Water – liquid and vapour phases

Water has a primary role in sustaining tissue activity and viability of both the parent plant and the developing caryopsis. The natural process of seed maturation involves a dehydration of the caryopsis during the final stages that culminates in abscission. Water stress on the intact plant can interact with temperature and influence both the rate of development and level of dormancy attained by the caryopsis. In its new environment, after abscission, the dormant seed may be exposed to a range of varying environmental factors, particularly moisture and temperature. For example, if the seed remains on the soil surface it will be exposed to diurnal cycles of desiccation and hydration according to the level of radiation from the sun and increased relative humidity at night. Alternatively seeds buried in soil can be exposed for long periods of time to excessive moisture or dryness. Variations in available moisture influence the persistence of dormancy. Excess of water can induce secondary dormancy in many grass species. Some optimum amount of water and suitable temperature will ultimately determine the time for germination. Seed coat structures can modify any of the above moisture variations by limiting ingress of water to particular rates or amounts (Chapter 2).

Interactions of moisture with temperature, controlling dormancy and germination, can occur in several ways. Temperature affects the relative humidity of the atmosphere.