Why do seeds germinate in spring? Surprisingly, the answer to this apparently simple question is still not fully understood, even in a plant family as extensive and as agronomically important as the legumes. The seeds of the legumes, in common with those of many other species (Box 1), have a mechanism of physical dormancy, with water-impermeable seed (or fruit) coats. Intriguingly, however, there is a water gap in the impermeable layer and it is this structure which formed the focus of research into dormancy-break in the work of Van Asshe et al. described in this issue (pp. 315–323). With water gaps also present in so many other species, this has very wide implications.
Table Box 1 . Physical dormancy
Species with water-impermeable seed (or fruit) coats – physical dormancy – occur in the following plant families (sensu APG, 1998; Baskin et al., 2000):
• Convolvulaceae (including Cuscutaceae)
• Dipterocarpaceae (subfamilies Montoideae and Pakaraimoideae, but not Dipterocarpoideae)
• Fabaceae (subfamilies Caesalpinioideae, Mimosoideae and Papilionoideae)
• Malvaceae (including Bombacacaceae, Sterculiaceae, and Tiliaceae)
Species with water-impermeable seed (or fruit) coats – physical dormancy – occur in some 15 plant families (sensuAPG, 1998), including the Fabaceae (subfamilies Caesalpinioideae, Mimosoideae, and Papilionoideae). This impermeability of the coat is caused by the presence of one or more palisade layers of lignified malphigian cells (macrosclereids) tightly packed together and impregnated with water-repellant chemicals (Rolston, 1978; Werker, 1980–81). An anatomical structure in the impermeable layer(s) functions as the ‘water gap’, seven types of which have been described (Baskin et al., 2000; water gaps have so far not been described in three families with physical dormancy – the Curcurbitaceae, Rhamnaceae, and Sapindaceae). In legumes the water-gap is the lens (Fig. 1).
How to open the water gap
Water gaps are closed at seed maturity (Fig. 1b), and then open in response to an appropriate environmental signal. The water gap is dislodged, or in the case of the lens the macrosclereids pull apart (Fig. 1c), thereby creating an entry point for water into the seed. Once open, water gaps cannot close. Since opening of the water gap is necessary for seeds with physical dormancy to germinate, this event indirectly controls when germination will occur. Thus, understanding how timing of germination of seeds with physical dormancy is controlled in nature means determining the environmental conditions required for the water gap to open.
As seeds with other kinds of dormancy have very specific temperature requirements for dormancy-break (Baskin & Baskin, 1998), it is logical that temperature should also be an important factor in breaking physical dormancy, and this certainly appears to be the case. For example, high and highly fluctuating temperatures promote dormancy break in impermeable seeds of Stylosanthes humilis and S. hamata (Fabaceae) during the hot, dry season in northern Australia (McKeon & Mott, 1982). Similarly a 15°C difference in amplitude of daily temperature fluctuations in an opening (gap) in a tropical rain forest in Vera Cruz, Mexico, promoted germination (67%) of seeds of Heliocarpus donnell-smithii (Malvaceae). Only 25% of the seeds germinated under the forest canopy, where the daily temperature fluctuation was 5°C (Vazquez-Yanes & Orozco-Segovia, 1982). Furthermore, many species whose seeds have physical dormancy appear after fire in the habitat (e.g. Iliamna spp. (Malvaceae)), and exposure of their seeds to temperatures of ≥ 70°C result in high germination percentages (Baskin & Baskin, 1997).
New insights into breaking physical dormancy
While temperature appears so important for breaking dormancy, the responses of seeds with physical dormancy to natural temperature regimes are not well understood. For example, how/why do seeds of some legumes in the temperate zone germinate only in spring? In a 2-yr germination phenology study on 14 herbaceous species of legumes, Van Assche et al. identified six species that germinated mainly in spring, and determined the temperatures required to break physical dormancy in their seeds.
Fresh seeds of five of the six legumes mostly did not germinate at constant (5, 10, 23, 30°C) or at alternating (20/10°C) temperatures. Furthermore, few seeds of five of the species germinated when they were kept on moist filter paper at 5°C for 2 months (simulating winter) and were then transferred to 23°C. In the sixth species, Trifolium pratense, germination ranged from 16 to 28%, regardless of treatment or test condition. In an experiment on five of the spring-germinating species, seeds of four of them chilled at 5°C for 2 months germinated to higher percentages at alternating (15/6, 20/10°C) than at constant (10, 23°C) temperatures, and in three species more seeds germinated at 15/6 than at 20/10°C. Seeds of Trifolium pratense germinated equally well at 10, 15/6, and 20/10°C. Significantly, Van Assche et al. showed that most seeds remained impermeable at 5°C and became permeable only after being subjected to 15/6 and/or 20/10°C, or to 10°C for T. pratense.
In another experiment, buried seeds of the 14 species were exposed to natural temperature regimes in Belgium, and at regular intervals for up to 28 months samples of each species were exhumed and tested for germination at constant (23°C) and at alternating (15/6, 20/10, 30/20°C) temperature regimes. The six spring-germinating species exhibited a peak of germination when exhumed in spring, but little or no germination occurred at other times of the year. The temperature regime simulating early spring (15/6°C) was optimal for germination for five of the six species, with 15/6 and 20/10°C being equally suitable for germination of the sixth. Depending on the species, little, or no, germination occurred at the constant temperature, regardless of the time of year seeds were tested.
Water gaps as environmental signal detectors
Why was seed germination of the six species restricted to spring? In general, germination of exhumed seeds in spring decreased with an increase in the alternating temperature regime; thus, germination in the field in summer is prevented by high temperatures. However, this does not explain why seeds exhumed in summer and tested at 15/6 and 20/10°C failed to germinate. In detailed studies on Melilotus albus seeds, Van Assche et al. showed that seeds failed to germinate if they were chilled at 5°C and then held at 20°C for 1 month before being transferred to 15/6°C. However, seeds of M. albus chilled at 5°C for 2 months and then moved directly to 15/6°C germinated to high percentages.
Thus, germination in spring requires that seeds be subjected to a sequence of two temperature regimes:
2Low alternating temperatures.
If the two temperature regimes are separated by a period of relatively high constant temperature, seeds lose the ability to respond to low alternating spring temperatures.
If seeds of these legumes are buried in soil and not exposed to alternating spring temperatures at the end of winter, they lose their ability to respond to low alternating temperatures. Therefore, although temperatures in late autumn are approx. 15/6°C, seeds would not be capable of responding to them and thus do not become permeable in autumn. Any seeds that fail to germinate in spring would be prevented from doing so until some following spring, after they are chilled and subsequently exposed to low alternating temperatures. Van Assche et al. found that 26–92% of the seeds of the six species were impermeable after 2.5 years of burial under natural temperature regimes.
Under natural temperature regimes, buried seeds of the six legumes did not cycle between physical dormancy and nondormancy, but there was cycling with regard to ability of seeds to respond to the second phase (i.e. low alternating temperatures) of the dormancy-breaking requirement. These discoveries help explain how timing of germination of seeds with physical dormancy is controlled in nature in temperate zones. As in other species whose seeds have physical dormancy, the conditions required for opening of the water gap ‘fine-tune’ germination of the species to the habitat. In the spring-germinating legumes, the two-step temperature requirements for opening of the water gap allow this special anatomical structure to act as a signal detector not only for the arrival of spring, but also for the depth of seeds in the soil.