Effects of zinc deficiency and phosphorus toxicity

Abstract
In some previous experiments, Zn deficiency has enhanced P accumulation in old leaves to toxic concentrations by enhancing the rate of P absorption. This research was undertaken to examine the relationship of Zn deficiency to P accumulation and toxicity in young wheat plants (Triticum aestivum L. ‘Gamenya’) grown in a complete nutrient solution with 1000 µM phosphate and either with or without 2.5 µM Zn. Mild Zn deficiency depressed shoot but enhanced root DM and this change was almost paralleled by P distribution. Severe Zn deficiency enhanced P concentration to 3 to 4% in old leaves and the marked necrotic symptoms were attributed to combined Zn deficiency and P toxicity. It depressed whole plant dry matter (DM) by 40% but P content by only 10%. It severely depressed DM distribution but enhanced P distribution to shoots thus doubling shoot P concentration. Short term and long term estimates indicate that mild Zn deficiency had only a transient effect in enhancing Pi absorption rate as severe Zn deficiency substantially depressed it. Thus, Zn deficiency enhanced P concentrations to toxic levels in old leaves through the cumulative effects of responses at three tiers of structural organization: (i) in the whole plant, it depressed DM more strongly than P content, (ii) between roots and shoots, it depressed that proportion of total DM which was present in shoots while increasing that of total P, (iii) within shoots, it concentrated P in old leaves by depressing new growth.

Corn plants (Zea mays L.) were grown in greenhouse pots on calcareous soil known to be low in available Zn. Phosphorus was applied to soil at rates of 0 to 520 ppm with concomittant addition of Zn decreasing from 40 to 0 ppm—in inverse concentration. Zinc deficiency occurred at high P and low Zn. Maximum growth occurred at 130 ppm P and 2.5 ppm Zn applied.

Distribution of Zn, Fe and Mn among leaf, stem and root tissue indicated that P and Zn fertilization had altered the mobility of these micronutrients within the plant. Mobility of Fe and Mn was increased by high P and low Zn. Zinc mobility varied opposite to that of Fe and Mn.

Applied P reduced Zn concentration in tissue but not uptake per plant. DTPA-extractable Zn in soil was not decreased by P applied, but did increase with Zn applied. DTPA-extractable Fe and Mn increased at high levels of applied P.

Zinc-deficient plants accumulated a large excess of Fe. Interference from excess Fe is suggested as contributing to physiological malfunction within Zn-deficient corn plants.