Acclimation of Stomatal Conductance to a CO₂-Enriched Atmosphere and Elevated Temperature in Chenopodium album

Abstract

Acclimation of stomatal conductance to different CO₂ and temperature regimes was determined in Chenopodium album L. plants grown at one of three treatment conditions: 23ºC and 350 μmol CO₂ mol^-1 air; 34ºC and 350 μmol mol^-1; and 34ºC and 750 μmol mol^-1. Stomatal conductance (g_s) as a function of intercellular CO₂ (C_i) was determined for each treatment at 25 and 35ºC, and these data were used to estimate gains of the feedback loops linking changes in intercellular CO₂ with stomatal conductance and net CO₂ assimilation. Growth temperature affected the sensitivity of stomata to measurement temperature in a pattern that was influenced by intercellular CO₂. Stomatal conductance more than doubled at intercellular CO₂ varying between 200 and 600 μmol mol^-1 as leaf temperature increased from 25 to 35ºC for plants grown at 23ºC. In contrast, stomatal conductance was almost unaffected by measurement temperature in plants grown at 34ºC. Elevated growth CO₂ attenuated the response of stomatal conductance to CO₂, but growth temperature did not. Stomatal sensitivity to C_i was extended to higher C_i in plants grown in elevated CO₂. As a result, plants grown at 750 μmol mol^-1 CO₂ had higher C_i/C_a at ambient CO₂ values between 300 and 1200 ¼mol mol-1 than plants grown at 350 ¼mol 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 μmol mol^-1 and 34°C. Both photosynthetic and stomatal loop gains acclimated to elevated CO₂ in proportion so that their ratio, integrated over the range of C_i in which the plant operates, remained constant. Water use efficiency (WUE) more than doubled after a short-term doubling of ambient 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 was maintained for all growth treatments.