Relationship Between Steady-State Gas Exchange, in vivo Ribulose Bisphosphate Carboxylase Activity and Some Carbon Reduction Cycle Intermediates in Raphanus sativus

Abstract

The relationships between CO₂ assimilation rate, RuP₂ carboxylase activity and sizes of the pools of ribulose 1,5-bisphosphate (RuP₂) and 3-phosphoglyceric acid (PGA) were examined using a freeze clamp device to rapidly freeze sections of attached leaves of R. sativus which previously had gas-exchange measurements made on them.

At high irradiance and ambient partial pressures of CO₂ and O₂, RuP₂ carboxylase was fully active in vivo. Activity was less at very low CO₂ pressures and at high CO₂ pressures, particularly when combined with low O₂ pressures. In vivo RuP₂ carboxylase activity and both RuP₂ and PGA pool sizes increased with increasing irradiance. RuP₂ pool sizes were high at low CO₂ pressures and decreased at high CO₂ pressures. PGA pool sizes, on the other hand, were low at low CO₂ and high at high CO₂ pressures.

A model of RuP₂ carboxylase-oxygenase (Rubisco) kinetics is used to examine the quantitative relationship between in vivo RuP₂ carboxylase activity, CO₂ assimilation rate and RuP₂ and PGA pools. The model predictions fit in vivo data, except at high CO₂ pressures, if it is assumed that RuP₂ does not bind tightly to the inactive enzyme form in vivo. It is shown that a large fraction of the RuP₂ and PGA pools may be chelated by magnesium in the stroma and that the high RuP₂ pools (e.g. at low irradiance) may represent an optimal concentration rather than be truly saturating. We conclude that RuP₂ pool sizes above Rubisco site concentration do not necessarily indicate a Rubisco limitation of photosynthetic rate.