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 droug ht stressed and nonstressed pearl millet [Pennisetum americanum (L.) L eeke] were measured across growth stages to determine if a pattern exi sts in Pn and Cg during the growing season and to evaluate the influen ce of air vapor pressure deficit (VPD(a)) on the seasonal variations o f Pn and Cg. Leaf photosynthesis and Cg were measured independently on pearl millet plants grown at the driest (drought stressed) and wettes t (nonstressed) ends of a line-source irrigation gradient system. Well defined and predictable variations in both Pn and Cg were found acros s two growing seasons. Leaf photosynthesis of the nonstressed plants d eclined from a maximum of 25.8 pmol m-2 s-1 at the flag leaf emergence (48 days after planting, DAP) to a minimum of 14.5 mumol m-2 s-1 at p hysiological maturity. Stomatal conductance of the nonstressed plants peaked at the flowering and early grain fill stages and declined as pl ants approached maturity. In contrast, Pn and Cg of the stressed plant s declined from a maximum at flag leaf emergence to a minimum at flowe ring and increased as plants approached maturity. High VPD(a) during t he flowering and grain fill stages induced stomatal closure and decrea sed Pn in the stressed plants. High mid-season VPD(a) did not induce s tomatal closure and did not reduce leaf photosynthesis in nonstressed plants. The lack of sensitivity of Pn to VPD(a) in the nonstressed tre atment suggests large air VPD such as that prevalent in southern Arizo na does not limit the growth of irrigated pearl millet by limiting CO2 assimilation.