Factors Affecting Prolonged Spore Viability in Herbarium Collections of Three Species of Pellaea

Abstract

Among lower vascular plants, the establishment of new individuals and populations is largely dependent on the transport of spores by prevailing winds (Tryon, 1970; Wagner, 1972). Since long distance dispersal may require prolonged exposure of spores to unfavorable conditions, spore longevity has a major impact on species migration and distribution. The degree to which spores remain viable for long periods also determines the applicability of many biosystematic techniques to herbarium specimens (Windham & Haufler, 1986). Although an understanding of spore longevity is thus critical to investigations of several biological phenomena, little is known about the factors affecting prolonged spore viability in ferns. Previous studies have shown that the spores of various pteridophyte species differ greatly in their ability to germinate after storage at ambient temperatures (Dyer, 1979). Two major groups of ferns have been identified on the basis of spore longevity (Lloyd & Klekowski, 1970): those with green spores containing active chlorophyll (mean viability of 48 days), and those with non-chlorophyllous spores (mean viability of 2.8 years). Most studies of the factors affecting spore longevity have compared these two groups. For example, Lloyd & Klekowski (1970) suggested that high water content, elevated respiratory rates, and the absence of a desiccation-resistant spore wall might account for the short viability of chlorophyllous spores. Since viability declines rapidly in Equisetum spores even when they are protected from desiccation (Hauke, 1963), the intensity of respiratory activity seems to be the limiting factor in this case. Although these studies may explain the lack of long-term viability in green spores, they shed little light on factors controlling longevity in the majority of fern taxa with nongreen spores. Depending on the species, non-chlorophyllous fern spores remain viable for periods ranging from several months (Lloyd & Klekowski, 1970) to nearly 100 years (Johnson, 1985). Both temperature and humidity are known to influence longevity (Okada, 1929), but a variety of other factors may also be involved. Although differences in respiratory rate seem relatively minor in this group (Okada, 1929), they may prove significant when calculated in terms of years or decades. Thus, ploidy level could be an important factor because polyploids tend to show lower respiratory rates (Levin, 1983). In addition, the lower surface/ volume ratios of polyploid spores would reduce the relative exposure of cytoplasm to the unfavorable environment surrounding the spore. For herbarium specimens traditionally used in spore longevity studies, this unfavorable environment may include a variety of physical and chemical treatments used for preservation and insect control. All of these factors have the potential to affect