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Phosphorus Accumulation and Toxicity in Leaves in Relation to Zinc Supply¹

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.

¹ Cooperative investigation, USDA-SEA-AR, U.S. Plant, Soil and Nutrition Lab., Ithaca, N.Y., and Dep. of Agronomy, Cornell Univ., Ithaca, N.Y. Dep. of Agronomy paper no. 1,328.

² Visiting Research Scientist and Senior Fulbright Scholar; Soil Scientist; Plant Physiologist; Visiting Scientist; Visiting Scientist; Soil Scientist; and Chemist, respectively. The first, fourth, and fifth Authors are now located at Murdoch University (Perth, Western Australia 6150), University of IFE (ILE-IFE, Nigeria), and MARDI (Serdang, Selangor, Malaysia), respectively.

Received for publication June 15, 1981. Accepted for publication September 14, 1981.

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