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Temperature conditions associated with the autumn softening of hard seeds of burr medic

G.B. Taylor

CSIRO Division of Animal Production, Private Bag, Wembley WA 6014

Patterns of seed softening during summer and autumn can have an important bearing on the extent of seed losses from annual legumes following false breaks of season. Observations of a significant proportion of burr medic,Medicago polymorpha cv. Serena seeds softening in April have been pursued in field and associated laboratory studies.

Methods

Newly ripened burrs of burr medic cvv. Serena and Santiago, barrel medic,M. truncatula cv. Cyprus, and subterranean clover,Trifolium subterraneum cv. Dalkeith were collected in November 1989 from replicated plots at Merredin, WA. Seeds were tested for permeability and then subjected to either a diurnal temperature fluctuation of 60/15°C, or to a constant temperature of 35°C for 24 weeks before re-testing for permeability. Seeds obtained from burr samples collected from the field on 1 March, 26 April and 24 May 1990 were also tested for permeability. Residual hard seeds from both field and laboratory were subjected to five diurnal cycles of 35/10°C and re-tested for permeability.

Results and discussion

Seed softening in the 1989/90 dry season was completed by 26 April (Table 1). Seeds of Serena and Cyprus softened most rapidly during the sampling interval between 1 March and 26 April when their proportions of soft seeds approximately doubled. Relative increase in soft seeds of Santiago and Dalkeith during this period was less. Treatment of residual hard seeds from the 1 March sampling at 35/10°C markedly increased the proportion of soft seeds in Serena and, to a lesser extent, Santiago but had negligible effect on residual hard seeds of other cultivars. Laboratory treatment at 60/15°C or 35°C, with brief supplementary treatment at 35/10°C, resulted in similar proportions of soft seeds as those obtained in the April field sampling.

Table 1. Soft seeds (%) from field and laboratory treatments. Mean s.e. 1.3.

Autumn seed softening in burr medics appears to be associated with soil surface temperatures in the vicinity of 35/10°C. Selection of such genotypes to avoid some of the germinable seed losses caused by false breaks could be worthwhile. Development of a laboratory technique for simulating field softening by combining treatment at 60/15°C, or 35°C, with supplementary treatment at 35/10°C appears feasible.

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