Expression Pattern of the Carrot EP3Endochitinase Genes in Suspension Cultures and in Developing Seeds1

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.