Viability of Lettuce Seeds

Abstract

Seeds which show orthodox storage characteristics conform to a common pattern of survival over a wide range of storage conditions which can be described by a single equation. Two aspects of this are that in a constant environment the life-spans of individual seeds in a population are normally distributed, and that there is negative linear relationship between log moisture content and log life-span. However, when orthodox seeds are fully hydrated they survive much longer than would be predicted by extrapolating from lower moisture contents; but the moisture content at which the change occurs has not previously been investigated. In this paper the viability of lettuce seeds was examined after various periods of hermetic storage at different moisture contents, temperatures and initial partial pressures of oxygen. At moisture contents below 15% the pattern of survival is typical of other orthodox seeds but above this value the responses change in four ways: instead of being deleterious, oxygen becomes beneficial to survival; instead of life-spans being normally distributed, they become skewed; the relative effect of temperature on decreasing longevity is slightly diminished; and the decrease in śeurvival period with increase in moisture content begins to become less marked, so that ultimately, above 20% to 30% moisture content, there is no further decrease in longevity. These results, which indicate substantial physiological changes at about 15% moisture content, are discussed in relation to the hypothesis, postulated by Villiers, that repair and turnover mechanisms are absent from dry seeds but are activated on hydration.