer wat er stress cycle (Su), and a spring plus summer water stress cycle (Sp + Su). For the Sp treatment, water application was progressively reduc ed from 100 to 20% of the control dose over a period of 27 days in spr ing. For the Su treatment, water application was progressively reduced over 23 days in summer, from 100 to 20% of the control dose. The Sp Su treatment comprised both the spring and summer drought stress cycl es. Pressure-volume (P-V) curves were constructed and stomatal conduct ances were determined far pear leaves from each treatment during the s pring and summer stress cycles. Leaf water potential (Psi(pi)(0)) and relative water content (R-0) at the turgor loss point of control leave s tended to decrease from spring to summer. Changes in leaf osmotic wa ter potential at full turgor (Psi(pi)(100)) and in symplast water frac tion (R-s) did not explain the seasonal decrease in Psi(pi)(0). The wa ter stress treatments had no effect on Psi(pi)(100), but R-s was reduc ed by the water stress treatments, particularly during the summer stre ss cycle of the Su and Sp + Su treatments. The decrease in R-s was cor related with an increase in the slope of the linear region of the P-V curve. Such a coupled adjustment would lead to increased water uptake capacity of water-stressed trees only under non-turgor conditions. Fur thermore, pear Leaves did not actively accumulate solutes. We conclude , therefore, that changes in leaf tissue water relations as a result o f leaf acclimation to water stress are unlikely to facilitate maintena nce of fruit productivity under drought.