Super-elevated CO2 interferes with stomatal response to ABA and night closure in soybean (Glycine max)

Studies have shown stomatal conductance (g(s)) of plants exposed to super-elevated CO2 (> 5000 mu mol mol(-1)) increases in several species, in contrast to a decrease of g(s) caused by moderate CO2 enrichment. We conducted a series of experiments to determine whether super-elevated CO2 alters stomata[ development and/or interferes with stomatal closure in soybean (Glycine max). Plants were grown at nominal ambient (400), elevated (1200) and super-elevated (10,000 mu mol mol(-1)) CO2 in controlled environmental chambers. Stomata[ density of the plant leaf was examined by a scanning electron microscope (SEM), while the stomatal response to the application of exogenous abscisic acid (ABA), a phytohormone associated with water stress and stomatal control, was investigated in intact growing plants by measuring the g(s) of abaxial leaf surfaces using a steady-state porometer. Relative to the control (400 mu mol mol(-1) CO2) plants, daytime stomatal conductance (g(s,day)) of the plants grown under 1200 and 10,000 mu mol mol(-1) CO2 was reduced by 38% and 15%, respectively. Dark period stomatal conductance (g(s),(night)) was unaffected by growing under 1200 mu mol mol(-1) CO2, but dramatically increased under 10,000 mu mol mol(-1) CO2, Stomatal density increased by 10% in the leaves of 10,000 mu mol mol(-1) CO2-grown plants, which in part contributed to the higher g(s), (night) values. Elevating [CO2] to 1200 mu mol mol(-1) enhanced ABA-induced stomatal closure, but further increasing CO2 to 10,000 mu mol mol(-1) significantly reduced ABA-induced stomatal closure. These results demonstrated that stomatal response to ABA is CO2 dependent. Hence, a stomatal failure to effectively respond to an ABA signal and to close at night under extremely high CO2 may increase plants susceptibility to other abiotic stresses. (c) 2008 Elsevier GmbH. All rights reserved.