ACCLIMATION OF STOMATAL CONDUCTANCE TO A CO2-ENRICHED ATMOSPHERE AND ELEVATED-TEMPERATURE IN CHENOPODIUM-ALBUM

Acclimation of stomatal conductance to different CO2 and temperature r egimes was determined in Chenopodium album L. plants grown at one of t hree treatment conditions: 23 degrees C and 350 mu mol CO2 mol(-1) air ; 34 degrees C and 350 mu mol mol(-1); and 34 degrees C and 750 mu mol mol(-1). Stomatal conductance (g(s)) as a function of intercellular C O2 (C-i) was determined for each treatment at 25 and 35 degrees C, and these data were used to estimate gains of the feedback loops linking changes in intercellular CO2 with stomatal conductance and net CO2 ass imilation. Growth temperature affected the sensitivity of stomata to m easurement temperature in a pattern that was influenced by intercellul ar CO2. Stomatal conductance more than doubled at intercellular CO2 va rying between 200 and 600 mu mol mol(-1) as leaf temperature increased from 25 to 35 degrees C for plants grown at 23 degrees C. In contrast , stomatal conductance was almost unaffected by measurement temperatur e in plants grown at 34 degrees C. Elevated growth CO2 attenuated the response of stomatal conductance to CO2, but growth temperature did no t. Stomatal sensitivity to C-i was extended to higher C-i in plants gr own in elevated CO2. As a result, plants grown at 750 mu mol mol(-1) C O2 had higher C-i/C-a at ambient CO2 values between 300 and 1200 mu mo l mol(-1) than plants grown at 350 mu mol mol(-1) CO2. The gain of the stomatal loop was reduced in plants grown at elevated CO2 or at lower temperature when compared to plants grown at 350 mu mol mol(-1) and 3 4 degrees C. Both photosynthetic and stomatal loop gains acclimated to elevated CO2 in proportion so that their ratio, integrated over the r ange of C-i in which the plant operates, remained constant. Water use efficiency (WUE) more than doubled after a short-term doubling of ambi ent CO2. However, the WUE of plant grown and measured at elevated CO2 was only about 1.5 times that of plant transiently exposed to elevated CO2, due to stomatal acclimation. An optimal strategy of water use wa s maintained for all growth treatments.