Small decreases in SBPase cause a linear decline in the apparent RuBP regeneration rate, but do not affect Rubisco carboxylation capacity

The response of net photosynthetic CO2 uptake (A) to increasing leaf interc ellular CO2 concentration (ci) was determined in antisense Nicotiana tabacu m plants, derived from six independent transformation lines, displaying a r ange of sedoheptulose-1, 7-bisphosphatase (SBPase) activities. The maximum in vivo ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxyla tion (V-c,V-max) and RuBP regeneration (J(max)) rates were calculated from the steady-state measurements of the A to ci response curves. In plants wit h reductions in SBPase activity of between 9% and 60%, maximum RuBP regener ation capacity declined linearly (r(2) = 0.79) and no significant.,m J chan ge in apparent in vivo Rubisco activity (V-c,V-max) was observed in these p lants. No correlation between V-c,V-max and a decrease in capacity for RuBP regeneration was observed (r(2) = 0.14) in the SBPase antisense plants. Th ese data demonstrate that small decreases in SBPase activity limit photosyn thetic carbon assimilation by reducing the capacity for RuBP regeneration.