Abstract
Flame grapefruit trees on Swingle citrumelo rootstock were planted in 1998. Phosphorus fertilizer was applied at 0, 48 (optimal), and 96 (high) kg P/ha in 1999-2000 to establish a range of soil-test P, and none was applied since Spring 2000. Potassium fertilizer was applied every year since planting at 0, 186 (optimal), and 372 (high) kg K/ha. The P and K treatments were applied in all factorial combinations. Nitrogen and other cultural practices followed the recommendations (www.CREC. IFAS.UFL.EDU) for citrus production. Fruit were harvested in January 2003 and 2004 equally from all sides of the tree from 5 blocks totaling 25 trees. Harvested fruit were hand-juiced and assayed for vitamin C, pigments, and sugar by an HPLC system. The results indicated that vitamin C and total sugar (sum of glucose, fructose, and sucrose) was higher in year 2004 than 2003 fruits. The highest vitamin C and sugar levels were in the treatment block without P and optimal K (0 × 186) while the lowest was found in the optimal P and non-K (48 × 0) block among the five treatments. This trend was reflected in sucrose concentration, but not in fructose and glucose concentrations in fruit juice. In turn, [beta]-carotene and lycopene were the highest in the optimal P and non-K (48 × 0) treatment during year 2003 and 2004, respectively. No significant difference was found among the rest of treatments. The study demonstrated that high level of P or K does not increase the fruit vitamin C, pigments, and sugar concentrations. Since Flame grapefruit is characterized and perceived by consumers as being rich in antioxidant compounds, and high in nutritional value, consideration should be given to using only the optimal fertilization rates. Increasing P and K fertilization rates does not increase the fruit internal nutritional value of the desired and healthful compounds found in Flame grapefruit.
