The Longevity of Seeds

Abstract

INTRODUCTION Ewart (1908) and Harrington (1972) provided summary lists of species with seeds that were short-lived, survived for moderately long periods, or survived for very long periods. Time is only one of several variables of concern, however; seed longevity is influenced markedly by the storage environment. Ambient environments in tropical regions are generally poorer for seed storage than those of temperate regions, and so the maintenance of viability during storage is generally a greater problem in tropical regions. However, there is no need to distinguish among crop species with regard to region of production or origin when considering the physiology of seed survival. The major difference transcends these areas; it is a consequence of different responses to desiccation. Roberts (1973) classified species into two categories with regard to seed storage behavior: orthodox and recalcitrant. Orthodox seeds tolerate considerable desiccation, and storage longevity is greatly increased thereby (Roberts, 1973). Similarly, considerable increase in orthodox seed longevity results from reduction in storage temperature, particularly since the seeds can be stored at subzero temperatures once they have been dried to a sufficiently low moisture content. These aspects of the physiology of orthodox seeds provide seedsmen with considerable scope to enhance storage life by environmental modification. If these responses are quantified, then the benefits to seed longevity can be assessed against the costs of modifying seed storage environments. In contrast, recalcitrant seeds do not survive desiccation (Roberts, 1973; Chin, 1988). In addition, the storage temperature cannot be reduced greatly because very moist seeds are damaged by cooling to subzero (and in some cases higher) temperatures. Thus, the scope for modifying seed storage environment is limited, and so it is difficult to improve the storage life of recalcitrant seeds beyond comparatively short periods (Ring and Roberts, 1979; Roberts et al., 1984). Fortunately, most crops have orthodox seeds (Ring and Roberts, 1979; Roberts, 1973; Roberts et al., 1984), including most