Phosphorus Accumulation and Toxicity in Leaves in Relation to Zinc Supply

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Abstract
The relationship of zinc (Zn) deficiency to phosphorus (P) toxicity was examined in ‘Emerald’ okra [Abelmoschus esculentus (L.) Moench] grown for 21 days (D21) in complete nutrient solution and then transferred to treatments with two levels of P (P1, P2 = 250, 2,000 µM P) and four levels of Zn (Zn1, Zn2, Zn3, Zn4 = 0, 0.25, 1, 2 µM added Zn).No symptoms of “little leaf” or “rosetting” developed in any plants. “Mottle leaf” symptoms developed as chlorotic and necrotic patches in the interveinal areas of the older leaves of Zn1 plants at P1 and in Zn1, Zn2, and Zn3 plants at P2. Thus, increasing the level of P induced symptoms which were eliminated by adding Zn.Treatments had little effect on dry matter at D45 and D63: at D111, low Zn depressed dry weight of P1 tops and even more markedly depressed dry weight of P2 tops, roots, and fruits.Increasing solution P increased the concentration and amount of P in all plant parts at all harvests. At D45 it also depressed Zn concentrations in all parts of Zn1 plants but in no other Zn treatment and in no treatment at later harvests.At Zn1, leaves had relatively low concentrations of Zn and high concentrations of P. Increasing levels of Zn enhanced their Zn concentrations and depressed their P concentrations; at D45 and D63 they also depressed the concentrations and total amounts of P in tops and whole plants but increased them in roots. Thus Zn deficiency markedly enhanced P absorption by roots, transport to tops, and accumulation in leaves.Concentrations of magnesium in plant organs responded to treatments in the same way as did P but to a smaller degree, particularly in the leaves. No other element measured (Ca, K, Na, Fe, Cu, Mn) responded in the same way.The development and intensity of symptoms in old leaves correlated closely with P concentrations. Leaves with faint symptoms had 0.8% P; those with marked symptoms had > 1.5% P; and dead leaves had nearly 5% P. The intensification of symptoms is attributed to accumulation of P to toxic levels in leaves.It is suggested that Zn deficiency interferes with P metabolism enhancing the amounts of P absorbed by roots and transported to tops: under conditions of high P supply, P accumulates to toxic levels in leaves inducing or accentuating symptoms resembling Zn deficiency. This effect of Zn on P metabolism in roots seems to explain previously puzzling observations in which P treatments enhanced symptoms of Zn deficiency without any reduction in Zn contents of plant tops: there is no need to invoke any effect of P in inactivating Zn in leaves.