Do Plants Derived from Seeds That Readily Germinate Differ from Plants Derived from Seeds That Require Forcing to Germinate? A Case Study of the Desert Mustard Lesquerella Fendleri
Cabin, Robert J., Evans, Ann S., Mitchell, Randall J., The American Midland Naturalist
ROBERT J. CABIN1,2, ANN S. EVANS AND RANDALL J. MITCHELL3 Department of Biology, University of New Mexico, Albuquerque 87131-1091
ABSTRACT.-We compared the performance of plants of the desert mustard Lesquerella fendleri derived from seeds that readily germinated ("natural" plants) with plants originating from seeds forced to germinate by the application of gibberellic acid which required an extended germination period ("induced" plants). Before transplanting from the greenhouse, induced plants were significantly larger in diameter but had significantly fewer leaves than natural plants. There were also significant differences between seed source populations, as well as seed source by germination treatment interactions, for both plant diameter and number of leaves. After transplantation to a desert shrubland site, there were highly significant differences in survivorship of natural and induced plants. Five months after transplantation, survival of natural plants (43.3%) was twice that of induced plants (21.3%). Natural plants transplanted beneath creosote bush shrubs were also larger in diameter than induced plants, while the converse was true for plants transplanted in the open intershrub areas. We argue that these results may be, at least partially, the result of genetic differences between seeds that readily germinate and seeds that remain dormant but viable under the same environmental conditions.
In many environments, within-year seed dormancy delays germination until a time which favors successful germination and subsequent plant establishment and reproduction. The time that a seed germinates within this favorable period often is the single most important variable in explaining variation in subsequent plant performance (Weiner, 1988). Many studies have shown that seeds that germinate early gain a competitive advantage over relatively late-germinating seeds (e.g., Ross and Harper, 1972; Howell, 1981; Waller, 1985; Firbank and Watkinson, 1987; Miller, 1987). However, seeds that germinate too early in the season face the risk of unpredictable and often fatal environmental conditions (e.g., frosts, droughts and temperature extremes). Thus a trade-off exists between delaying germination until a stable favorable period, and initiating germination soon enough within a favorable period to gain a competitive advantage over later germinating seeds (Silvertown, 1988).
In addition to this within-year dormancy difference, seeds of many species also exhibit between-year seed dormancy, in which a fraction of the seeds within a given environment remain dormant throughout an entire season in which other conspecific seeds germinate. This type of dormancy has led to the creation of persistent soil seed banks in most of the world's major ecosystems (reviewed by Leck et al., 1989; Thompson, 1992). Numerous theoretical models have also suggested that this between-year dormancy may balance the risk of local extinction from germination in unfavorable years with the risk of missing good years by remaining dormant (e.g., Cohen, 1966; MacArthur, 1977; Venable and Lawlor,1980; Brown and Venable, 1986; Venable and Brown, 1988).
Both within- and between-year seed dormancy are often broken by a particular combination of specific environmental cues [e.g., temperature, light and soil moisture; see reviews in Mayer and Poljakoff-Mayber (1975) and Baskin and Baskin (1989)]. Seeds of some species may have evolved the ability to "predict" favorable establishment periods by using dormancy-breaking environmental cues that are also correlated with subsequent conditions favorable for their growth and reproduction. Such seeds may thus be able to germinate in particular years and seasons within years that best match their particular ecophysiology; that is, over evolutionary time, germination and postgermination traits may have adaptively coevolved (Evans and Cabin, 1995). For example, in relatively dry years, individuals with more xerophytic traits (e. …