RuP2 pool size indicated by CO2 assimilation following the abrupt loss of light

Measurement of the changes in CO₂ uptake by single leaves following the abrupt onset of darkness were made on sugarbeets (Beta vulgaris L.) and (Phaseolus vulgaris L.) The shape of the CO₂ dark response curve was analyzed with respect to the reaction kinetics of CO₂, RuP₂ and RuP₂ carboxylase. It was concluded that the net uptake of CO₂ in the dark from a 1% O₂ atmosphere can be approximately related to the pool size of the RuP₂ substrate in the chloroplasts of C₃ plants. This information was combined with CO₂ levels and decay rates of the response curves to infer changes in carboxylase activity. Preliminary data are presented showing the relative concentration changes in RuP₂ as light intensity decreases and as water stress increases. The method may prove useful in studies of plant response to environmental stresses.