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 Main subject - july/aug 2005
continuation: Seed Dormancy
Dormancy duration varies very much among the species. Some species, specially vegetables and grasses for example, have a short duration dormancy (usually from three to six months), so that the time interval between seed harvest and sowing is enough to have dormancy ceased when planting. In this case, the farmer does not need to use treatments to overpass dormancy before sowing because it has been naturally overpassed with storage. However, for some temperate grasses, the establishment of pastures is difficult by the marked dormancy of the seeds. It is the case for Brachiaria dyctioneura, cv. Llanero, whose seeds may show dormancy until one year after their harvest, causing problems during pasture establishment.
The immersion of grasses seeds in concentrated sulphuric acid has been considered efficient for dormancy reduction. This kind of scarification has been applied, in commercial scale, later.
On the other hand, in seeds from given species such as arboreous vegetables, grasses or used as green fertilizer and some fruit trees from temperate climate (peach, persimmon, apple, pear, plum), dormancy can be more marked during a year or more, several times making necessary the use of specific treatments before sowing in order to suit the stand. Such treatments aim at standardizing seed germination and seedling emergence, avoiding failures in the final stand.
The knowledge of dormancy causes or mechanisms helps both in defining the need to use a specific treatment to overpass them or not, and defining the more efficient method for each specie.
In order to make the comprehension easy, it can be basically considered that dormancy has three mechanisms, namely:
1) physical dormancy, related to seed coat permeability to water;
2) physiological dormancy, related to physiological processes that block embryo growth; and
3) morphological dormancy, related to embryo immaturity.
Some species show a water-impermeable coat due to the presence of lignin, suberin and other compounds, being necessary to break it off or make it more permeable, what can be achieved by means of scarification treatments with the use of sandpaper, thick sand, immersion in hot water or abrasive chemicals (acids as sulphuric and chloridric, or bases). There physical-nature mechanism is typical of some families, such as leguminous family, whose seeds often have to be submitted to specific treatments before sowing in order to overpass dormancy.
In physiological dormancy, the embryo, although physically structured and complete, does not germinate due to reasons such as unsuitable hormonal balance, coat impermeability to gas exchange (oxygen and/or carbonic gas) or the presence of inhibitor chemical compounds. Overpassing this dormancy involves hormonal modifications at the embryo, i. e., both inhibitors content reduction and germination-promoting phitohormones synthesis. Thus, methods that act preventing the action of the first ones, or increase promoters content are the most recommended. Direct seed soaking in GA solution can be efficient for some species.
For some seeds from temperate climates, a period of exposition to low temperatures in humid substrate is enough to promote the change needed in the hormonal balance. Such treatment is known as cold stratification because the seeds are put in layers in the humid substrate. For example, mild climate species seeds such as peach, apple, pear, plum, persimmon, Acer spp. and Pinus spp. must be stratified in humid substrate at 5°C (freezer or cool chamber) for periods of 30 over 90 days, depending on genotype, so that dormancy can be overpassed. Each cultivar, depending on its origin, and mainly on their characteristics, requires an optimum stratification period. Seeds of other species, such as grasses, require sudden temperature shifts or thermal shock to defeat physiological dormancy; in this instance, daytime temperatures higher than 30°C and nighttime temperatures under 20°C, are the most indicated.
Dry warm weather or temperatures relatively high (over 35°C) also help overpassing dormancy in some species from rainforest, Savannas and cereals (rice, wheat), being recommended a pre-drying treatment with air flow at 35-40°C for 5 to 7 days.
Dormancy can be also caused by inhibitor chemical compounds present in different structures of the seed that, when translocated to the embryo, inhibit its growth. As most of those compounds are water-soluble ones, it is easy to wonder how this dormancy is overpassed in nature: rain water or snow melting, leaches such compounds. So, seeds wash in current water during a given period, is an efficient method for overpassing dormancy of seeds that present this mechanism, as beet roots, rose, pequi, some pepper species, Caatinga species.
Tissues around the embryo or the tissues of the embryo itself, can restrict the intake of O2 and the release of CO2, interfering on seed respiration and consequently blocking embryo growth. Such gas impermeability is ascribed to phenolic compounds existing in the seed wrap, which retain O2, thus reducing the availability of this gas to the embryo. This mechanism occur in cereal seeds (rice, oats, barley) as well as in fodder grasses. For its surpass, a chemical scarification treatment (H2SO4) is used in laboratory before performing the germination testing in fodder grasses seeds for partial or total removal of the "coat" (palled and lemma) which is the structure responsible for O2 retention. Such method, however, is not indicated to every specie presenting this kind of dormancy. For example, for rice it is recommended seeds immersion in water or sodium hypochlorite before performing the germination testing.
In many species, as maté/Paraguay tea, araticum, peach, apple, and plum morphological dormancy occurs, that is, seeds are dispersed with a morphologically immature embryo. In order to let the seed germinate it is needed a certain period of time that varies according to the species, until the complete development of the embryo. Such dormancy mechanism is also known as embryo immaturity or rudimentary embryo. Seeds stratification is the most indicated method for promoting embryo development. For maté/ Paraguay tea seeds it is recommended the stratification in humid sand for 150 days. In species from Rosaceae family as peach, plum, and apple, besides morphological dormancy physiological dormancy is also present, that is, after the complete development of the embryo, it will not germinate yet due to the physiological obstenction caused by growth inhibitor hormones; in this case cold stratification (30 to 90 days) is also recommended, which will promote embryo maturation and its hormonal balance shift.
In nature each dormancy mechanism is overpassed by different agents. For example, soil organic matter acids and/or the ones from animals digestive trait that spread seeds contribute to make seed wrap permeable to water; the heating caused by fire or clearings opened in the forest can also act in this sense. Cold, characteristic of a rigorous winter, can cause physical shifts in the seed, unblocking embryo growth. Inhibitor compounds present in the seeds are washed by rain water or snow melting.
Seeds located on the soil at an unsuitable depth, if not endowed with germination blocking mechanisms, would have their reserves consumed before the plantlet could reach soil surface, during their germination.
Final Comment
Dormancy has an important ecological meaning, providing seeds with resistance to animals ingestion, heat, cold, fire as well as to the remaining agents, while interferes in the dynamics of natural populations, since it relates to plants adaptation to heterogeneity of the different ecosystems, allowing plant seeds survival and assuring open areas to be quickly colonized. Plant communities regeneration from seeds depends, at a great extent, on the ability the seed has to recognize whether the media in which it is favors its descent survival. Thus, ecological succession process, that is the way through which both vegetation and forests get regenerated, only takes place due to the capacity of the seeds from different species and the different successional stadia that wait the proper occasion to germinate.
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