Seed priming is a process of regulating the germination process by managing the temperature and seed moisture content, the seed is taken through the first biochemical processes within the initial stages of germination.
The priming process regulates the seed’s temperature and moisture content, bringing the seed closer to the point of germination. The process involves advancing the seed to an equal stage of the germination process, to enable fast and uniform emergence when planted.
Seed priming is a pre-sowing treatment which leads to a physiological state that enables seed to germinate more efficiently. The majority of seed treatments are based on seed imbibition allowing the seeds to go through the first reversible stage of germination but do not allow radical protrusion through the seed coat. Seeds keeping their desiccation tolerance are then dehydrated and can be stored until final sowing. During subsequent germination, primed seeds exhibit a faster and more synchronized germination and young seedlings are often more vigorous and resistant to abiotic stresses than seedlings obtained from unprimed seeds. Priming often involves soaking seed in predetermined amounts of water or limitation of the imbibition time. The imbibition rate could be somehow controlled by osmotic agents such as PEG and referred to as osmopriming. Halopriming implies the use of specific salts while “hormopriming” relies on the use of plant growth regulators. Some physical treatments (UV, cold or heat…) also provide germination improvement thus suggesting that priming effects are not necessarily related to seed imbibition. A better understanding of the metabolic events taking place during the priming treatment and the subsequent germination should help to use this simple and cheap technology in a more efficient way.
Several methods of seed priming have been developed in order to invigorate seeds and alleviate the environmental stresses. A common feature of water-based priming techniques, which distinguishes them from other pre-sowing treatments, is partial seed pre-hydration and the activation of early germination events in seed. Priming efficiency is affected by many factors and strongly depends on treated plant species and chosen priming technique. Physical and chemical factors such as osmotica and water potential, priming agent, duration, temperature, presence or absence of light, aeration, and seed condition also influence priming success and determine germination rate and time, seedling vigor, and further plant development
Hydropriming is the simplest method of seed priming, which relies on seed soaking in pure water and re-drying to original moisture content prior to sowing. No use of additional chemical substances as a priming agent makes this method low-cost and environmentally friendly. The main disadvantage of hydropriming is uncontrolled water uptake by seeds. This is a consequence of free water availability to seeds during hydropriming, so that the rate of water uptake depends only on seed tissue affinity to water
One of the commercially used types of hydropriming is the system named “drum priming”, patented in the early 1990s. In this technique, seeds are gently rotating in drum and gradually hydrated by addition of water in vapor form. Drum priming allows seed imbibition in a controlled manner and could be an attractive alternative to conventional hydropriming. Specially designed apparatus enables monitoring of the seed weight, precise regulation of time, and water amount during hydration process, what ultimately results in an appropriate and uniform moisture level of the seeds
Osmopriming involves soaking seeds in an osmotic solution with low water potential instead of pure water. Due to low water potential of osmotic solutions, water enters seed slowly which allows gradual seed imbibition and activation of early phases of germination but prevents radicle protrusion. Usually, water potential of the priming agent varies from −1.0 down to −2.0 MPa . However, values of water potential together with duration of the priming treatment should be always adjusted to species, cultivar, and sometimes seed lot. Different compounds are used in osmopriming procedure including polyethylene glycol (PEG), mannitol, sorbitol, glycerol, and inorganic salts such as NaCl, KCl, KNO3, K3PO4, KH2PO4, MgSO4, and CaCl2. Priming with salt solutions is often referred to as “halopriming”. Most common chemical employed in osmopriming treatment is PEG, mainly owing to its specific characteristic. Large molecular size of PEG prevents its penetration into the seed thus avoiding induction of potential cytotoxic effect and reduction of osmotic potential within seed
Solid matrix priming
Solid matrix priming (SMP, matriconditioning), in which water uptake by seeds is controlled, has been developed as an alternative method to osmopriming because of the high cost of osmotic agents and technical problems with aeration. During solid matrix priming, seeds are mixed and incubated with a wet solid water carrier for a certain period. Afterward, seeds are separated from the matrix, rinsed, and back-dried. The use of solid medium allows seeds to hydrate slowly and simulates natural imbibition process occurring in the soil. To successfully accomplish SMP, materials utilized as matrices should possess specific physical and chemical features such as low matrix potential, minimal water solubility, high water holding capacity and surface area, no toxicity to seeds, and ability to adhere to seed surface. In fact, vermiculite, peat moss, charcoal, sand, clay, and some commercially offered substrate such as Celie or Micro Cell are exemplary solid carriers applied in solid matrix priming. In order to obtain the best priming performance, time of treatment and optimal water content must be determined separately for each matrix
During hormopriming, seed imbibition occurs in the presence of plant growth regulators, which can have direct impact on seed metabolism. The following regulators are commonly used for hormopriming: abscisic acid, auxins, gibberellins, kinetin, ethylene, polyamines, and salicylic acid (SA). Gibberellic acid (GA3) and PEG priming improved photosynthetic properties, antioxidant system, seedling emergence, and growth of white clover on heavy metal polluted soil
Biopriming involves seed imbibition together with bacterial inoculation of seed. As another priming method, this treatment increases rate and uniformity of germination, but additionally protects seeds against the soil and seed-borne pathogens. Hydration of seeds infected with pathogens during priming can result in a stronger microbial growth and consequently impairment of plant health. However, applying antagonistic microorganisms during priming is an ecological approach to overcome this problem. Moreover, some bacteria used as biocontrol agents are able to colonize the rhizosphere and support plants in both direct and indirect ways after the germination stage. It was found that biopriming is a much more effective approach to disease management than other techniques such as pelleting and film coating. Nowadays, the use of biopriming with plant growth-promoting bacteria (PGPB) as an integral component of agricultural practice shows great promise. In pearl millet, biopriming with Pseudomonas fluorescens isolates enhanced plant growth and resistance against downy mildew diseases. Biopriming with rhizobacteria improved germination parameters of radish seeds under saline conditions.
Chemical priming refers to seed treatment with different chemical solutions used as priming agents. This approach includes priming with wide range of both natural and synthetic compounds such as antioxidants (ascorbic acid, glutathione, tocopherol, melatonin, and proline), hydrogen peroxide, sodium nitroprusside, urea, thiourea, mannose, selenium, chitosan, fungicide etc. Positive impact of chemical priming with various priming agents in a wide range of environmental conditions was indicated by numerous studies. Seed priming with β-amino butyric acid increased drought and salt tolerance of green gram. Application of ascorbic acid as a seed priming agent induced drought and salt resistance of wheat.
Nutripriming is a technique in which seeds are soaked with solutions containing the limiting nutrient instead of pure water. The idea of this method is to obtain nutritional effect together with biochemical advantages of priming in order to improve seed quality, germination parameters, and seedling establishment. Seed priming with Zn improved productivity of chickpea and wheat, germination and early seedling growth of rice, development and root growth of maize seedling exposed to low root zone temperatures, while K-priming brought favorable effect on growth and nutrient status of cotton seedling under saline conditions. Some nutripriming techniques are commonly used by seed companies in the process of seed production and preparation for growers.
Seed priming and agriculture;
Pre-sowing priming induces a particular physiological status in seeds and has emerged as a promising strategy to improve plant behavior in the field. There is a strong interest for farmers and seed companies to find suitable cheap priming treatments but also to precisely identify the agronomical properties improved as a result of priming in cultivated species.