Disorders producing symptoms mainly on the older leaves

Toxicities

Salinity is caused by high concentrations of salts in the soil solution. The chlorides and sulphates of sodium and magnesium are the most damaging of these salts to plants²³.

While soils containing large amounts of soluble salts are not common in New Zealand²⁷, there are at least three situations where salinity could limit kiwifruit production in this country: First, in orchards sited on soils recently affected by sea-water, such as the land near Napier which was raised above sea level during the 1931 earthquake. Appreciable quantities of soluble salts are still present, particularly in the subsoil²⁷.

Second, in orchards in exposed coastal areas, especially on the west coast of the North Island, where large quantities of sodium chloride are blown inland in sea spray. In 1953 it was found that quantities of sodium chloride deposited on soils near the coast were as high as 388 kg/ha/yr (Table 8)⁶. Even at a distance of 60 kilometres inland sodium chloride additions were over 60 kg/ha/yr.

The third situation is one which has caused the greatest damage to kiwifruit to date. This involves the use of saline bore water for irrigation. Districts affected by this problem include the eastern Bay of Plenty and Gisborne where the concentrations of sodium and chloride in the bore waters range from 230 to 860 mg/l and 475 to 1585 mg/l respectively.

In the field kiwifruit affected by excess salts are typically stunted with small and dull bluish-green leaves (Photo 8a). Symptoms appear first on the older leaves which may be cupped downwards (Photo 8b). Scorching of the leaf margins may also occur. As the disorder becomes more pronounced, progressively younger leaves are affected with the new growth being much paler green than normal (Photo 8c). Wilting symptoms are seldom observed. This disorder appears to be more associated with excess sodium than with chloride in the cases so far investigated.

In solution culture experiments in which effects of excess sodium (100 mg/l) were studied in the absence of excess chloride, the leaves produced were small, leathery, cupped downwards, and bluish in colour, (Photo 8d). These symptoms were prominent on salt affected plants in the field (Photos 8a and 8b).

When excess chloride was provided in the absence of excess sodium, the first observable symptom was the development of a bronze discolouration of the leaf margins on the older leaves (Photo 8e). Small necrotic patches may also appear between the veins of the older leaves (Photo 8f). This necrosis spreads quickly to occupy most of the leaf surface except a narrow zone each side of the major veins (Photo 8g). The young leaves tend to be pale, and upward rolling of leaf margins or downward cupping of the leaves may occur but leaf size is not noticeably reduced (Photo 8h). In solution culture, chloride affected plants undergo premature defoliation but this has not been observed for kiwifruit affected by salinity in the field.

Healthy plants sampled in the field at mid season usually contain less than 0.05 per cent sodium and between 0.8 and 2.0 per cent chloride in the leaf dry matter. Solution culture experiments indicate that symptoms of excess chloride do not usually appear until the concentration in the youngest fully expanded leaves exceeds 7 per cent of dry matter.

In the field, the concentrations of chloride in the leaves of plants severely affected by salinity have been found to range from 1 to 3 per cent of the dry matter (unpublished data), further suggesting that chloride is not the major factor involved in salinity problems in the field. Normally kiwifruit leaves have 1-2% chloride levels that are of dry matter. Potential problems can arise at chloride levels about 2.5% dry matter.

Paradoxically, the concentration of sodium remains quite low in the leaves of affected plants, always less than 0.12 per cent of the dry matter.

It appears that kiwifruit effectively exclude sodium from the aerial tissues, although they may accumulate excessive concentrations in the roots. As with many other plant species, the mechanisms whereby sodium interferes with the metabolism of the plant are not well understood.

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