Photosynthetic and metabolic acclimation to low growth temperatures were studied in Arabidopsis (Heynh.). Plants were grown at 23 degrees C and then shifted to 5 degrees C. We compared the leaves shifted to 5 degrees C for 10 d and the new leaves developed at 5 degrees C with the control leaves on plants that had been left at 23 degrees C. Leaf development at 5 degrees C resulted in the recovery of photosynthesis to rates comparable with those achieved by control leaves at 23 degrees C. There was a shift in the partitioning of carbon from starch and toward sucrose (Suc) in leaves that developed at 5 degrees C. The recovery of photosynthetic capacity and the redirection of carbon to Suc in these leaves were associated with coordinated increases in the activity of several Calvin-cycle enzymes, even larger increases in the activity of key enzymes for Suc biosynthesis, and an increase in the phosphate available for metabolism. Development of leaves at 5 degrees C also led to an increase in cytoplasmic volume and a decrease in vacuolar volume, which may provide an important mechanism for increasing the enzymes and metabolites in cold-acclimated leaves. Understanding the mechanisms underlying such structural changes during leaf development in the cold could result in novel approaches to increasing plant yield.