Disorders producing symptoms mainly on the younger leaves

Toxicities

Zinc toxicity is characterised by a severe reduction in dry matter yield. Both internode elongation of the stem and leaf expansion are retarded. Symptoms appear first on the young immature leaves as a pale green chlorosis (Photo 19a). A marginal chlorosis may also appear on the older leaves. As the effects of the toxicity become more pronounced the young expanding leaves turn uniformly yellow except for the main veins and the midrib which remains conspicuously green (Photo 19a).

Although similar symptoms are produced by deficiencies of nitrogen and iron, an important difference is the appearance of a red pigment in the vascular tissues of the older leaves. Because of weathering, this pigment may change to a purple colour in leaves of plants growing in the field (Photos 19a and 19b). Deposition of this pigment has been noted in other plant species affected by excess zinc³⁴.

To date only one case of zinc toxicity has been observed in kiwifruit grown in New Zealand. In this particular case, thermal bore water naturally high in zinc (3.6 mg/l) was used for irrigation. Toxicity symptoms were associated with concentrations of zinc in the youngest fully expanded leaves in excess of 1100 µg/g dry matter; usually zinc concentrations in the leaves of healthy plants sampled mid season range from 15 to 28 µg/g dry matter.

Studies with other plant species have shown that high concentrations of zinc can induce iron deficiency¹⁸. This effect may account for the symptoms of zinc toxicity being similar to those of iron deficiency (Photo 19c), (see also section on iron deficiency).

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19a	19b	19c