Acclimation of Photosynthesis and Stomatal Conductance to Elevated CO₂ in Canopy Leaves of Wheat at Two Nitrogen Supplies

Abstract

The great majority of the studies on photosynthetic acclimation have focused on upper-sunlit leaves and little attention has been paid to the acclimatory responses of lower-shaded canopy leaves. In this study we assessed the acclimatory responses of photosynthesis and stomatal conductance (gs), as well as chlorophyll content (Chl) and ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) activity in flag and lower canopy leaves of wheat growing in polyethylene tunnels set at ambient (360μ mol mol^-1) and elevated (700μmol mol^-1) CO₂ , and under two levels of N supply. The rate of photosynthesis, gs, transpiration (E), Chl and Rubisco activity of sunlit upper- (flag) and shaded lower-canopy leaves were significantly (P<0.05) lower in elevated relative to ambient CO₂ -grown plants. All the parameters were lower in N deficient plants, except gs, and all declined with leaf position at both growth CO₂ . The acclimatory responses of An and gs to elevated CO₂ , evaluated as the ratio of An (or gs) measured at 700μmol mol^-1 of elevated to ambient CO₂ -grown plants, were enhanced in N deficient plants; with N supply the acclimatory responses were less pronounced in lower canopy leaves relative to flag leaf. It is concluded that in elevated CO₂ , photosynthetic capacity and stomatal conductance were reduced in sunlit upper- and shaded lower-canopy leaves, and that N supply reduced acclimation, particularly in lower- canopy, shaded leaves