Nitrogen Nutrition of Young Citrus Fruit with Special Reference to Asparagine

Asparagine was the predominant constituent of the free amino acids in young fruit of Citrus unshiu Marc. In early August, asparagine increased exponentially with total N in the fruit peel, suggesting that it can be used as an indicator of the nitrogen status of young fruit. 15N data showed that the primary assimilation of ammonium was catalyzed by glutamine synthetase, and asparagine was also actively formed in the early stages of nitrogen assimilation. Nitrate assimilation occurred to a lesser extent than ammonium assimilation in the hour-order experiment using excised fruit. In the day-order experiment, however, nitrate taken up via the roots was assimilated in greater amounts than ammonium. 14C-asparagine was rapidly metabolized to different compounds, such as aspartate, acidic and neutral compounds, and alcohol-insoluble compounds. However, considerably less CO2 was formed from asparagine than from aspartate. Our data show that asparagine is not a dead-end-product but plays an important role in the supply of both N and C for the synthesis of other amino acids and protein in young fruit.