The aim of this study was to determine the response of photosynthetic carbon metabolism in spinach and bean to low temperature. (a) Exposure of warm-grown spinach and bean plants to 10-degrees-C for 10 days resulted in increases in the total activities of a number of enzymes, including ribulose 1,5-bisphosphate carboxylase (Rubisco), stromal fructose 1,6 bisphosphatase (Fru 1,6-P2ase), sedoheptulose 1,7-bisphosphatase (Sed 1,7-P2ase), and the cytosolic Fru 1,6-P2ase. In spinach, but not bean, there was an increase in the total activity of sucrose-phosphate synthase. (b) The CO2-saturated rates of photosynthesis for the cold-acclimated spinach plants were 68% greater at 10-degrees-C than those for warm-acclimated plants, whereas in bean, rates of photosynthesis at 10-degrees-C were very low after exposure to low temperature. (c) When spinach leaf discs were transferred from 27 to 10-degrees-C, the stromal Fru 1,6-P2ase and NADP-malate dehydrogenase were almost fully activated within 8 minutes, and Rubisco reached 90% of full activation within 15 minutes of transfer. An initial restriction of Calvin cycle fluxes was evident as an increase in the amounts of ribulose 1,5-bisphosphate, glycerate-3-phosphate, Fru 1,6-P2, and Sed 1,7-P2. In bean, activation of stromal Fru 1,6-P2ase was weak, whereas the activation state of Rubisco decreased during the first few minutes after transfer to low temperature. However, NADP-malate dehydrogenase became almost fully activated, showing that no loss of the capacity for reductive activation occurred. (d) Temperature compensation in spinach evidently involves increases in the capacities of a range of enzymes, achieved in the short term by an increase in activation state, whereas long-term acclimation is achieved by an increase in the maximum activities of enzymes. The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 10-degrees-C, may be factors contributing to its poor performance at low temperature.
