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.