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A Rapid Technique for Measuring Breakdown Rate of Hardseededness in Subterranean Clover

USDA-ARS Crop Sci. Res. Lab., Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762

^* Corresponding author.

An impermeable seedcoat or hardseededness is the most important germination regulation mechanism in subterranean clover (Trifolium subterraneum L.). Hardseededness breakdown rate has been evaluated by exposing hard seed to fluctuating temperature (60/15 °C) for 6 mo. Our objective was to compare four methods for evaluating breakdown rates of hardseededness from four subterranean clover cultivars and to determine if the Weibull model accurately described the loss of hardseededness with time. The Weibull model accurately described the loss of hardseededness with time of 24 legume genotypes with diverse softening rates from data reported in the literature. Hard seed from greenhouse-grown plants of ‘Nungarin’, ‘Meteora’, ‘Trikkala’, and ‘Mount Barker’ were tested for hardseededness breakdown rate in 1988 and 1989. Hardseededness breakdown rate was determined by using diurnal fluctuating temperature methods of 60/15 °C in Australia or 47/25 °C in the USA and constant temperature method of 50 or 60 °C followed by 7 d of 60/15 °C fluctuating temperature. The Weibull model accurately described the loss of hardseededness with time of all treatments (S_y.x = 4.35). Meteora was the slowest to soften followed by Nungarin, Mt. Barker, and Trikkala. The U.S. temperature treatment softened seed the slowest followed by Australia's, 50 and 60 °C. All temperature methods ranked cultivars in the same order for hardseededness breakdown rate. Variation among cultivar softening rate was measured in 112 fewer days by the constant temperature methods compared to the fluctuating temperature methods.

Received for publication February 18, 1992.