Seasonal accumulation of mineral nutrients by kiwifruit 2. Fruit

Abstract

SUMMARYChanges in the concentration, distribution and quantity of macro‐ and micronutrients in the skin and flesh of fruit from 30 mature kiwifruit vines (Actinidia deliciosa var. deliciosa cv. Hayward) were recorded over a growing season in two high‐producing orchards.The concentration of copper, iron, nitrogen, phosphorus, potassium, sulphur and zinc in the flesh (seeds inclusive) declined sharply during the first 8 weeks of growth, reaching values which remained relatively constant, or declined only gradually, until harvest. For boron, calcium, magnesium and manganese flesh concentrations declined steadily throughout the season.Concentrations of nitrogen, phosphorus and potassium in the flesh were generally higher, and those of calcium manganese and zinc consistently lower, than corresponding concentrations in skin over the entire maturation period (23 weeks). Flesh concentrations of copper, iron, magnesium and sulphur exceeded those in the skin but only during the early stages of fruit development (up to 11 weeks after pollination). Boron concentrations in flesh and skin were similar during the season.The quantity of all nutrients increased throughout the season to be at a maximum by harvest The rate of accumulation of each element was greatest during the first 8 weeks of growth corresponding to the cell division phase of fruit development. There was 61–75% of the total calcium, manganese and zinc content taken up during this time, compared with 37–54% of the content of other nutrients. In the subsequent 15 weeks of development rates for nutrient accumulation declined. In the case of boron, calcium, magnesium, manganese, potassium and zinc there was minimal accumulation in the final 6 weeks prior to harvest.A mass How‐vascular transport model based on carbon and water influxes was used to predict the relative importance of phloem and xylem supply, and to partition nutrient accumulated by fruit over two time intervals; 2–8, and 8–17 weeks of growth. The model implies that where transpiration rates and phloem carbon concentrations are both high (i.e. approx. 6 ml water fruit−1 day−1 early in the season or 2 ml water mid season, and > l00 mg C ml−1), transport of nutrients occurs primarily via phloem. The only exceptions under these conditions are calcium, manganese and zinc which are translocated preferentially in xylem.