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 (c_i) 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,max) and RuBP regeneration (J_max) rates were calculated from the steady-state measurements of the A to c_i response curves. In plants with reductions in SBPase activity of between 9% and 60%, maximum RuBP regeneration capacity declined linearly (r² = 0.79) and no significant change in apparent in vivo Rubisco activity (V_c.max) was observed in these plants. No correlation between V_c,max and a decrease in capacity for RuBP regeneration was observed (r²=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.