Environmental conditions for seed germination
Before seeds can germinate, they need to encounter a favourable set of environmental conditions. Seeds need water, a suitable temperature and sometimes light to germinate. Most seeds also need oxygen.
The first phase of water uptake during germination is a purely physical process. All dry seeds, whether alive or dead, will absorb moisture. This process of rapid water uptake is called imbibition. Some seeds might have an impermeable seed coat which needs to be ruptured before they can imbibe water. There is little or no further uptake of water until the first visible sign of germination, when the root emerges from the seed. After this point, the seedling usually develops rapidly and this is accompanied by a further sharp increase in water uptake.
Imbibition is an essential step in seed germination, but seeds can ‘drown’ if there is too much water and some seeds can be damaged if imbibition is too fast. Commercial growers usually germinate seeds in loam-free compost. Other materials such as sand or vermiculite that offer good water-holding properties but are free-draining are ideal for raising plants to the seedling stage. In seed testing laboratories, absorbent paper is commonly used. In this case seeds are simply sown on the wetted surface and then either enclosed in air-tight boxes or the paper is rolled and then sealed in polythene bags. At the MSB, plain water agar is usually used for germination testing.
Most seeds germinate equally well in the light or dark. Although many people believe that seeds germinate best in the dark, this is only the case for some species from very dry habitats. Many plants actually require light for germination and this is particularly true for small-seeded annuals.
Requiring light to germinate gives seeds two advantages:
- It prevents the germination of seeds that are deeply buried in the soil. Seedlings from deeply buried seeds are likely to use up their food reserves and die before they reach the soil surface.
- It prevents seeds from germinating when they are shaded by surrounding plants that would compete for sunlight.
Even light-requiring seeds may be stopped from germination if they are exposed to continuous high intensity light. For this reason, the Millennium Seed Bank uses incubators with low intensity cool-white fluorescent tubes that are switched on for 8 or 12 hours per day.
All seeds have a minimum and maximum temperature at which they will germinate. The optimum temperature for a particular species is the temperature that enables all seeds to germinate in the shortest time. Many seeds only germinate if they experience daily temperature shifts. This would normally occur on or near the soil surface, as the soil warms up during the day and cools at night. This is a common trait amongst small-seeded species that also require light for germination.
Like the requirement for light, the need for alternating temperatures acts to prevent the germination of deeply buried seeds. Deeply buried seeds under more or less constant temperature conditions will not germinate at all. The MSB uses alternating temperature incubators for germination testing. 25/10°C (8 hours / 16 hours) is often used for temperate seeds and 35/20°C for tropical species.
Seeds of some aquatic plants are able to germinate without oxygen, but most seeds germinate best when oxygen is available. Usually, germination is reduced if carbon dioxide is raised above normal atmospheric levels or if oxygen is depleted.
Nitrates and smoke are naturally occurring chemical factors that are also important environmental triggers for germination in some species.
- Baskin, C. C. and Baskin, J. M. (1998) Seeds: Ecology, Biogeography and Evolution of Dormancy and Germination. Academic Press.
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