The activity of key enzymes of carbon metabolism- RBP-carboxylase, PEP-carboxylase, NADP-malate dehydrogenase was studied during primary periods of vegetation in C3- and C4-plants under conditions of optimal water supply and soil moisture deficit. The key enzymes of C4-photosynthesis-PEP – carboxylase and NADP-malate dehydrogenase were established to be more stable in amaranth leaves under conditions of water stress than the Calvin cycle enzymes. However in wheat leaves Calvin cycle enzymes- RBPcarboxylase and carbonic anhydrase were more stable under water stress in comparison with the C4- enzymes