Diurnal changes in photosynthetic parameters of Populus tremuloides, modulated by elevated concentrations of CO2 and/or O3 and daily climatic variation

The diurnal changes in light-saturated photosynthesis (Pn) under elevated CO2 and/or O3 in relation to stomatal conductance (gs), water potential, intercellular [CO2], leaf temperature and vapour-pressure difference between leaf and air (VPDL) were studied at the Aspen FACE site. Two aspen (Populus tremuloides Michx.) clones differing in their sensitivity to ozone were measured. The depression in Pn was found after 10:00 h. The midday decline in Pn corresponded with both decreased gs and decreased Rubisco carboxylation efficiency, Vcmax. As a result of increasing VPDL, gs decreased. Elevated [CO2] resulted in more pronounced midday decline in Pn compared to ambient concentrations. Moreover, this decline was more pronounced under combined treatment compared to elevated CO2 treatment.The positive impact of CO2 on Pn was relatively more pronounced in days with environmental stress but relatively less pronounced during midday depression. The negative impact of ozone tended to decrease in both cases.