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 investi gated in greenhouse-grown eastern cottonwood (Populus deltoides Bartr. ). Two clones from southern Ohio (Ohio Red) and eastern Nebraska (Plat te) were selected based on their differences in dehydration tolerance. Plants were either watered every day (control) or preconditioned by w atering 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 M Pa; net photosynthesis and stomatal conductance were reduced to 13 and 9% of control, respectively. Eighteen hours after rewatering, photosy nthesis recovered (103% of control), while stomatal conductance was 60 % of control. Net photosynthesis of Platte was reduced to 43% and stom atal conductance to 32% of control (PSI(w) -0.21 MPa), and neither rec overed fully when rewatered. After six dry-wet cycles (TRT 2), net pho tosynthesis of Ohio Red was reduced by 50%, though PSI(w) was -0.48 MP a. Clones showed an osmotic adjustment of -0.2 MPa after three (Platte ) and six cycles (Ohio Red). When all preconditioned plants were stres sed for 10 days, PSI(w) was -1.05 MPa and plants had negative net phot osynthesis and no osmotic adjustment. Net photosynthetic rate of Ohio Red recovered (100% of control) on the second day of rewatering (stoma tal conductance 68%), while Platte had not recovered (71%) by the four th day (stomatal conductance 95%). These differences suggest that reco very of photosynthesis was limited mainly by stomatal factors in Ohio Red and nonstomatal factors in Platte. The preconditioning treatment a lso seems to have benefitted Ohio Red.