Ecophysiology of dormancy: Response to the environment and dormancy cycling in the seedbank
In species adapted to regions of seasonal drought and dry soils, physiological changes recorded during dry after-ripening storage may reflect a natural mechanism, which can control the annual germination timing in the wild (Finch-Savage and Leubner-Metzger, 2006). In addition, there can be an annual rhythm of germination, which in Mesembryanthemum nodiflorum, a desert plant of the Aizoaceae (Caryophyllid clade), is retained after more than 30 years of dry storage. When seeds are in the imbibed state, dormancy may be broken by exposure to high summer temperatures.
In temperate regions, seeds may cycle through different depths of dormancy in the seed bank for prolonged periods, but may also persist in a state that only requires light to terminate dormancy and induce germination. Dormancy is therefore a moving target continuously reacting to the environment and adjusting the conditions required for germination. Induction and breaking of non-deep PD in the variable natural environment is therefore a continuous, and often gradual, process where the temperature range (window) permissive for germination, and/or sensitivity to light and nitrate etc., widens then narrows on a continuous scale. This conditional dormancy is recognised in the classification of five types of non-deep PD.
Whether the window opens from low to high or high to low temperatures (type 1 and 2, respectively) not only determines the classification type but also the environment to which it is adapted. The former is adapted to germination in the autumn (winter annual) and latter to germination in the spring (summer annual); however, these changes in permissive temperatures and rate of germination with loss of dormancy will occur at different rates in individual seeds within the population.
Some species may also have seeds that are physiologically heterogeneous. Thus, as the ambient temperatures continue to change secondary dormancy can be induced to close the window and prevent inappropriate late germination of seeds that are slower to loose dormancy. In some cases, the mechanism can be further complicated as diurnal temperature variation (alternating temperatures) is required to terminate dormancy and promote germination and this may be further influenced by other environmental factors, particularly light. Both these signals may, for example, indicate a position close enough to the soil surface to enable seedling emergence. Fluctuations in soil water content can also affect the dormancy status of seed banks. In all cases, the completion of germination of the non-dormant seed is dependant of the availability of sufficient moisture. In dryer climates water may also fulfil other roles such as diluting salinity, which would otherwise inhibit germination. |
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