Low Moisture Content Limits to Relations Between Seed Longevity and Moisture

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Abstract
Discontinuities at low moisture contents in the otherwise logarithmic relations between seed longevity and seed moisture content (%, f. wt basis) in hermetic storage at 65 °C were detected at 2–0% in groundnut (Arachis hypogaea L.), 3·5% in onion (Allium cepa L.), 4·5% in sugar beet (Beta vulgaris L.), 4·6% in barley (Hordeum vulgare L.), 5·3% in chickpea (Cicer arietinum L.) and wheat (Triticum aestinum L.), and 5·6% in cowpea [Vigna unguiculata (L.) Walp.]. In contrast, the equilibrium relative humidity of seeds at these values was similar, varying between 9·9% (onion and sugar beet) and 11·5% (wheat). The mean value was 10·5% r.h. (s.e. 0.2). There was no significant (P > 0·05) effect of further reduction in seed moisture content below these critical values on longevity, except in wheat (P < 0·005), in which there was a further increase in longevity. In soya bean [Glycine max (L) Merrill], the logarithmic relation continued down to the lowest moisture content investigated, 3·3% (11·4% equilibrium relative humidity). Above the critical value, seed longevity in groundnut showed the least sensitivity to increase in percentage moisture content, while barley showed the greatest, the values of the viability constant Cw (slope of the negative logarithmic relation between longevity and moisture) being 4·089 (s.e. 0·278) and 5·966 (s.e. 0·325), respectively. These differences in the value of Cw among the eight crops were significant P < 0·005), whereas the relative sensitivity of seed longevity to change in equilibrium relative humidity above the critical moisture content did not differ significantly among the eight (P > 0·10) and was equivalent to a doubling of longevity for each 8·7% reduction in equilibrium relative humidity. Accordingly it is concluded that the relative effect of water potential on seed longevity can be considered to be the same for these and probably also for many other orthodox species.