Acclimation of photosynthesis and stomatal conductance to elevated CO2 in canopy leaves of wheat at two nitrogen supplies

The great majority of the studies on photosynthetic acclimation have focused on uppersunlit 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 (360 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) CO, 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 CO, evaluated as the ratio of An (or gs) measured at 700 mu mol 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 CO, 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.