Response of maize (Zea mays L.) on yield, physiology and stomatal behaviour under two different elevated CO<sub>2</sub> concentrations. Do these anatomical changes affect the physiology of the C4 crop plant under high CO<sub>2</s

Rising CO₂ concentration in the atmosphere is a matter of global concern and poses apprehension about how plants will adapt to the changing environment. Various studies have proved that under high CO₂ levels, plant physiology alters and affects plant functioning. However, under elevated CO₂, the stomatal characters and their relation with physiological responses are still not yet clear. To find out these changes in the stomatal parameters at ambient and two elevated CO₂ (550 ppm and 700 ppm) levels, four genotypes of maize (<i>Zea mays</i> L.) viz. DHM-117, Harsha, Varun and M-24 were grown in open-top chambers. In the study, it was observed that the stomatal density increased, stomatal size altered, stomatal conductance (g_s) and transpiration rate (T_r) decreased under elevated CO₂ (eCO₂) while photosynthetic rate (Pn), water use efficiency (WUE), yield and biomass, of which especially the reproductive biomass increased. Under eCO₂, stomatal and physiological changes were genotypic and CO₂ concentration specific. Increased stomatal density at eCO₂ was mainly due to increased abaxial stomatal density. The improved P_n and reduced T_r at 550 ppm improved the WUE in the plants, while this response was not observed at 700 ppm. These results elucidate that this C4 crop responded positively to up to 550 ppm of CO₂ concentrations, and beyond this, the impact was minimal.