Despite large differences in total dry matter at final harvest, no significant differences (p≤0.05) were detected in the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) or in its regulation by carbamylation between tobacco genotypes selected for survival at low CO 2 and their source cultivar. Irrigated and water stressed plants of these genotypes differed in the diurnal pattern of Rubisco activity in their leaves. In irrigated plants Rubisco was not fully active even in the presence of saturating irradiance in the middle of the day. The maximum total activity reached during the day was higher in water stressed plants than in irrigated ones. Through the day decarbamylation of lysine 201 increased progressively in water stressed plants but remained less significant in the regulation of Rubisco activity than tight binding inhibitors