Modulation of Rubisco Activity during the Diurnal Phases of the Crassulacean Acid Metabolism Plant Kalanchoëdaigremontiana

Abstract

Abstract The regulation of Rubisco activity was investigated under high, constant photosynthetic photon flux density during the diurnal phases of Crassulacean acid metabolism in Kalanchoëdaigremontiana Hamet et Perr. During phase I, a significant period of nocturnal, C4-mediated CO2 fixation was observed, with the generated malic acid being decarboxylated the following day (phase III). Two periods of daytime atmospheric CO2 fixation occurred at the beginning (phase II, C4–C3 carboxylation) and end (phase IV, C3–C4 carboxylation) of the day. During the 1st h of the photoperiod, when phosphoenolpyruvate carboxylase was still active, the highest rates of atmospheric CO2 uptake were observed, coincident with the lowest rates of electron transport and minimal Rubisco activity. Over the next 1 to 2 h of phase II, carbamylation increased rapidly during an initial period of decarboxylation. Maximal carbamylation (70%–80%) was reached 2 h into phase III and was maintained under conditions of elevated CO2 resulting from malic acid decarboxylation. Initial and total Rubisco activity increased throughout phase III, with maximal activity achieved 9 h into the photoperiod at the beginning of phase IV, as atmospheric CO2 uptake recommenced. We suggest that the increased enzyme activity supports assimilation under CO2-limited conditions at the start of phase IV. The data indicate that Rubisco activity is modulated in-line with intracellular CO2 supply during the daytime phases of Crassulacean acid metabolism.