Changes in some Calvin Cycle Enzymes of the Tomato during Acclimation to Irradiance

The activities of three Calvin cycle enzymes, RuBPc (E.C. 4.1.1.39), 3PGA phosphokinase (E.C. 2.7.2.3) and NADP-G3P dehydrogenase (E.C. 1.2.1.13), and the cytoplasmic enzyme PEPc (E.C. 4.1.1.31) together with soluble protein and chlorophyll were measured in extracts from young tomato leaves during acclimation to a change in irradiance. Leaf area and fresh weight were also measured to show changes due to growth during treatments. Soluble protein had doubled on a unit leaf area basis 7 d after transfer from 100 μmol quanta m−2s−1PAR (low light) to 400 μmol quanta m−2s−1PAR (high light). During this period the protein/chlorophyll ratio rose from 4⋅6 to 10, RuBPc activity almost doubled and PEPc almost trebled. Following the reverse transfer from high to low light, soluble protein decreased by 30% after 7 d and the protein/chlorophyll ratio fell from 12 to 5⋅6. There was no change in RuBPc activity 3 d after transfer from high to low light while PEPc activity decreased by over 30%. There was no decrease in the activity of 3PGA phosphokinase or NADP-G3P dehydrogenase 1 d after transfer to low light, but decreases were apparent after 3 d. The extracted kinase and dehydrogenase when fully activated were able to phosphorylate and reduce 3PGA at more than 2⋅5-fold its calculated rate of synthesis in the leaf. The data are discussed in relation to changes in the CO2 exchange of the leaf.