Water stress decreases the transfer conductance of Douglas-fir (Pseudotsuga menziesii) seedlings

We tested the hypothesis that transfer conductance (g(i)) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings is reduced by water stress. Seedlings were irrigated with a solution of 25% polyethylene glycol so as to impose water stress rapidly, thereby limiting acclimatory responses. Transfer conductance was measured pre-treatment and post-treatment by two methods. Water stress reduced net photosynthesis by 20-50%. The initial slope of the rate of photosynthesis (A) over the intercellular carbon dioxide (CO2) concentration (C-i) response was reduced by water stress, indicating that reduced photosynthesis was not wholly accounted for by reduced stomatal conductance. The carbon isotope and chlorophyll fluorescence methods both indicated that water stress decreased g(i). From isotopic measurements with 1% O-2, gi was 0.076 +/- 0.009 (mean +/- SE) mol m(-2) s(-1) in well-watered seedlings and 0.044 +/- 0.004 mol m(-2) s(-1) in water-stressed seedlings. Fluorescence estimates of g(i) were 0.08 +/- 0.01 mol m(-2) s(-1) in well-watered seedlings and 0.044 0.004 mol m(-2) s(-1) in water-stressed seedlings. The drought-induced reduction in g(i) was responsible for the reduction in slope of the A/C-i response, and thus there was no difference in the slope of the A over the chloroplastic CO2 concentration (C-c) response between treatments and no indication of impaired mesophyll metabolism. These data illustrate that impairments of mesophyll metabolism can be revealed only from analysis of the A/C-c response.