ULTRAVIOLET-IRRADIATION EFFECTS ON SEROTINOUS LEUCADENDRON LAUREOLUM SEEDS - ALTERED SEED PHYSIOLOGY AND ULTRASTRUCTURE, AND SEEDLING PERFORMANCE

Dry seeds of Leucadendron laureolum (Lam.) Fourc. (Proteaceae) were ex posed for different intervals (range: 7 to 84 days) to visible, UV-A a nd UV-B radiation of different biologically effective dose (range: 0 t o 11.43 kJ m(-2) d(-1)). Changes in seed germination, physiology and u ltrastructure, and residual UV effects on seedling performance, were e xamined. Germination was depressed in seeds following short (7-day) ex posures to UV radiation. This depression was intensified with increase d UV exposure dose, and most pronounced at shorter UV-B wavelengths. A lso glutathione reductase (GR) activities increased in seeds exposed t o shorter UV-B wavelengths, but these were unaffected by irradiation d ose level in the UV-B range. Electrolyte leakage rates from UV-irradia ted seeds were unaltered, which indicated that germination depression did not result from intrinsic membrane damage. The reversal of germina tion depression (W-induced dormancy) in UV-irradiated seeds by red lig ht pointed to the possible involvement of phytochrome in this photo-re sponse. Germination depression disappeared in seeds after 56-days irra diation, possibly due to photoreceptor damage by excess UV light. At t his stage, all UV irradiated seeds, irrespective of treatment waveleng th or dose level, exhibited increased electrolyte leakage rates, which indicated membrane perturbation. Also, increased GR activities were o bserved in irradiated seeds, but these were proportionately smaller in seeds exposed to shorter wavelength UV-B radiation (9.1 to 35.8% incr ease) than longer wavelength UV-A (73.4% increase) and visible (97.7% increase) radiation. This implied a metabolic limitation for scavengin g of free radicals and peroxides in aging seeds exposed to UV-B radiat ion, which pointed to accelerated seed deterioration. It was indirectl y supported by ultrastructural evidence of sub-cellular damage (Lipid coagulation and plasmalemma withdrawal from cell walls) in embryonic t issues of seeds after 84 days UV-B exposure, and reflected in decrease d leaf numbers, photochemical efficiencies, and foliar chlorophyll a a nd carotenoid levels in seedlings cultured from these seeds.