ASSOCIATION OF HIGH LIGHT AND ZINC-DEFICIENCY IN COLD-INDUCED LEAF CHLOROSIS IN GRAPEFRUIT AND MANDARINE TREES

The effect of varied light intensity on seasonal changes in levels of chlorophyll, ascorbic acid, lipid peroxidation and mineral nutrients was studied in leaves of 5-year-old <<star ruby>> grapefruit (Citrus paradisi Macf.) and 12-year-old <<fairchild>> mandarin (Citrus reticulata Blanco) trees in orchards in Southern Turkey. In the leaf canopy, light intensity was reduced by partial shading of trees from the south and southeast sides by using a green plastic screen. Measurements were carried out in spring-cycle and autumn-cycle leaves between August 1992 to April 1993 at about monthly intervals. Concentration of chlorophyll remained more or less constant in shaded parts of trees between August and April, whereas in light exposed parts during this period (winter month) the chlorophyll concentration markedly declined and severe, mainly interveinal chlorosis developed. During the winter months, levels of ascorbic acid and lipid peroxidation increased in light-exposed leaves, but not in shaded leaves. There was a close relationship between chlorosis and levels of ascorbic acid and lipid peroxidation. Of the mineral nutrients (Zn, Fe, Mn, Cu, K, Ca, P) measured in the leaves only Zn was clearly in the deficient range. During the winter months, chlorotic (light-exposed) as well as green (shaded) leaves contained about 8 mg Zn kg(-1) leaf dry matter, indicating that trees possibly suffer from Zn deficiency. The results demonstrate a major role of high light intensity in the development of low temperature-induced leaf chlorosis in field-grown citrus species and indicate a possible involvement of Zn deficiency in this chlorosis.