Interactions between Zinc and other Plant Nutrients
Authors: Deep Mohan Mahala*, Rajendra Kumar Yadav

Introduction

Zinc is essential micronutrients for proteins production in plants; also zinc is main composition of ribosome and is crucial for their growth. Zinc is vital element in biochemical processes and has a chemical and biological interaction with other elements. Although genotypic factors are important for determining either the tolerance or susceptibility of a plant to zinc deficiency, it is soil features which are responsible for low available zinc supply. In general, the soils are generally associated with zinc deficiency in plants mainly due to the factors such as neutral to alkaline in reaction (pH >7.4), high CaCO3 content or in subsoil exposed by removal of the topsoil during leveling or by erosion, coarse texture soil with a low organic matter (SOM) status, permanently or intermittently waterlogged soil, high available phosphate status, acid soil of low zinc status developed on highly weathered parent material. Here we discuss only the interaction of Zn with other soil elements which affect availability to plants. Following interactions of Zn with other nutrients are important:

1. Zinc-Phosphorus Interactions:

Excessive phosphate levels in soils are one of the most important causes of zinc deficiency in crops. However, although this interaction with phosphate has been recognized for several years, the actual mechanisms responsible are still not completely understood.

Marschner (1993) discussed that, in general, plant uptake of zinc decreases sharply, often beyond a certain limit which can be ascribed to dilution effects due to more growth enhancement, with an increase in the soil content or supply of fertilizer phosphorus. However, extractable zinc in soils is either not at all, or only slightly decreased by a increased phosphorus supply. In acid tropical soils, the risk of phosphorus induced zinc deficiency increases when phosphorus is applied with liming. Liming will overcome toxicity caused by aluminium and thereby increase root growth but the sharp drop in zinc concentrations in the soil solution combined with higher zinc requirements due to enhanced shoot growth requires the extra supply of zinc to prevent a reduction in growth with high phosphorus and lime supply Marschner (1993).

Loneragan and Webb (1993) distinguish two different kinds of zinc-phosphorus interactions:

1) Those in which increasing phosphorus applications decline concentrations of zinc in the shoot.

2) Those in which increasing phosphorus applications do not decline zinc concentrations in the shoot.

The most usual type of zinc-phosphorus interaction is the type 1, where phosphate cause a decrease in zinc concentrations and this generally occurs where the supply capacity of the soil for both zinc and phosphorus are marginal, so the addition of phosphatic fertilizer advances the growth sufficiently to cause the dilution of zinc concentrations in plant tissues to levels which enhance zinc deficiency.

However, there are other conditions in which zinc deficiency has been induced by phosphorus without the dilution of zinc in plant shoots. It is most likely that phosphorus may either depressed zinc absorption by roots or the translocation of zinc from roots to the shoots.

There are four probable mechanisms by which phosphorus can reduce the absorption of zinc from soils:

i) High concentrations of phosphorus may suppressed the infection of roots by Arbuscular mycorrhizae (AM)

ii) Cations with phosphate salts can inhibit zinc absorption from solution.

iii) H+ generated by phosphate salts can inhibit zinc absorption from solution.

iv) Phosphorus can enhances the adsorption of zinc onto soil constituents.

The role of AM in the uptake of phosphorus by plants is well known. The AM effectively increase the area of the absorbing surface of root and this not only affects the absorption of phosphorus but all other elements. So when the phosphorus concentrations are relatively high the suppression of the mycorrhizae will have the effect of reducing the uptake of other ions such as Zn2+. Cereals and grasses, which have finely branched root systems, generally do not rely heavily on AM for the absorption of nutrients. However, plants with relatively coarse, poor branched root systems and few root hairs, such as legumes, may acquire a considerable proportion of their nutrients through AM fungi.

The circumstances in which phosphorus directly hamper zinc absorption are difficult to substantiate. In plant there are several mechanisms by which phosphorus can affect the mobility as well as availability of zinc.

These include:

- Inhibition of translocation of zinc from roots to shoots,

- Reduction in the amount of soluble zinc,

- Binding of zinc by phosphorus-containing phytate,

- Leakage of phosphorus from membranes.

In situations where phosphorus causes the symptoms of zinc deficiency without decreasing zinc concentrations, it is considered that the increasing phosphorus concentrations within the plant increase the plant's internal zinc requirement (which is a "phosphorus-enhanced zinc requirement").

In some situations, applications of phosphatic fertilizers can bring about an increase in zinc concentrations in plants. These can be explained by an increase in acidity in the root environment (and high uptake of zinc) or to zinc impurities in the phosphatic fertilizer. Fertilizers, such as superphosphate contain appreciable amounts of zinc (and cadmium), which, also acidify the soil due to their elevated sulphate (SO42-) content.

Many researchers have used phosphorus/zinc (P/Zn) ratios for the identification of zinc deficiency, but in general, they have not been widely acceptable. These ratios can vary for species and experimental conditions.

It is widely acknowledged that zinc deficiency can enhance phosphorus toxicity in plant shoots. In many instances, it was the phosphorus concentration in leaves which was correlated with the symptoms, but not the zinc content. The high concentrations of phosphorus at low levels of zinc is considered to be responsible for the "phosphorus-enhanced zinc requirement syndrome". In some crops such as potato, okra and cotton, low zinc with increased phosphorus supplies induced phosphorus toxicity by enhancing phosphate uptake into the plant, causing phosphate to accumulate preferentially in the leaves, likely by reducing the translocation of phosphorus out of the leaves.

2. Zinc-Nitrogen Interactions

Nitrogen affect the zinc status of crops by both assisting plant growth and by changing the pH of the root surrounding. In soils, nitrogen is the main factor for limiting growth and yield and therefore, improvements in yield have been found through positive interactions by applying both, nitrogen and zinc fertilizers. Crops often respond to nitrogen and zinc together but not to zinc alone. The application of nitrogen fertilizer in the absence of zinc can lead to zinc deficiency through a dilution effect caused by an increase in growth due to the nitrogen application. However, this may be a negative interaction if other micronutrients, such as Cu are also low in the soil. Nitrogen enhanced growth can cause a dilution in copper concentration which is then aggravated by applied zinc. This problem is really a zinc-copper interaction aggravated by nitrogen (Kirk and Bajita, 1995).

Nitrogenous fertilizers such as ammonium sulphate ((NH4)2SO4) have a marked acidifying effect on soils and so increase the availability of zinc to crops in soils with high pH status. Whereas, nitro-chalk (Ca(NO3)2) increase the soil pH and lower down the zinc availability.

3. Interactions of Zinc with other Macronutrients:

Several macronutrients, including calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na) are known to hamper the absorption of Zn by plant roots in solution culture experiments, but in soils their principal effect seems to be through their influence on soil pH. Applications of gypsum (CaSO4. 2H2O) which lower down the soil pH from 5.8 to 4.6 increased the Zn content of plants, but the equivalent amount of Ca applied as calcium carbonate (CaCO3), which increased the pH from 5.7 to 6.6, lowered the zinc in plants. K and Mg have been shown to inhibit Zn absorption in solutions with low levels of Ca, but once the Ca concentration was increased, the effects disappeared.

4. Interactions of Zinc with other Micronutrients:

Zinc is known to interact with copper, iron, manganese and boron.

a) Zinc-copper interactions occur through:

i) Competitive absorption inhibition (due to copper and zinc sharing a common site for root absorption),

ii) Copper affects the redistribution of zinc within plants.

Where soils are deficient in either element, application of the other will aggravate the deficiency in the plant. In copper deficient plants, the senescence of the oldest leaves and the export of nitrogen, copper and zinc from them was slowed down compared with plants with adequate copper.

b) Iron-zinc interactions are as complex as those between zinc and phosphorus. Higher zinc supplies to plants have been observed to improve the iron status, to decrease it, and to have no effect on it.

Under iron deficiency, zinc absorption into plants and concentrations in shoots can be much increased. In dicotyledons, the mechanism involved is probably that of acidification of the rhizosphere (Strategy I reaction) to iron deficiency. In cereals, phytosiderophores released in the soil (Strategy II reaction) to reduce iron deficiency have been found to enhance the mobilization of zinc from a calcareous soil. However, phytosiderophores did not improve zinc absorption into the root in the same way as they did for Fe. Effects of these mechanisms on Zn absorption would be additional to any effect Fe might have in directly inhibiting Zn absorption.

Root exudates from Zn deficient plants have been shown to be capable of mobilizing more iron from Fe2+ hydroxides than the exudates from Zn adequate plants. High Mn in combination with high Fe may hamper the absorption of Zn by rice in flooded soils and enhance Zn deficiency in rice.

Zinc-deficient plants can absorb high concentrations of boron (B) in a similar way to Zn deficiency enhancing phosphorus toxicity in crops and this is probably due to weakened membrane function in the root.

References:

1. Brown, P. H., I. Cakmak and Q. Zhang (1993) Form and function of zinc in plants. Chap 7 in Robson, A.D. (ed.) Zinc in Soils and Plants, Kluwer Academic Publishers, Dordrecht. 90-106.

2. Kirk, G. J. D. and J. B. Bajita (1995) Root-induced iron oxidation, pH changes and zinc solubilisation in the rhizosphere of lowland rice. New Phytologist, 131, 129-137.

3. Loneragan, J. F. and M. J. Webb (1993) Interactions between Zinc and Other Nutrients Affecting the Growth of Plants. Chap 9 in Robson, A.D. (ed.) Zinc in Soils and Plants, Kluwer Academic Publishers, Dordrecht. pp 119-134.

4. Marschner, H. (1993) Zinc Uptake from Soils, Chap 5 in Robson, A.D. (ed.) Zinc in Soils and Plants, Kluwer Academic Publishers, Dordrecht. pp 59-78.


About Author / Additional Info:
Ph.D. Scholar, Division of Soil Science and Agricultural Chemistry, IARI, New Delhi