Fate of annual pasture legumes seeds on a two-way thermogradient plate

Abstract

Effects of diurnally alternating temperatures (5/5–45/45°C) were examined on a two-way thermogradient plate for non-dormant seeds of 14 annual pasture legumes. Seed fates (germination, temperature induced non-viability and temperature induced dormancy) were determined from daily seed counts over a 14-day period on-plate and a further 14 days after removal from the plate. These data clearly demonstrate the existence and extent of seed fates other than germination. Maximum dormancy occurred over a broad range of temperatures for seeds of 'Trifolium subterraneum L.' var. 'subterraneum' (Katzn. et Morley) Zohary and Heller cv. Woogenellup (40/35–30/25°C), Astragalus hamosus L. cv. Ioman (40/25–30/5°C) and 'Trifolium hirtum' All. cv. Hykon (35/30–30/5°C). In contrast, maximum dormancy occurred over a narrow temperature range for 'Medicago truncatula' Gaertn. var. 'truncatula' cv. Paraggio (35/30–30/25°C), M. aculeata Willd. var. inermis (Aschers.) Heyn (30/25 and 30/20°C), and M. minima L. (35/20–35/5°C) seeds. Generally, non-viability was highest in all legumes when temperatures were greater than 35/30°C and not significantly different from the maximum value (100%) at temperatures greater than 40/30°C. At temperatures less than 35/30°C non-viability was less than the maximum (P<0.05) for seeds of all legumes, except 'M. truncatula' cvs. Jemalong and Sephi, 'M. aculeata', and Vicia villosa ssp. 'dasycarpa' (Ten.) Cav. cv. Namoi. Compared with the other subterranean clover cultivars germination levels T. subterraneum var. 'brachycalycinum' (Katzn. et Morley) Zohary and Heller cv. Clare seeds were not significantly different from the maximum value at higher temperatures. In all annual medics, except M. aculeata germination was not significantly different to the maximum at temperatures greater than 25/20°C, with high germination (P<0.05) occurring at 30/25°C in Jemalong and 'M. scutellata' (L.) Mill cv. Sava and at 30/20°C in Paraggio, Sephi, 'M. minima' and 'M. polymorpha' L. Maximum germination in Ioman and Hykon seeds occurred over a broader temperature range (35/20–15/10C and 30/20–15/5°C, respectively) than in Namoi (30/15C and 25/20–25/5°C). Probable implications of these data for field emergence of non-dormant seeds and soil seed reserves are discussed.