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

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Abstract
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 (Vc,max) and RuBP regeneration (Jmax) 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 (r2=0.79) and no significant change in apparent in vivo Rubisco activity (Vc,max) was observed in these plants. No correlation between Vc,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.