Assimilation and Metabolism of Carbon and Nitrogen In Fruiting Spur Leaves of ‘Gala' Apple (Malus domestica) In Response to Nitrogen Supply

Seven-year-old ‘Gala’/M.26 trees grown in sand culture were provided with 8.8, 26.4, 52.7, or 105.4 g N per tree in a modified Hoagland’s solution via fertigation and the cropload was adjusted to 8 fruits per cm² trunk cross-sectional area at 10 mm king fruit size via hand thinning. Leaves on fruiting spurs were sampled during rapid fruit cell expansion period for measuring activities of key enzymes and profiling primary metabolites. Both light-saturated CO₂ assimilation and dark respiration increased curvilinearly with increasing N supply. A total of 28 key enzymes and 46 metabolites in carbon and nitrogen metabolism were measured. Activities of most enzymes showed a curvilinear or linear increase as N supply increased. Of the carbohydrates measured, sorbitol, sucrose, 3-phosphoglycerate, glucose 1-phosphate, myoinositol, maltitol, xylitol, erythritol showed a curvilinear increase whereas starch, fructose, glucose 6-phosphate and fructose 6-phospharte showed a curvilinear decrease as N supply increased. Concentrations of all the amino acids increased curvilinearly or linearly with increasing N supply. Concentrations of organic acids responded to increasing N supply to a much lesser degree: phosphoenolpyruvate, oxaloacetate, and pyruvate remained unchanged with only a slight drop observed at the lowest N supply for malate, citrate, 2-oxoglutarate, succinate and fumarate. These results indicate that, in response to increasing N supply, carbon assimilation and the key enzymes in glycolysis and tricarboxylic acid cycle are all up-regulated to increase carbon flux in coordination with nitrogen flux while maintaining the concentrations of most metabolites in glycolysis and tricarboxylic acid cycle relatively stable.