The seeds also “sleep”

Some seeds of certain plants of economic value and of many wild plants, considered viable, do not always germinate when placed in favorable environmental conditions. They have a persistent rest period and are called dormant. When seeds have normal intrinsic conditions and remain at rest due to the absence of favorable environmental conditions, they are said to be quiescent.

Apparently, dormancy evolved as a species survival mechanism for certain climatic conditions. For example, wild plants that develop in the desert only germinate when subjected to a sufficient amount of water to remove some chemical inhibitors from their integuments: for seedling growth or plants from temperate climates, they will only germinate at an appropriate time so that the period of winter does not coincide with a more vulnerable stage (seedling phase).

The drastic reduction in physiological activities integrated in the dormancy process is commonly related to the development of external protective tissues and a reduction in the hydration of the cytoplasm: as a result, dormant seeds are much more resistant to unfavorable environmental conditions and, therefore, more efficient for the perpetuation of the species, delaying germination and distributing it over time.

Seeds can remain viable for prolonged periods of time in a dormant state, and can thus survive for several years, decades and even centuries under favorable conditions. Pioneer species such as samaúma, embaúba, pau balsa, wild cotton and paricá germinate quickly after the formation of clearings, due to the increase in light.

Most cultivated plants such as corn, wheat and beans, due to genetic selection and improvement processes, do not have prolonged dormancy: they germinate immediately when they are sown, which is an important agricultural advantage.

Several theories have been studied to explain the mechanism of dormancy in seeds, however, as it presents peculiarities for different species, it is difficult to generalize about its causes. One of the theories is that of Khan, who proposed that seed dormancy would be controlled by growth-regulating substances, with gibberellins being responsible for promoting germination. Cytokinins would have a "permissive" action or nullify the effect of inhibitory substances, but without promoting germination. The inhibitors would have the function of preventing germination. Dormancy would result not only from an excess of inhibitors, but also from the absence of gibberellins (in the absence of inhibitors), or the absence of cytokinins (in the presence of inhibitors).

More recent work, for example, with Arabidopsis thaliana mutants advocates the existence of genes controlling dormancy induction (RDO1 and RDO2) that are linked to the level of abscisic acid and gibberellin. The role of small heat shock proteins (sHSPs) on Arabidopsis seed dormancy has also been hypothesized. The mechanism of regulation of dormancy and germination through phytochrome, in positive photoblastic seeds, is not yet well understood. Recent studies have demonstrated that this process is regulated by the expression of genes that encode hydroxylases that catalyze the final biosynthesis of gibberellins.

It is important that the mechanisms of dormancy are well known in order to find appropriate ways to overcome it. Many factors necessary for the germination of some species are exactly the same as those necessary to overcome dormancy, such as the need for light in positive photoblastic seeds. Furthermore, more than one type of dormancy may be present in a given seed. The essence of unraveling the mechanisms of dormancy lies in the ability to distinguish cause and effect.

Ana da Silva Ledo

Researcher at Embrapa Tabuleiros Costeiros, Sergipe, specialist in Biotechnology/Fruitculture