EFFECTS OF WATER-STRESS PRECONDITIONING ON GAS-EXCHANGE AND WATER RELATIONS OF POPULUS-DELTOIDES CLONES

The effect of water stress preconditioning on gas exchange was investigated in greenhouse-grown eastern cottonwood (Populus deltoides Bartr.). Two clones from southern Ohio (Ohio Red) and eastern Nebraska (Platte) were selected based on their differences in dehydration tolerance. Plants were either watered every day (control) or preconditioned by watering every 3 (TRT 1) or 4 days (TRT 2). After three dry-wet cycles (TRT 2), predawn leaf water potential (PSI(w)) of Ohio Red was -0.32 MPa; net photosynthesis and stomatal conductance were reduced to 13 and 9% of control, respectively. Eighteen hours after rewatering, photosynthesis recovered (103% of control), while stomatal conductance was 60% of control. Net photosynthesis of Platte was reduced to 43% and stomatal conductance to 32% of control (PSI(w) -0.21 MPa), and neither recovered fully when rewatered. After six dry-wet cycles (TRT 2), net photosynthesis of Ohio Red was reduced by 50%, though PSI(w) was -0.48 MPa. Clones showed an osmotic adjustment of -0.2 MPa after three (Platte) and six cycles (Ohio Red). When all preconditioned plants were stressed for 10 days, PSI(w) was -1.05 MPa and plants had negative net photosynthesis and no osmotic adjustment. Net photosynthetic rate of Ohio Red recovered (100% of control) on the second day of rewatering (stomatal conductance 68%), while Platte had not recovered (71%) by the fourth day (stomatal conductance 95%). These differences suggest that recovery of photosynthesis was limited mainly by stomatal factors in Ohio Red and nonstomatal factors in Platte. The preconditioning treatment also seems to have benefitted Ohio Red.