The biological mechanism of seeding and seed priming technology
An article that provides a detailed introduction to the common knowledge of flowering, focusing on the biological mechanism of seeding and seed priming technology. Next, the editor will introduce this to netizens.
Seeds are developed from fertilized ovules and are detachable organs that can survive after leaving the mother plant. The strict survival cycle of plants starts from the division of the fertilized egg into an embryo, but it is currently customary to consider seeds as the starting point of the plant's survival cycle. Therefore, seeding naturally becomes the main method for people to propagate plants.
From seed to plant, it first goes through the germination stage. The physiological activity of air-dried seeds is extremely weak and is basically in a static state, known as the dormant state. When seeds absorb sufficient water and are under suitable conditions, viable seeds can germinate. Germination can be described and defined from three disciplinary perspectives:
Morphological perspective: Germination is the process in which viable seeds absorb water, the embryo grows and breaks through the seed coat boundary, and forms a seedling. It is usually considered that the radicle breaking through the seed coat is a sign of germination.
Physiological perspective: Germination is the process in which dormant or dormant seeds absorb water and transition from a relatively static state to a physiological activity state, respiration increases, stored substances are decomposed and transformed into substances that can be used by the embryo, causing the embryo to grow.
Biochemistry and molecular biology perspective: The essence of germination is that factors such as water and temperature cause the expression of certain genes in seeds and the activation of enzymes, leading to a series of biochemical reactions related to embryo growth.
Seed germination requires three stages, which are:
Stage 1 - Imbibition period: This stage mainly relies on the absorption of water by the colloidal components in the seed, transforming the original gel-like substances into sol-like substances. At this time, the chains of many macromolecules can expand, and the cells of the seed return to their original structure and function. It is worth noting that the water absorption in this period is unrelated to whether the seed is viable, that is, dead seeds can also experience the imbibition period, showing seed swelling.
Stage 2 - Slow water absorption period: After the water absorption in the first stage, the hydration level of the protoplast in the seed has tended to be saturated; the increase in cell turgor pressure and the restriction of the seed coat hinder further water absorption. At this point, the seed water absorption shows a transition from rapid to slow absorption. During this stage, various enzyme proteins in the seed cells resume activity, dormant DNA resumes activity, and begin to transcribe messenger RNA. At the same time, a large number of enzyme-catalyzed reactions are initiated, expressing various proteins. In the meantime, the seed embryo cells begin to synthesize and secrete gibberellin (GA), promoting the decomposition of endosperm to provide raw materials for biochemical reactions in the embryo cells. As a large amount of endosperm decomposition material enters the embryo cells, the osmotic pressure of the embryo cell fluid rises, which accelerates water absorption, so the cells enter the third stage of germination.
Stage 3 - Growth water absorption period: Due to the acceleration of cell metabolism and the rise of osmotic pressure in the embryo cells, the water absorption rate accelerates progressively from the slow absorption in the second stage. At this time, the nucleic acids, proteins, and other vital substances in the radicle and plumule have a vigorous metabolism, and the growth of the embryo cells causes visible germination in the appearance of the seed coat. When the radicle breaks through the seed coat, aerobic respiration strengthens, and the growth of new organs is rapid, showing a continuous increase in seed osmotic water absorption and fresh weight.
Throughout the germination process, many mechanisms are involved in regulation, and it is currently believed that gibberellin levels play a major role: in the second stage of germination, the embryo cells begin to secrete gibberellin, which promotes the decomposition of the endosperm and is transported into the embryo cells. At this point, the endosperm acts as a nutritional "source," with small-molecule substances from decomposition transported into the cytoplasm of the embryo cells to form a "pool," which then provides raw materials for numerous biochemical reactions. The resolution of substances in the "source" determines the amount of small-molecule substances in the "pool"; the amount of stored substances in the "pool" affects the synthesis of gibberellin, which in turn affects the decomposition of substances in the "source." The "source" and "pool" interact and coordinate to ensure that nutrients are effectively utilized by cells, which is the famous "source-pool" theory. This theory has important guiding significance for the biological utilization of endosperm and seed priming.
The conditions for seed germination are mainly water, temperature, gas composition, and light: water is the most important factor in germination, as the degree and state of seed water absorption directly lead to germination rate and the health of seedlings. Temperature and other factors also have a significant impact on seed water absorption; within a certain range, the higher the temperature, the faster the water absorption and germination. However, excessive temperature can easily lead to the overproliferation of microorganisms, affecting germination, so the temperature is generally controlled within a moderate range, with most succulents preferring to germinate at 25~35 degrees Celsius. The amount of salt substances in water also determines the effect of water absorption; too much salt can prevent seeds from effectively absorbing water, while also inhibiting cell respiration and even causing seed decay. Oxygen is also an essential germination condition for seeds; a low-oxygen gas environment will cause seed cells to suffocate and die, so increasing the oxygen content (greater than 10%) is beneficial for seed germination. Light factors mainly determine the synthesis and secretion of gibberellin, which in turn determines the effective utilization of endosperm and ultimately the health of seedlings. Therefore, moderate light is conducive to seed germination and seedling health. The exogenous use of gibberellin can replace the light required in the second and third stages of germination to some extent, that is, it can germinate in the dark, but the seedlings still need light after germination to effectively mobilize photosynthesis.
Seed priming technology is based on the above biological mechanism of seed germination and aims to promote seed germination, improve the stability and uniformity of germination time, reduce the standard deviation of germination time, and improve the resistance and quality of seedlings, as well as their nutritional status. Priming mainly achieves this through osmotic regulation, temperature regulation, gas regulation, and hormone regulation.
Based on osmotic regulation, liquid priming controls the speed and degree of water supply to cells to control the speed and uniformity of seed germination. It mainly considers using high colloidal osmotic pressure solutions or appropriate crystal osmotic pressure salt solutions to regulate seed water absorption. The commonly used chemical is PEG, a high molecular polymer with high chemical stability that does not pass through the cell wall, thus not affecting cell biochemical reactions. PEG solutions regulate the degree and state of cell water absorption through colloidal osmotic potential, allowing seed water absorption to tend to be stable and synchronized, ultimately improving germination rate and uniformity, which is much more effective than using a single water浸泡 priming. Salt solution priming is also commonly used, for example, calcium chloride, potassium nitrate, and other substances can change the water potential of the solution to reduce cell osmotic potential, making cell water absorption more stable, and these salt molecules can also enter cells, having a positive effect on metabolic activity.
Solid matrix priming is a relatively common seed priming method, which mainly relies on the seed absorbing water to reach equilibrium from the solid-phase carrier to achieve priming. The commonly used matrices are vermiculite, porous clay, synthetic calcium silicate, and filter paper.
Use of antimicrobial agents: The impact of microorganisms on seed germination is significant, with some pathogenic microorganisms inhibiting seed germination and also harming newly emerged seedlings, causing decay. At the same time, the reproduction of microorganisms is also conducive to the breeding of pests, consuming seedlings. Therefore, antimicrobial treatment is a key part of seed priming, with the primary measure being the disinfection of seeding-related items and seed disinfection. Some oxidative chemicals such as hydrogen peroxide, potassium permanganate, and peroxyacetic acid have significant antibacterial effects and can also abrade the seed coat and provide some oxygen for seed germination. Hydrogen peroxide is a chemical substance with no residue after decomposition, so it is a preferred chemical reagent for seed priming, commonly used at a concentration of 0.1~1%, soaking seeds for 1~24 hours, which can greatly promote seed germination. In addition, the "PEG combined with antibiotics" priming seeds studied in recent years can also achieve good results, usually involving adding broad-spectrum antibiotics such as tetracycline, penicillin+streptomycin, sulfonamides, and macrolides to a certain concentration of PEG solution. Antifungal agents are also essential and are usually added to the water during the germination process, with carbendazim and metalaxyl being the preferred options. The seed of the succulent plant Baobab is a typical type of seed sensitive to fungi, rich in lipids that favor fungal growth. Effective use of antifungal substances is necessary to improve its germination rate.
Application of hormones: Gibberellin can promote the decomposition of endosperm and acts on the second to third stages of seed germination, making it an indispensable hormone for seed germination. The exogenous use of gibberellin can break seed dormancy, promote germination, and improve seed resistance, usually at a concentration of 10~100ppm for spraying. Auxins can promote cell elongation and radicle development, while cytokinins can promote embryo cell division and plumule development. These two hormones can also promote seed germination and can be appropriately used, usually selecting 1-Naphthylacetic acid (NAA) 10ppm, 6-Benzylaminopurine (6BA) or Kinetin (KT) 10~50ppm for spraying.
Application of cell signaling molecules: This field has been deeply studied and gradually applied in practice in recent years, focusing on the biochemical study of salicylic acid. Salicylic acid is an important biomarker molecule that, along with jasmonic acid and abscisic acid, constitutes the three major signaling molecules of the plant cell transduction system. Its main function is to induce the expression of resistance genes in cells, improving the resistance and quality of seedlings. The appropriate use of salicylates or acetylsalicylic acid (aspirin) in the third stage of germination can effectively improve the quality of seedlings, commonly used at a concentration of aspirin 10~50ppm for spraying during the seedling stage.
Application of cell cycle drugs: Due to the differences between seeds and seed coats, seed germination is often uneven, which is disadvantageous for unified control and seedling raising. How to improve the consistency of seed germination is one of the important issues to be solved by seed priming. Various priming techniques can improve consistency to some extent; if further improvement in consistency or uniformity is needed, chemical drugs can be used for control. The preferred substances are cell cycle controllers, such as "hydroxyurea," a commonly used drug for the treatment of chronic myeloid leukemia. Usually, soaking seeds with 100ppm can significantly improve the uniformity of germination. Unfortunately, this substance is a chemical mutagen, and improper control of concentration and time can cause chromosomal abnormalities or gene mutations.
Biological priming: This field is currently at the forefront, using harmless microorganisms as a coating on the surface of seeds. By utilizing the antagonistic effects between microorganisms, it inhibits the harm caused by pathogenic microorganisms. At the same time, these beneficial microorganisms can provide effective nutrition and water for seeds, promoting germination and improving the resistance of seedlings. A successful example is using a 1.5% methylcellulose solution containing fluorescent pseudomonas to coat seeds, shrinking water back for 2 hours after coating, and absorbing water for 20 minutes at 23 degrees Celsius before sowing. Seeds primed in this way show almost no diseases or lodging.
There are many physiological and biochemical changes during the seed priming process, and these changes together achieve the priming effect. It is worth noting that artificial seed priming can not only improve the germination rate but also improve the uniformity and resistance, making seedlings strong and laying a good foundation for later growth and development. For enthusiasts, using seed priming substances appropriately on the basis of correct seeding is beneficial for breeding. Currently, there is a vacancy in the market for seed priming products, which requires enthusiasts to understand some knowledge of seed germination and select and adjust seed priming agents themselves. Admittedly, seed priming is a "double-edged sword" and is also a practice-based on experience. Beginners should be cautious when using seed priming substances.
The above () provides you with a detailed introduction to the biological mechanism of seeding and seed priming technology for reference by netizens.