The Effects of CO2 on the Photosynthetic Fixation of CO2 and the Activity of Ribulose-1,5-Bisphosphate Carboxylase in Single-Rooted Soybean Leaves under Sink-Limited Conditions

Abstract

Ribulosebisphosphate carboxylase activity of single- rooted soybean leaves was lowered under sink-limited con- ditions. The low photosynthetic and enzymatic activities of the sink-limited leaves returned to the high levels, similar to those of the control leaves, within 30 min of the treatment with high CO2. turing them under continuous light or with cooling of the root system. These results provided good evidence for the end-product inhibition of photosynthesi s, a concept that was first proposed by Boussingault (1868). A subsequent study using this model plant showed that the sink-limited condition caused a reduction in the ac- tivity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) that could be extracted from the leaves. This reduction in activity was reversed by incubation of the extract with bi- carbonate in the presence of MgCl 2 (Sawada et al. 1990). Inorganic phosphate was also effective in reversing the re- duction in the activity of RuBPcase in the presence of ex- tremely low levels of endogenous CO2 (Sawada et al. 1990, 1992). It was suggested that the reduction in photosynthetic activity in the sink-limited leaves might have been due to the deactivation of RuBPcase, caused by a decrease in the level of Pj in the chloroplast stroma as a consequence of a significant increase in levels of various phosphorylated in- termediates in the photosynthetic carbon cycle. This sugges- tion is supported by the recently reported result that Pj stim- ulated the binding of activator CO2 to RuBPcase that had been purified from spinach leaves and activated this en- zyme at a physiological concentration (5-7 fjM) of CO 2 in chloroplast stroma that is in equilibrium with atmospheric CO2 (Anwaruzzaman et al. 1995).