Keywords

Reserve mobilization; Chemical composition; Growth seedlings

Short Communication

Germination is a key process in plant metabolism responsible for embryo growth and development into a complete plant [1]. From the physiological point of view, germination comprises four phases [2]:

• Water imbibition;

• Cell stretching;

• Cell division and;

• Cell differentiation into tissues.

Knowledge about seed biology and germination process of each species is fundamental to understanding the establishment of a plant community, as well as its survival and natural regeneration [3]. When considering the germination process of a seed, knowledge about the mechanisms related to seed dormancy assumes a relevant role [4]. On the one hand, dormancy has an ecological function, since it constitutes a survival mechanism of the species, ensuring its viability until the environmental conditions are adequate for the establishment and growth of the seedling [5]. On the other hand, it is an impediment to the early germination, damaging the production of seedlings on a large scale [2].

The dormancy is usually associated with intrinsic factors related to the seed itself, such as hardness and impermeability of the integument to water and gases, immature embryos, inhibitors and abiotic factors such as temperature, light, humidity and substrate [6].

To identify the method used to break dormancy, it is essential to identify the triggers [7]. The temperature has been considered as one of the main responsible for both the germination speed and the final percentage of germination, as it affects especially the water absorption rate, reactivate the metabolic reactions, fundamental to the processes of reserve mobilization and to the seedling growth [8]. Seeds of many species require daily fluctuation of temperature to germinate properly. Although this requirement is associated with seed dormancy, temperature alternation may accelerate germination in non-dormant seeds [7].

Another factor that has been studied is the light, which exerts great influence on the germination of seeds, being the embryo responsible for the perception and translation of the luminous stimulus [5]. Many cultivated species are indifferent to light to germinate, however, the light stimulus is quite variable in seeds of various wild species, there are species whose seeds are positively or negatively affected, and seeds that are not affected by light [9].

The knowledge of the chemical composition of the seeds is also of fundamental importance from a physiological point of view and considering the pre and postharvest management practices, since the accumulated reserves are responsible for the supply of nutrients and energy necessary for the full demonstrating the vital functions of the seeds, besides directly affecting the storage potential and determining the targeting of procedures adopted during post-harvest artificial drying [4]. Therefore, variations in chemical composition are related to seed performance, including during the induction and dormancy exceedance stages [10].

It is important to emphasize that oilseeds have a lower storage potential than amylaceae, due to the lower chemical stability of lipids in relation to starch [1]. The high protein content may also contribute to the reduction of seed storage potential due to the high affinity of this substance to water [10]. In oilseeds, the main endosperm reserve is lipid, which is in form of Triacylglycerol (TAG) stored in organelles called of lipid bodies or oleosomes [11]. The TAG present in the lipid bodies are initially cleaved by lipases, release fatty acids into glyoxysomes and are subsequently degraded by the ßoxidation enzymes, producing acetyl-CoA [12]. Acetyl-CoA is converted into sucrose through the glyoxylate cycle and gluconeogenesis, where they have two key enzymes: malate synthase (Msy) and isocitrate lyase (ICL), both act on the lipid metabolism stored in oilseeds. Activity these enzymes increases during germination, obtaining maximum values when the highest proportion of degraded lipids occurs and in the sucrose synthesis, which is transported to the embryonic axis and serves as energy and carbon support for root growth [3-11].

Despite the importance of lipid catabolism in glyoxysomes during germination, this process is responsible for the potential production of reactive oxygen species - ROS [13]. The amount of ROS is closely regulated by the balance between production and elimination, playing a dual role in the seed physiology: by one side, they behave as cellular signals and may even act as a break in the dormancy of orthodox seeds [14]. On the other hand, they can accumulate as toxic products under stress conditions interfering with cellular homeostasis [12].

The obtaining of this information plays a fundamental role in the development of correct protocols and reliable analyzes, since they base the basic procedures for the drying and the storage of the seeds.

References

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