EFFECTS OF TEMPERATURE, LIGHT AND GIBBERELLIC-ACID ON THE GERMINATIONOF SEEDS OF 43 SPECIES NATIVE TO WESTERN-AUSTRALIA

Species native to the southwest of Western Australia, representing a r ange of plant families, life-history strategies, fire-response syndrom es, seed-store types and seed weights, were tested for viability using tetrazolium chloride and for germination under combinations of consta nt temperatures of 15 degrees C or 23 degrees C, constantly dark or 12 h diurnal white-light conditions, and with, or without, addition of g ibberellic acid (GA(3), 50 mg/l). Species previously known to require a heat-shock treatment to overcome dormancy due to an impervious testa were pre-treated prior to imposition of temperature, light and GA(3) conditions. The test environmental conditions related to differences b etween winter and autumn temperatures and surface and buried seed germ ination positions of post-fire habitats. The viability of the selectio n of native Western Australian species ranged from 0 to 100 %, averagi ng 71 %. For all taxa, no combination of temperature, light and gibber ellic acid treatment induced all viable seeds to germinate. The greate st percentage germination achieved in any combination of treatments av eraged 71 % of all viable seeds for all species. Larger seeds (> 10 mg seed weight) tended to have greater viability percentages, but no ove rall patterns of viability or germinability were attributable to seed storage syndrome, strategy of fire recovery response or life-form type . Germination of most of the dominant tree representatives (Eucalyptus calophylla, E. diversicolor, E. erythrocorys, E. gomphocephala, and E . patens) was indifferent to the trial conditions of temperature, ligh t and GA(3). However, Eucalyptus marginata showed reduced germination in the light, which was overcome with GA(3). GA(3) also overcame the i nhibition resulting from exposure to light in some understorey species (e.g. Allocasuarina campestris, Regelia ciliata, Xanthorrhoea gracili s and X. preissii). Germination of many hard-seeded, understorey shrub and herbaceous perennial species, especially those with small (< 10 m g) seeds (e.g. Bossiaea ornata, B. aquifolium and Acacia drummondii ss p. candolleana) was greater at the lower trial temperature and in the dark. Some large (> 10 mg) seeded, understorey species (e.g. Acacia ex tensa, Kennedia coccinea, K. prostrata, Hovea trisperma and Hardenberg ia comptoniana) germinated in high percentages in both temperatures, b ut maximum germination percentages still tended to be at 15 degrees C. Large-seeded species were less sensitive to exposure to light compare d to the smaller seeded species. The largest seeded species tested, Pa raserianthes lophantha, germinated best in the warmer incubation tempe rature and in the light. The ecological significance of the tests woul d be that species which have seed dormancy mechanisms capable of delay ing germination until the cool temperature, winter rainy period of thi s mediterranean-type climate would be more likely to survive than if g ermination followed summer rain showers or the first, intermittent rai ns of autumn. Burial of seeds becomes more important if germination oc curs when rains first begin as this period has less available soil moi sture and temperatures are high. Also survival of seedlings could be e nhanced if germination of seed was restricted to the positions protect ed from high light, higher temperatures and lower soil moisture by the presence of a forest canopy. Therefore, seeds which have an ability t o sense the presence of a previous fire in the habitat, conditions in light environment and appropriate temperature level have an adaptive a dvantage to time emergence to situations of time and space where survi val is maximized. Variation in viability and germination percentages w ere apparent in some cases where more than one seed collection of avai lable for testing, indicating that further aspects, such as seed age, maturity at collection, storage conditions and depth of seed dormancy, remain to be considered.