Nitrogen Metabolism of ‘Gala' Apple Fruit in Response to Nitrogen Supply

Amino acid profiles and activities of several key enzymes in nitrogen and amino acid metabolism in ‘Gala’ apple fruit were determined to understand how nitrogen metabolism in apple fruit responds to N supply. Seven-year-old ‘Gala’/M.26 trees were grown in sand culture and provided with a total of 8.8, 26.4, 52.7 or 105.4 g actual N per tree from early May to harvest via fertigation in Hoagland’s solution. As N supply increased, fruit N concentration at harvest increased linearly whereas leaf N concentration at regular leaf sampling time showed a curvilinear increase. Concentrations of soluble proteins, total free amino acids, and the proportion of total free amino acids accounted for by asparagine (Asn) in fruit increased with increasing N supply throughout fruit development. At harvest, fruit Asn concentration showed a linear response to N supply, and the increase was about 10 times more sensitive than fruit N concentration. The elevated Asn level at higher N supply corresponded with higher asparagine synthase activity, and may also be related to the enhanced transport of Asn into fruit. As N supply increased, glutamine synthetase, NADH-glutamate synthase, glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase increased whereas glutamate dehydrogenase decreased, all of which contributed to the increased free amino acids in fruit tissue. GOT activity increased during fruit growth and in response to N supply, which paralleled with developmental increases and N-effected increases of some aspartate family amino acids such as aspartate and isoleucine, and threonine, confirming that GOT plays an important role in the synthesis of aspartate family amino acids. These results indicate that amino acid metabolism in apple fruit is up-regulated in response to increasing N supply and asparagine may serve as a sensitive indicator of fruit N status.