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Soils of litchi orchards in China are commonly deficient in nitrogen and potassium. The cultivar Feizixiao litchis planted in a typical acidic upland orchard, which is low in nitrogen and potassium, were used as a subject in field experiments with different ratios of potassium to nitrogen (K2O:N = 0.6, 0.8, 1.0, 1.2, and 1.4). Field experiments were conducted from 2009 to 2012. The effects of K2O:N ratio on the yield, quality, and storability of litchi were investigated and discussed. Results indicated that with the increase of K2O:N ratio, fruit yield initially increased and then decreased, and litchi had the highest yield when K2O:N was 1.2. When K and N fertilizers were applied at the ratio of 1.2, litchi had a better fruit quality with higher vitamin C content, soluble sugar, and soluble solid. With the increase of K2O:N ratio, healthy fruit rate initially increased and then decreased. This rate reached the maximum value when K2O:N was 1.2. Meanwhile, fruit-rotting rate, peel-browning index, cell membrane permeability, and peroxidase (POD) activity decreased at first and then increased and reached the minimum value when the K2O:N ratio was 1.2. Therefore, litchi fruit had the highest yield, better quality, and best storage property when K2O:N was 1.2. Thus, this ratio is recommended for the main litchi production areas in China.
Free amino acid (FAA) profile is an important indicator of the quality of fruit and fruit product. Foliar nutrient diagnosis has been used for crop yield prediction for decades but not for fruit quality evaluation. Concentrations of 11 leaf nutrients including N, P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, and B at stages of terminal shoot maturation and fruit development and fruit FAA profile at harvest were examined in longan in South China. The relation between leaf nutrient and fruit FAA was then investigated by multiple stepwise regression analysis. Foliar N content was greatest among the nutrients among the detected elements at both stages. Twenty-nine FAAs were determined in longan flesh, with alanine (19.9%), γ-aminobutyric acid (17.5%), glutamic acid (15.2%), and asparagine (10.7%) as the main components. Flesh individual FAA, essential amino acid (AA), umami-, and sweet and bitter taste AA strongly depended on foliar nutrients. However, the relation between flesh FAAs and foliar nutrients varied with FAA species. Leaf N was the dominant indicator for most pulp FAAs at two growth stages, while other nutrients (e.g., B, Zn, P, K, Ca, Mg) also played versatile roles on flesh FAAs. This work provides a novel tool to predict fruit FAAs via foliar nutrient diagnosis, which supports the practicality of producing specific target fruit or improving fruit quality through regulation of fertilization strategies in fruit production.