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

Studies have shown stomatal conductance (gs) of plants exposed to super-elevated CO2 (>5000 μmol mol−1) increases in several species, in contrast to a decrease of gs caused by moderate CO2 enrichment. We conducted a series of experiments to determine whether super-elevated CO2 alters stomatal 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 μmol mol−1) CO2 in controlled environmental chambers. Stomatal 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 gs of abaxial leaf surfaces using a steady-state porometer. Relative to the control (400 μmol mol−1 CO2) plants, daytime stomatal conductance (gs,day) of the plants grown under 1200 and 10,000 μmol mol−1 CO2 was reduced by 38% and 15%, respectively. Dark period stomatal conductance (gs,night) was unaffected by growing under 1200 μmol mol−1 CO2, but dramatically increased under 10,000 μmol mol−1 CO2. Stomatal density increased by 10% in the leaves of 10,000 μmol mol−1 CO2-grown plants, which in part contributed to the higher gs,night values. Elevating [CO2] to 1200 μmol mol−1 enhanced ABA-induced stomatal closure, but further increasing CO2 to 10,000 μmol 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.