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 intercellular CO2 concentration (ci) was determined in antisense Nicotiana tabacum plants, derived from six independent transformation lines, displaying a range of sedoheptulose-1, 7-bisphosphatase (SBPase) activities. The maximum in vivo ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (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 with reductions in SBPase activity of between 9% and 60%, maximum RuBP regeneration capacity declined linearly (r(2) = 0.79) and no significant.,m J change in apparent in vivo Rubisco activity (V-c,V-max) was observed in these plants. 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. These data demonstrate that small decreases in SBPase activity limit photosynthetic carbon assimilation by reducing the capacity for RuBP regeneration.