Species Survival Mechanisms

For seeds of some species of temperate climate, a period of exposure to low temperature in a moist substrate is sufficient to promote the necessary change in hormonal balance. This treatment is known as cold stratification, as the seeds are placed in layers in the moistened substrate. For example, seeds from temperate species such as peach, apple, pear, plum, persimmon, Acer spp. and Pinus spp. they must be stratified in a moist substrate, at 5°C (refrigerator or cold room), for periods of 30 to 90 days, depending on the genotype, so that dormancy is overcome. Each cultivar, depending on its origin and, mainly, its genetic characteristics, requires an optimal period of stratification. Seeds of other species, such as forage grasses, require sudden changes in temperature or thermal shock so that physiological dormancy is overcome; in this case, daytime temperatures above 30°C and nighttime below 20°C are the most suitable.

Dormancy can also be caused by inhibitory chemical compounds present in different structures of the seed which, when translocated to the embryo, inhibit its growth. Since most of these compounds are water soluble, it is easy to imagine how this form of dormancy is overcome in nature: rainwater or snow melt leaches such compounds. Thus, washing seeds in running water, for a certain period of time, is an efficient method to overcome the dormancy of seeds that have this mechanism, such as beetroot, rose, pequi, and some species of peppers.

In several species, such as yerba mate, araticum, peach, apple and plum, there is morphological dormancy, that is, the seeds are dispersed with the morphologically immature embryo. For the seed to germinate it is necessary a certain period of time, varying with the species, until the complete development of the embryo. This dormancy mechanism is also known as embryo immaturity or rudimentary embryo. Seed stratification is the most suitable method to promote embryo development. For yerba mate seeds, stratification in wet sand for 150 days is recommended. In species of the Rosaceae family, such as peaches, plums, apples, in addition to morphological dormancy, physiological dormancy is also present, that is, after the embryo is fully developed, it will still not germinate due to the physiological blockage caused by growth-inhibiting hormones; in this case, cold stratification (30 to 90 days) is also recommended, which will promote the maturing of the embryo and modify its hormonal balance.

In nature, each dormancy mechanism is overcome by different agents. For example, acids from soil organic matter and/or those from the digestive tract of seed dispersing animals contribute to making the seed envelope permeable to water; the heat caused by fire or by the opening of a clearing in the forest can also act in this direction. The cold, characteristic of a severe winter, can cause physiological changes in the seed, unblocking the embryo’s growth. Inhibitor compounds present in the seeds are washed away by rain or thaw water.

Seeds located in the soil at an inadequate depth, if they were not equipped with germination blocking mechanisms, when germinating, their reserves would be consumed before the seedling reached the soil surface. Only when they are in a situation where red radiation predominates (under direct sunlight or located 2-3 cm deep in the soil), will the positive photoblastic seeds germinate, as such radiation determines changes in the embryo’s metabolism, directing it towards development .

Dormancy has an important ecological significance, giving seeds resistance to ingestion by animals, heat, cold, fire and other agents and interfering in the dynamics of natural populations, as it is related to the adaptation of plants to the heterogeneity of different ecosystems, allowing the survival of plant species and ensuring that open areas are colonized quickly. The regeneration of plant communities from seeds depends, to a large extent, on the seed’s ability to “recognise” whether the environment in which it finds itself is favorable to the survival of its offspring. Thus, the process of ecological succession, which is how vegetation and forests regenerate, only occurs thanks to the capacity of seeds of different species and different successional stages “wait” for the right occasion to germinate.