Adaptation of tobacco plants to elevated CO2: influence of leaf age on changes in physiology, redox states and NADP-malate dehydrogenase activity

Transgenic tobacco plants (Nicotiana tabacum L. cv. Xanthi) with altered ch loroplast NADP-malate dehydrogenase (NADP-MDH) content were grown under amb ient or under doubled atmospheric CO2 in order to analyse the effect of ele vated CO2 on the redox state of the chloroplasts. Since large differences e xist between the individual leaves of tobacco plants, gas exchange characte ristics, enzyme capacities and metabolite contents were measured separately for each leaf of the plants, Large variations between leaves of different age were found in nearly every parameter analysed, and the differences betw een younger and older leaves were, in most cases, larger than the differenc es between comparable leaves at ambient or elevated CO2. For all parameters (chlorophyll fluorescence, P700 reduction, NADP-MDH activation) that are i ndicative for the redox situation in the electron transport chains and in t he chloroplast stroma, more oxidized values were determined under elevated CO2. The increased redox state of ferredoxin, observed at ambient condition s in the NADP-MDH-under-expressing plants, disappeared under elevated CO2. It was concluded that the reduced rate of photorespiration under elevated C O2 decreases the amount of excess electrons. Interestingly, this lowered no t only the activation state of NADP-MDH, but also the expression of the enz yme in the wild-type plants. The results are discussed with respect to a po ssible interaction between stromal reduction state and gene expression.