International Water & Irrigation

23 In general, nutritional needs of persimmon trees change with the tree age and also during the annual productive period. The nutritional requirements of persimmon trees varies according to phonological growth stage, namely budbreak, shoot growth, flowering, fruit set, fruit development, flower induction, ripening, accumulation of reserves and entry into dormancy. Fertilization after harvest is highly recommended to improve bud sprouting and flower-bud initiation in the next season. Sprouting is a short and intense phase (around 15 days) during which the nutrients are exclusively taken from the nutritional reserves accumulated during the previous year. Leaf carbohydrate at the beginning of flower bud initiation and nitrogen content of 1-year-old wood at the time of flower differentiation are the main determining factors of the number and quality of the flowers that will be present in the trees in the following season. Light postharvest applications of N and K may prevent early leaf drop ensuring enough reserves in the tree for a satisfactory performance of the crop during the following season . Substantial change in the nutrient content of shoots occurs during early season growth following budbreak indicating the importance of mobilization for the tree reserves during these first stages. Photosynthetic activity starts fifteen days after budbreak (Hino et al., 197 4 ). The timing of fertilization can be delayed to coincide with the onset of photosynthetic activity. It is also recommended to provide a large part of the annual N and P requirements, as well as other micronutrients such as zinc or manganese, which usually are deficient in persimmon trees. During the productive phase, from June to October, persimmon trees require adequate nutrition to ensure good fruit growth. A ratio of 20-40 leaves fruit+ is required to achieve good fruit quality. An excess of N fertilizer has been reported to cause fruit drop as well as a reduction in fruit firmness, sugar content and postharvest storage time. Plant nutrition status monitoring is recommended to avoid over-fertilization. Optimum leaf N content values one month before harvest should be in a range between 1.8-2.5% (dm), while Ca/N and Ca/K should be higher than 1.5 to ensure a good fruit quality. Conclusions The studies on the assessment of RDI strategies in persimmon trees, highlight that further research is still needed in order to elucidate the physiological mechanisms involved in fruit drop, as well as to explore how different water stress levels might affect fruit drop in the ‘Rojo Brillante’. Other topics that need to be addressed in the future to achieve a more efficient nutrient management of persimmon trees are: • Study of the effects of specific mineral nutrition programs on fruit quality and astringency; • Salinity and chloride phytoxicity management; • Study of the physiological responses of persimmon trees to climate change (heat spells, low winter chilling). ■ Table Three: Fertilization program for the astringent persimmon cultivar “ Rojo Brillante” Month Nitrogen (kg N ha 1 ) Phosphorous (kg P 2 0 5 ha 1 ) Potassium (kg K 2 0 ha 1 ) Magnesium (kg Mg0 ha 1 ) March 9 4 6 0.8 April 18 8 10 1.2 May 27 12 12 1.6 June 36 16 18 2.4 July 36 16 37 5 August 36 16 37 5 September 18 8 30 5 Total 180 80 150 20 explore how different water stress levels might affect fruit drop in the ‘Rojo Brillante