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Boron contamination in the agricultural lands is an important problem for Western Turkey, which has rich boron deposits. This study was carried out in Nazilli regions upon orange (Citrus sinensis L. Osbeck) plants irrigated with relatively high boron laden channel waters (LCI) and with well waters (LWI) which contain lower amounts of boron. The leaves of the plants irrigated with channel water were found to contain twice the amount of boron compared with those irrigated with well waters. Boron content of leaves in both groups were approximately ten times the boron content in the soil on which they are grown. In the leaves of the plants irrigated with channel waters there were approximately 50% chlorophyll loss and higher chlorophyll a/b and caretonoid/chlorophyll ratios. In the excessive boron containing leaves was found higher soluble protein and carbohydrate contents, but lower determined free proline value. In plants that irrigated with high boron laden channel water significantly lower α-tocopherol content and two fold higher ascorbate concentration were determined. The lower activities of catalase and glutathione reductase enzymes and higher total superoxide dismutase activity were measured in high boron content leaves. The retardation of growth due to boron toxicity can be attributed to the chlorophyll loss and inhibition of the carbohydrate metabolism. Boron at toxic level may cause the cell membrane lipids to be damaged by the free radicals by decreasing the α-tocopherol levels. The increase in the ascorbate concentration may have a vital role in the protection of the inner cell structures against the boron toxicity.

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