Influence of UV‐B Radiation on Early Seedling Growth and Translocation of 65Zn from Cotyledons in Cotton

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Abstract
The influence of UV‐B radiation from filtered or unfiltered fluorescent sunlamps on early seedling growth and translocation of 65Zn from cotyledons to the shoot was examined in two cultivars of cotton, Acala and Gregg. Ten‐day‐old seedlings which had been irradiated in the greenhouse for 6 h continuously each day for 14 days with 0.81 or 1.61 W × m‐2 UV‐B radiation under two unfiltered FS‐40 sunlamps, showed pronounced phytotoxic damage. This was characterized at first by bronzing and glazing of the cotyledons and later by upward curling of the leaves and abscission. Leaf expansion, dry matter accumulation, and mobilization of 65Zn from the cotyledons was severely impaired in the young developing shoot under unfiltered UV‐B radiation. A significant stress response also was observed in seedlings exposed to 0.61 W × m‐2 UV‐B radiation through a polystyrene filter and 0.73 W × m‐2 UV‐B radiation through a cellulose‐acetate filter. This stress response was characterized by the formation of a red pigment in the petioles of the cotyledons, reduced leaf expansion, and reduced transport of 65Zn. Control seedlings exposed to 0.03 W × m‐2 UV‐B radiation through a mylar filter were green, had maximum leaf size and dry‐matter accumulation, and had the greatest percentage of 65Zn translocated from the cotyledons.