A short-term exposure of cucumber plants to rising atmospheric CO2 increases leaf carbohydrate content and enhances nitrate reductase expression and activity

Nitrate reductase (NR; EC 1.6.6.1) is the first enzyme of the nitrate-assimilatory pathway and is regulated transcriptionally and post-translationally by several metabolic and environmental signals. To investigate whether NR is controlled by the rate of photosynthetic CO2 assimilation in cucumber (Cucumis sativus L.), intact plants were exposed, after the dark period, to light under different atmospheric CO2 concentrations (100, 400 and 2,000 muL L-1) for 2 h. The in-vivo rates of net CO2 assimilation correlated with atmospheric CO2 concentrations. The CO2-fixation rate under 2,000 muL L-1 CO2 was 2.4- and 5.4-fold higher than under 400 and 100 muL L-1, respectively. Stomatal conductances and transpiration rates were almost identical after the 2-h light period under the various CO2 concentrations tested. Increasing atmospheric CO2 concentrations caused concomitant increases in the contents of starch and soluble sugars in the leaves and a decrease in the nitrate content. The activity and activation state of NR were both higher under elevated CO2 than under low CO2. High CO2 also enhanced NR-gene expression in the leaves. Sugars were supplied via roots to intact carbohydrate-starved plants and NR mRNA levels were analysed after 7 h. Fructose markedly stimulated NR-gene transcription in both leaves and roots. It is concluded that, in cucumber plants, the rate of CO2 assimilation controls the rate of nitrate assimilation by modulation of NR expression and activity, and that sugars are presumably involved as regulatory metabolites.