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 w as studied in leaves of 5-year-old <<star ruby>> grapefruit (Citrus pa radisi Macf.) and 12-year-old <<fairchild>> mandarin (Citrus reticulat a Blanco) trees in orchards in Southern Turkey. In the leaf canopy, li ght intensity was reduced by partial shading of trees from the south a nd southeast sides by using a green plastic screen. Measurements were carried out in spring-cycle and autumn-cycle leaves between August 199 2 to April 1993 at about monthly intervals. Concentration of chlorophy ll remained more or less constant in shaded parts of trees between Aug ust and April, whereas in light exposed parts during this period (wint er month) the chlorophyll concentration markedly declined and severe, mainly interveinal chlorosis developed. During the winter months, leve ls of ascorbic acid and lipid peroxidation increased in light-exposed leaves, but not in shaded leaves. There was a close relationship betwe en chlorosis and levels of ascorbic acid and lipid peroxidation. Of th e 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 a bout 8 mg Zn kg(-1) leaf dry matter, indicating that trees possibly su ffer from Zn deficiency. The results demonstrate a major role of high light intensity in the development of low temperature-induced leaf chl orosis in field-grown citrus species and indicate a possible involveme nt of Zn deficiency in this chlorosis.