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 chloroplast NADP-malate dehydrogenase (NADP-MDH) content were grown under ambient or under doubled atmospheric CO2 in order to analyse the effect of elevated CO2 on the redox state of the chloroplasts. Since large differences exist between the individual leaves of tobacco plants, gas exchange characteristics, 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 between younger and older leaves were, in most cases, larger than the differences between comparable leaves at ambient or elevated CO2. For all parameters (chlorophyll fluorescence, P700 reduction, NADP-MDH activation) that are indicative for the redox situation in the electron transport chains and in the chloroplast stroma, more oxidized values were determined under elevated CO2. The increased redox state of ferredoxin, observed at ambient conditions in the NADP-MDH-under-expressing plants, disappeared under elevated CO2. It was concluded that the reduced rate of photorespiration under elevated CO2 decreases the amount of excess electrons. Interestingly, this lowered not only the activation state of NADP-MDH, but also the expression of the enzyme in the wild-type plants. The results are discussed with respect to a possible interaction between stromal reduction state and gene expression.