A comparison of the low-moisture-content limit to the logarithmic relation between seed moisture and longevity in twelve species

Discontinuities in the relation between seed longevity and seed storage moisture content (% f.wt basis) were detected at low moisture contents in the Compositae, Cruciferae, Gramineae and Leguminosac. In all 12 species investigated, relations between seed longevity and moisture content above a critical value were logarithmic, the value of the viability constant Cw (relative sensitivity of longevity to moisture) varying significantly (P < 0.025) amongst the species, from 4.5 in rape (Brassica napus) to 6.3 in mung bean (Vigna radiata). Ten of the 12 species showed no significant variation in longevity with further reduction in moisture content below a critical value, but in rice (Oryza saliva) and mung bean significant (P < 0.05), though only slight, negative and positive logarithmic relations, respectively, were detected between longevity and moisture contents below the critical value. The critical seed moisture content varied between 2.4% in niger (Guizotia abyssinica) and 6.2% in pea (Pisum sativum). Comparison between two cultivars of timothy (Phleum pratense) showed estimates of Cw and the critical moisture content to be invariant within the species. Variation in critical moisture content amongst species was correlated (P < 0.01) with variation in the value of Cw. The equilibrium relative humidity, and thus the water potential of the seeds at their various critical moisture contents, was similar, mean 10.6% r.h. (s.e. 0.3). Accordingly, interspecific variation in the values of Cw and critical moisture content is ascribed to differences in seed composition. Moreover, in each species the critical moisture content coincided with the inflection point of the isotherm between seed moisture and equilibrium relative humidity. It follows that variation in longevity with variation in seed moisture content above the critical value results from variation in the weakly bound fraction of the water held within the seeds, while it appears that removal of the tightly bound water fraction does not influence the rate of seed deterioration. Above the critical moisture content we detected no significant difference (P > 0.25) in the relative sensitivity of seed longevity to change in equilibrium relative humidity amongst the 12 species, this being equivalent to a doubling of longevity for each 8.4% reduction in equilibrium relative humidity, but further work is required to corroborate this tentative conclusion.