Seasonal trends in leaf photosynthesis and stomatal conductance of drought stressed and nonstressed pearl millet as associated to vapor pressure deficit

Single leaf photosynthesis (Pn) and stomatal conductance (Cg) of drought stressed and nonstressed pearl millet Pennisetum americanum (L.) Leeke] were measured across growth stages to determine if a pattern exists in Pn and Cg during the growing season and to evaluate the influence of air vapor pressure deficit (VPD_a) on the seasonal variations of Pn and Cg. Leaf photosynthesis and Cg were measured independently on pearl millet plants grown at the driest (drought stressed) and wettest (nonstressed) ends of a line-source irrigation gradient system. Well defined and predictable variations in both Pn and Cg were found across two growing seasons. Leaf photosynthesis of the nonstressed plants declined from a maximumof 25.8 mol m^–2 s^–1 at the flag leaf emergence (48 days after planting, DAP) to a minimum of 14.5 mol m^–2 s^–1 at physiological maturity. Stomatal conductance of the nonstressed plants peaked at the flowering and early grain fill stages and declined as plants approached maturity. In contrast, Pn and Cg of the stressed plants declined from a maximum at flag leaf emergence to a minimum at flowering and increased as plants approached maturity. High VPD_a during the flowering and grain fill stages induced stomatal closure and decreased Pn in the stressed plants. High mid-season VPD_a did not induce stomatal closure and did not reduce leaf photosynthesis in nonstressed plants. The lack of sensitivity of Pn to VPD_a in the nonstressed treatment suggests large air VPD such as that prevalent in southern Arizona does not limit the growth of irrigated pearl millet by limiting CO₂ assimilation.Abbreviations Cg stomatal conductance - DAP days after planting - Pn leaf photosynthesis - VPD_a air vapor pressure deficit - VPD_1-a leaf to air vapor pressure deficit Contribution of the Arizona Agricultural Experimental Station. Research supported in part by INTSORMIL/USAID.