Effects of CO2 on stomatal conductance:: Do stomata open at very high CO2 concentrations?

Potato and wheat plants were grown for 50 d at 400, 1000 and 10 000 mu mol mol(-1) carbon dioxide (CO2), and sweetpotato and soybean were grown at 1000 mu mol mol(-1) CO2 in controlled environment chambers to study stomatal conductance and plant water use. Lighting was provided with fluorescent lamps as a 12 h photoperiod with 300 mu mol m(-2) s(-1) PAR. Mid-day stomatal conductances for potato were greatest at 400 and 10 000 mu mol mol(-1) and least at 1000 mu mol mol(-1) CO2. Mid-day conductances for wheat were greatest at 400 pmol mol(-1) and least at 1000 and 10 000 mu mol mol(-1) CO2. Mid-dark period conductances for potato were significantly greater at 10 000 mu mol mol(-1) than at 400 or 1000 mu mol mol(-1), whereas dark conductance for wheat was similar in all CO2 treatments. Temporarily changing the CO2 concentration from the native 1000 mu mol mol(-1) to 400 mu mol mol(-1) increased mid-day conductance for all species, while temporarily changing from 1000 to 10 000 mu mol mol(-1) also increased conductance for potato and sweetpotato. Temporarily changing the dark period CO2 from 1000 to 10 000 mu mol mol(-1) increased conductance for potato, soybean and sweetpotato. In all cases, the stomatal responses were reversible, i.e. conductances returned to original rates following temporary changes in CO2 concentration. Canopy water use for potato was greatest at 10 000, intermediate at 400, and least at 1000 mu mol mol(-1) CO2, whereas canopy water use for wheat was greatest at 400 and similar at 1000 and 10 000 mu mol mol(-1) CO2. Elevated CO2 treatments (i.e. 1000 and 10 000 mu mol mol(-1)) resulted in increased plant biomass for both wheat and potato relative to 400 mu mol mol(-1), and no injurious effects were apparent from the 10 000 mu mol mol(-1) treatment. Results indicate that super-elevated CO2 (i.e. 10 000 mu mol mol(-1)) can increase stomatal conductance in some species, particularly during the dark period, resulting in increased water use and decreased water use efficiency. (C) 1999 Annals of Botany Company.