Accumulating epidemiological evidences indicate that citrus phytochemicals have prevented chronic diseases such cancer and heart diseases. To enhance nutraceutical levels, field experiments were conducted using `Ruby Red' grapefruit on Carrizo citrange rootstock to evaluate the effects of variable fertilizer rates on nutraceutical contents. The trees received annual nitrogen rates from 0 to 280 kg·ha-1 (using a 1 N: 0.25 P: 1 K blend) under optimal irrigation schedule. Subsamples of fruit were analyzed for nutraceutical levels. HPLC analysis showed that naringin concentrations of the fruit collected from the trees treated with different levels of nitrogen differ significantly, and naringin levels decreased with increased nitrogen levels. Fruit from the control treatment had 1316 mg·mL-1 of naringin compared to the fruit collected from 280 kg N/ha per year trees (1056 mg·mL -1). A similar trend was observed with tasteless flavonoid naringenin rutinoside (narirutin). Total vitamin C [ascorbic acid (AA) plus dehydroascorbic acid (DHAA)] content from the fruit collected decreased with the nitrogen levels increased. These results demonstrate that increased fertilizer rates have an influence on the nutraceutical levels; therefore, there is a potential for further investigations on fine tuning the preharvest production programs to improve the nutritional value of the fruit.
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