The effect of temperature on the hardseed content of some annual legumes grown on the northern slopes of New South Wales

Abstract

Two laboratory experiments were conducted to assess the effects of constant and fluctuating temperatures on the hardseed content of a range of legume species. In the first experiment the effects of constant (10/10,25/25,40/40¦C) and fluctuating temperatures (40/10,40/25¦C) on the rate of breakdown of the hardseed of 15 legumes was examined over a 12-month period. The second experiment investigated the hardseed level of seven of these legumes stored at temperatures of 25/25,40/25,60/25¦C over 11 months. A method of analysing these data is presented which describes the hardseed profile over time and allows the rates of change to be compared. For seed collected in a summer rainfall environment fluctuating temperatures of 40/10¦C significantly reduced the hardseed level of Medicago aculeata Willd. and M. scutellata (L.) Mill, cv. Sava, Trifolium subterraneum ssp. subterraneum Katzn. et Morley cv. Woogenellup and T. subterraneum ssp brachycalycinum Katzn. Et Morley cv. Clare. A 40/25¦C regime reduced the hardseededness of M. minima (L.) Bart, M, aculeata, M. truncatula Gaertn. cv. Sephi, T. subterraneum ssp, subterraneum Katzn. et Morley cvv. Nungarin and Woogenellup and cv. Clare. At 60/25¦C the hardseed level of all legumes declined over time. Further at 60/25¦C hardseed contents declined at a significantly faster rate than for those at 40/25¦C. Field observations of the number of seedlings emerging in the summer after seed set followed a similar pattern to that expected from the laboratory studies. From these studies it is proposed that, provided their seed set is adequate, cultivars with comparatively low levels of hardseed may perform better than expected when grown in a summer rainfall environment, particularly when they are either grown in association with native grasses or have a high proportion of buried burrs. Under these conditions temperatures of around 40/25¦C could be expected.