Accumulation of macro- and micronutrients and nitrogen demand-supply relationship of 'Gala'/'Malling 26' apple trees grown in sand culture

Abstract

Six-year-old 'Gala'/'Malling26' ('M.26') apple (Malus xdomestica Borkh.) trees grown in sand culture were provided with a total of 30 g of N per tree as enriched 15N-NH4NO3 in Hoagland's solution via fertigation to determine the magnitude and seasonal patterns of accumulation of macro- and micronutrients and the demand-supply relationship of N. Crop load was adjusted to 8.2 fruit/cm2 trunk cross-sectional area, at king fruit diameter of 10 mm by hand-thinning. At each of seven key developmental stages throughout one annual growth cycle, four trees were excavated and destructively sampled for complete nutrient analysis. Nutrient concentrations in leaves and fruit fell within the recommended optimal range, and the fruit yield was 18.8 kg/tree (equivalent to 52.45 t·ha-1') with an average fruit weight of 181 g. The net accumulation of N, P, K, Ca, Mg, and S from budbreak to fruit harvest was 19.8, 3.3, 36.0, 14.2, 4.4, and 1.6 g/tree, respectively, and that for B, Zn, Cu, Mn, and Fe was 93.6, 60.9, 46.5, 184.8, and 148.7 mg/tree, respectively. Nutrient accumulation by new growth (fruit plus shoots and leaves) accounted for over 90% of the net gain for N, P, K, Mg, S, and B in the whole tree and a large proportion of the net gain for Ca, Zn, Mn, and Fe (from 58.1% for Zn to 87.2% of Fe) from budbreak to fruit harvest. Differential nutrient accumulation patterns were found in shoots and leaves and fruit. The most rapid accumulation of all nutrients in shoots and leaves took place during active shoot growth from bloom to the end of shoot growth. The accumulation pattern of most nutrients corresponded well with the accumulation of dry matter, with continued accumulation observed only in total Ca and Mn in shoots and leaves after the end of shoot growth. Nutrient accumulation in fruit largely followed its dry matter accumulation, and a large proportion of the nutrient accumulation (from 58.1% for Zn to 77.4% of K) occurred from the end of shoot growth to fruit harvest. At harvest, fruit contained more P, K, B, and Fe, whereas shoots and leaves had more N, Ca, Mg, S, Zn, and Mn. Most of the N demand by new growth at bloom was provided by tree reserve N. Remobilization of N from perennial parts of the tree was found to support rapid fruit expansion from the end of shoot growth to fruit harvest. The most rapid uptake from current season's N supply occurred from bloom to the end of shoot growth, corresponding to the highest tree N demand. At harvest, 62.4% of the total N in new growth was in shoots and leaves, with the balance in fruit. Reserve N and current season's N uptake each contributed about 50% to the total N in the whole tree at harvest.