Abstract

The photosynthetic capacity of leaves of N-sufficient plants of Spinacia oleracea L. increases following transfer a constant temperature of 10°C for 10 d compared to plants maintained at 25°C. The effects of nitrogen nutrition on this low temperature acclimation have been investigated in respect of CO2 assimilation, the activites and activation states of key enzymes and the partitioning of recently fixed carbon. N-deficiency greatly restricted acclimation of photosynthetic CO2 assimilation to low temperature at both ambient and at saturating CO2 concentrations, indicating a restriction on acclimatory changes in both ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and the reactions of ribulose-1,5-bisphosphate regeneration. Nitrogen limitation led to an increase in the partitioning of recently-fixed carbon into starch. Total protein increased during acclimation in both N-sufficient and N-deficient leaves and was much less affected than were the activities of enzymes. Increases in the activation state of Rubisco and the stromal fructose-1,6-bisphosphatase occurred in response to low temperature, but increases in the activities of Rubisco, sucrose-phosphate synthase or the cytosolic fructose-1,6-bisphosphatase could not be sustained in N-deficient plants throughout the period of acclimation, although the activities of these enzymes declined less precipitately than in non-acclimated N-deficient plants. These data are all consistent with the view that increases in the activities of key enzymes of carbon assimilation are a pre-requisite for acclimation to low temperature and that these increases are restricted under N-limitation.