Photosynthetic enzymes and carbohydrate metabolism of apple leaves in response to nitrogen limitation

One-year-old apple (Males domestica Borkh. cv. Gala) trees were supplied twice weekly for 5 weeks with 500 ml of a modified Hoagland's solution at a nitrogen (N) concentration of 0, 5, 10, or 15 mM. Both CO2 assimilation and stomatal conductance decreased with decreasing leaf N. However, the calculated intercellular CO2 concentration increased as leaf N decreased. The chlorophyll a/b ratio remained unchanged as N supply decreased, except for a slight drop at 0 mM N. On a leaf area basis, the activities of key enzymes in the Calvin cycle [ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), stromal fructose-1,6-bisphosphatase (FBPase)] and those in end-product synthesis [cytosolic FBPase, aldose-6-phosphate reductase (A6PR), sucrose phosphate synthase (SPS), and ADP-glucose pyrophosphorylase (AGPase)] decreased linearly with decreasing leaf N. Contents of glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P) increased slightly as leaf N decreased from 2.39 g m(-2) to 1.31 g m(-2), then decreased in the lowest N leaves. The ratio of G6P/F6P remained unchanged over the leaf N range examined. The content of 3-phosphoglycerate (PGA) decreased linearly with decreasing leaf N. Starch content increased with decreasing leaf N both at dusk and pre-dawn. However, the contents of sorbitol, glucose, fructose, and sucrose decreased or remained unchanged as leaf N decreased. In conclusion, N limitation leads to accumulation of starch, but not soluble carbohydrates in apple leaves. Our data are consistent with the notion that N limitation restricts CO2 assimilation by directly limiting the activities of Rubisco and other enzymes, not by indirect feedback repression via sugar accumulation.