Ecophysiological effects of light quality and nitrate on seed germination in species from Western Australia

Germination occurs usually in response to multiple environmental cues. Seed s with the ecophysiological ability to simultaneously sense the previous pr esence of fire and appropriate levels of temperature, light and soil nitrat e could restrict germination to postfire, winter and competition-free micro habitats, where the potential for seedling survival is enhanced. Germinatio n responses of 16 species with a range of life forms, fire responses and se ed weights were determined under controlled conditions of 15 degrees C temp erature, a 12 h light cycle, exposure to I g L-1 nitrate solution, and six conditions of light quality (white, blue, yellow, red, far-red light and da rkness). Germination in Oenothera stricta, a weedy naturalized ephemeral, a nd two small-seeded indigenous Asteraceae species of mulga woodlands, Leuco chrysum fitzgibbonii and Craspedia sp., were enhanced by white, yellow or r ed light compared with germination achieved in the dark, or under far-red o r blue light. In red light, KNO3 further enhanced germination of these posi tively photoblastic species. The germination response of Trachyandra divari cata, a naturalized herb of sandy, seaside locations, and several native ja rrah forest legumes (four Acacia species, Bossiaea aquifolium, Gompholobium marginatum and Sphaerolobium vimineum) proved to be negatively photoblasti c. Of these seven negatively photoblastic herb and shrub species, exposure to KNO3 overcame the inhibition of light only in the resprouter species, Ac acia lateriticola. In the serotinous, negatively photoblastic tree species, Corymbia calophylla and Eucalyptus marginata, KNO3 seemed to be required b efore the negative response to light exposure was recorded. A dose-curve ex periment on two positively photoblastic and three negatively photoblastic s pecies indicated that although KNO3 exposure affected germination in all sp ecies, different concentrations of KNO3 (0, 0.5, 1, 2, and 5 g L-1) produce d different levels of response. Detailed studies with additions of KNO3 (1 g L-1) and the growth hormone, gibberellic acid (GA(3); 50 mg L-1), showed that increased germination percentages of the positively photoblastic speci es, Oenothera stricta, occurred in the light, but blocking endogenous gibbe rellic synthesis with paclobutrazol, or adding exogenous GA(3) or KNO3 had no effect on the light-induced germination levels. In the negatively photob lastic species Trachyandra divaricata, additions of KNO3 and GA(3) had no i nfluence on the germination inhibition induced by exposure to light nor did blocking endogenous GA synthesis. The 16 species growing naturally in West ern Australia, Australia show a range of germination responses to environme ntal conditions, but depending on their natural habitat, the ecophysiology of each species appears to be optimized for subsequent seedling survival.