© 1997 Heron Publishing—Victoria, Canada
Effects of water stress cycles on turgor maintenance processes in pear leaves (Pyrus communis)
Jordi Marsal and Joan Girona
Area de Tecnologia Frutícola, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Avda, Alcalde Rovira
Roure, 177, 25198 Lleida, Spain / Received May 1, 1996
Summary
Effects of water deficits on leaf turgor maintenance processes were analyzed for pear trees (Pyrus communis L. cv. “Barlett”) grown in 120-liter containers. Four irrigation treatments were applied: a well-watered control treatment,
a spring water stress cycle (Sp), a summer water stress cycle (Su), and a spring plus summer water stress cycle (Sp + Su).
For the Sp treatment, water application was progressively reduced from 100 to 20% of the control dose over a period of 27
days in spring. 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 cycles. Pressure–volume (P–V)
curves were constructed and stomatal conductances were determined for pear leaves from each treatment during the spring and
summer stress cycles. Leaf water potential (Ψπ0) and relative water content (R0) at the turgor loss point of control leaves tended to decrease from spring to summer. Changes in leaf osmotic water potential
at full turgor (Ψπ100) and in symplast water fraction (Rs) did not explain the seasonal decrease in Ψπ0. The water stress treatments had no effect on Ψπ100, but Rs was reduced by the water stress treatments, particularly during the summer stress cycle of the Su and Sp + Su treatments.
The decrease in Rs was correlated 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. Furthermore, pear leaves did
not actively accumulate solutes. We conclude, therefore, that changes in leaf tissue water relations as a result of leaf acclimation
to water stress are unlikely to facilitate maintenance of fruit productivity under drought.
Keywords:
deficit irrigation, leaf water potential, stomatal conductance, tissue elasticity, turgor loss.
