Effects of prolonged drought stress and nitrogen deficiency on the respiratory O2 uptake of bean and pepper leaves

We analyzed the combined effects of mild drought stress and severe nitrogen (N) deprivation on respiration of acclimated mature leaves of beans (Phaseolus vulgaris L. cv. Garrofal) and peppers (Capsicum annuum L., pure line B6). Rates of oxygen uptake were measured polarographically, and inhibitors were added to the closed cuvette to compare the effects of environmental stress on the cytochrome (cyt) and alternative pathways of mitochondrial respiration. Dark oxygen uptake was decreased by the water deficit treatment in both plants, and in the case of N limitation leaf respiration rates (RD) of peppers were also reduced. RD of leaves of beans and peppers grown under N-limiting conditions did not follow the decrease in leaf N concentration, since RD expressed per unit of tissue N was considerably higher in the N-stressed leaves. Values obtained with specific inhibitors of the two terminal oxidases of mitochondrial respirations suggested that the cyt pathway of respiration was affected by mild drought and severe N stress. When plants were exposed to both environmental stresses, leaf respiration response was similar to that under N limitation, in this case the most severe stress.