Seed priming is a pre-sowing treatment in which seeds are soaked in an osmotic solution that allows them to imbibe water and go through the first stage of germination.
The three types of seed priming used for treating seeds are: 1. Osmo Priming (Osmo Conditioning) 2. Hydro Priming (Drum Priming) 3. Solid Matrix Priming (Solid Matrix Conditioning).
1. Osmo Priming:
Seed are incubated in well aerated solutions with a low water potential, and afterward washed and dried. The low water potential of solution can be achieved by adding osmotic like mannitol, polyethylene glycol (PEG, a chemically inert compound), or salt like KCl, KNO3, K3PO4.
Osmopriming gives better results in many vegetable crops (reported greater germination of pepper seeds, after they were soaked in a solution containing NaNO3, MnSO4 and MgCl2). Ells (1963) that reported use of K3PO4 and KNO3 solution improved the germination rate of tomato at low temperatures (below 20°C). He also coined the term “vigorization” where he suggested that the vigour of seeds has been improved by the priming process.
Problems of Osmopriming:
i. Problem with aeration of solution when priming large seeded species or large quantities of seed. Because considerable volume of solution is required per seed.
ii. Cost of osmotic such as PEG is high.
iii. Osmotica must be removed from the seeds before they are dried otherwise it may affect initial seedlings.
2. Hydro Priming:
Seed are hydrated to predetermined water content over a one day period, by placing them inside a horizontal rotating drum into which water vapours are released. The drum is monitored for seed water content and controlled the production of water vapours. Hydrated seeds are kept in this drum for further 14 days before being sown or dried to facilitate storage.
Drum primed seeds germinate faster and more uniformly than seed primed by osmopriming. e.g. drum primed seeds of 10 cultivars of leek, germinated faster and more uniformly than seeds primed for the same period by placing them on paper saturated with a solution of polyethylene glycol (PEG).
3. Solid Matrix Priming (SMP):
It is the incubation of seeds in a solid insoluble matrix vermiculite, cross linked high polymers, with a limited amount of water for a prescribed period of time. The moisture content of the mixture is brought to a level just below that required for radical protrusion.
Ideal Characteristics of Soild Matrix Priming Agent:
i. No Phyto-toxicity
ii. Higher water holding capacity
iii. Remain friable at different moisture content.
iv. Easy to remove from seeds after treatments.
Beneficial Effects of SMP:
i. When cucumber seeds during SMP mixed with growth regulator and fungicide (Metalaxyl and Thiabendazole) improvement takes place in germination percentage and rate.
ii. SMP treatment reduced the time between sowing and emergence of tomato, onion, and carrot seeds compared to nonprime seeds.
iii. SMP minimize aeration problem and facilitates the incorporation at other types of product into mix.
Factors Affecting the Seed Priming:
i. Aeration
ii. Light
iii. Duration of priming process
iv. Temperature
v. Osmotic potential
vii. Dehydration after priming
viii. Seed storage.
Several factors must be controlled during the priming treatment in order to improve seed performance. Heydecker et al. (1975) demonstrated that proper aeration of the osmotic solution was essential to living seeds and helped synchronize seed germination.
Onion seeds primed in an PEG solution, using enriched oxygen, had greater percent germination after treatment compared to seeds primed in non-aerated solutions. Aeration of the solution, however, modified the optimal duration of priming. Aeration of priming solution lessened soaking time to achieve greater germination percentage at 35°C.
The beneficial effects of seed priming could be modified by light quality during treatment. When seeds of endive were primed in KNO3 solution under continuous red light, they had greater germination percentage and more uniform germination than seeds primed in the dark.
Seed priming in K3PO4 solution improved germination of ‘Valmaine’ lettuce at high temperature regardless of light conditions. Celery seeds primed in a solution of PEG+BA had more germination at high temperature when the priming treatment was done under light than in the dark.
iii. Duration of Priming Process:
The ideal duration of priming process varies according to type of osmotic, osmotic potential of solution, temperature during the treatment and species. If radical protrusion occurs during seed priming, irreversible embryo damage could be expected during dehydration after priming. In tomato, one week of priming in PEG 6000 solution is ideal seed priming period to increase germination rate. Spinach seeds required 14 days of priming to improve emergence compared to nonprime seeds.
iv. Temperature:
Duration of seed priming depends on the species and seed quality. However, temperature during treatment and osmotic potential of solution affect the length of soaking period. If the soak temperature is maintained below the optimum range for rapid germination, radical growth during priming may be restricted.
At 25°C a high percentage (63%) of carrot seeds germinated in priming solution (-0.5 MPa PEG) whereas no germination was observed with solution at 15°C. Temperature during priming may alter the soaking period in many species. Seed priming for 14 days at 15°C was more effective in improving seed germination of tomato, carrot and onion than 14 days at 25°C.
Osmotic potential of solution is another factor affecting the length and effectiveness of priming treatment. Seed priming in high osmotic potential solutions may allow rapid seed germination and radical growth because seed water contents become too great. Tomato seed germinated more rapidly when osmotic potential of the solution was -0.58 or -0.86MPa compared to -1.19 or -1.49 MPa.
vi. Seed Quality:
A differential response to seed priming in seed lots of the same species has been reported by numerous authors. Seed vigour is another factor influencing the responses to seed priming. Priming did not alleviate thermo-dormancy of artificially aged seeds of ‘Great Lakes’ and ‘Montello’ lettuce, but was effective for non-aged seeds. High vigour and seeds free from pathogen are both essential prerequisites for good priming results.
vii. Dehydration after Priming:
To achieve the maximum potential of priming, strict control of the entire previous factor is required. However, following priming, dehydration and subsequent seed storage become crucial components of this technology for commercial application. To facilitate handling and storage, the seed should be dried to an acceptable moisture level for long term storage, after the priming treatment.
Water uptake during priming is influenced by osmotic potential of the solution, type of osmoticum, duration of treatment and the physical/chemical characteristics of seeds. Dehydration after priming involve removal of a large quantity of moisture from the seed. Although temperature and rate of dehydration following priming are both crucial factors affecting seed quality of primed seeds.
Tomato seeds dried at 30°C after priming but RH was not specified. Cabbage seed dried to their original moisture content at 7°C and 45% RH, onion and watermelon seeds dried to original fresh weight at 15°C. Lettuce dried at 21°C and 40% RH.
After seed priming and dehydration, seeds usually are stored before planting. The conditions and duration of storage is important. Good seed storage facility is therefore, a basic requirement for primed seeds.
The effect of storage conditions after priming has been studied; however, more information about seed dehydration conditions after priming and their potential effects on seed behaviour are limited.
In commercial vegetable farming desired plant population and a high probability of successful establishment is important. The most essential phases of precision planting is the singling of seeds for exact placement depth and spacing in the soil with a uniform coverage and at reasonable planting speed. Singling is difficult for small seeds which are low in density and irregular shape. To overcome these difficulties, seed pelleting and coating are advocated.
Seed pelleting is the process of enclosing a seed into a small quantity of inert materials just large enough to produce a globular unit of standard size to facilitate precision planting. The inert material creates natural water holding media and provides small amount of nutrients to young seedlings.
Seed pelleting is also mechanism of applying needed materials in such a way that affect the seed or soil at the seed- soil interface. Thus, seed pelleting provides an opportunity to package, effective quantities of materials such that they can influence the micro-environment of each seed.
In South Asia, practicing of seed pelleting is limited, neither commercial nor government seed organizations has realized its importance. Whereas, in developed countries, it is very common and most of the seeds are sold with coating only. However, now attention has been invited on the importance of seed pelleting.