Impact of atmospheric CO2 on growth, photosynthesis and nitrogen metabolism in cucumber (Cucumis sativus L.) plants

SummaryExpression and activity of nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) were analysed in relation to the rate of CO2 assimilation in cucumber (Cucumis sativus L.) leaves. Intact plants were exposed to different atmospheric CO2 concentrations (100, 400 and 1200 μL L−1) for 14 days. A correlation between the in vivo rates of net CO2 assimilation and the atmospheric CO2 concentrations was observed. Transpiration rate and stomatal conductance remained unaffected by CO2 levels. The exposure of the cucumber plants to rising CO2 concentrations led to a concomitant increase in the contents of starch and soluble sugars, and a decrease in the nitrate content in leaves. At very low CO2, NR and GS expression decreased, in spite of high nitrate contents, whereas at normal and elevated CO2 expression and activity were high although the nitrate content was very low. Thus, in cucumber, NR and GS expression appear to be dominated by sugar levels, rather than by nitrate contents.