Seed Priming

Seeds require water, oxygen and a suitable temperature for germination. Water uptake follows a three-phase pattern with an initial rapid uptake or imbibition (phase 1), followed by a lag period (phase 2) and then a second increase in water uptake associated with seedling growth (phase 3). Seeds are tolerant of desiccation during phases 1 and 2 but frequently intolerant of it in phase 3 (Taylor et al., 1998).

Water uptake may be either uncontrolled or controlled. In the former, water is freely available and not restricted by the environment. The seeds may be soaked or placed on moistened blotters. Soaking can involve the total immersion of seed in water with or without artificial aeration. Provided the seeds are viable and not dormant, with a sufficient oxygen supply available and a suitable temperature, germination will proceed. The process is arrested at a specific time, inhibiting the onset of phase 3.

Homogeneity of field establishment from either direct-drilled crops or emergence from seed sown into modules increases the ultimate uniformity of crop growth and concentrates the harvest into a shorter period of time, increasing efficiency and decreasing costs. Where brassicas are grown below or above their optimal temperatures for germination and establishment, direct-seeded crops tend to be drilled at high rates, leading to the need for subsequent thinning to achieve efficient final stand densities. Alternatively, seed priming techniques may be employed.

Priming is a controlled hydration process followed by redrying that allows the metabolic activities of germination to commence but not reach the stage of radicle emergence. Priming using polyethylene glycol (PEG) increased the germination rate of smaller seeds compared with larger ones. When seeds are rehydrated after priming, the germination rate is increased and, in some cases, the temperature range for continued germination may be expanded. Two forms of priming treatments are available, either osmotic or matric. In the former, seeds are incubated in solutions of either low molecular weight inorganic salts such as potassium nitrate, sodium chloride or potassium phosphate or high molecular weight, non-penetrating solutes such as PEG at a water potential that is low enough to inhibit germination.

This technique has been applied successfully to several vegetable Brassica crops. In particular, it will increase seedling vigour in cold, moist soils. Matric priming utilizes moistened solid carriers such as the clay mineral, vermiculite or calcium silicate to hydrate seeds. In several vegetable crops, matric-primed seeds germinate more rapidly compared with those that have been osmoti-cally treated.

To achieve osmotic priming, seeds are placed on two layers of filter paper saturated with the priming agent and sealed in containers, such as Petri dishes, for up to 7 days in darkness. For matric priming, water and calcium silicate are mixed thoroughly and placed in a sealed container for 24 h before adding the seeds. The containers are rotated every 12 h to ensure the uniform mixing of the carrier and seed for 7 days. After priming by either method, the seeds are washed for 2 min in tap water to remove either the osmotica or the solid carriers, rinsed in distilled water and blotted dry. The seed is then dried with forced air at 3 7°C for 2 min and finally reduced to a moisture content of 5-6% on a dry weight basis by being placed over silica gel in a desiccator. The seeds may then be coated with thiram (tetramethylthiuram disulphide) or other approved fungicides and insecticides, and sealed into tin-foil packages or held in glass or plastic bottles at 4°C.

Aerated hydration for periods up to 24 h at 20°C was used by Mehra et al. (2003) as an adjunct to priming with PEG for B. juncea and B. rapa seed. There appeared to be increased resistance to water and salt stress in B. juncea and an extension of the range of temperatures for successful germination for B. rapa when aerated hydration was used.

Improving the prediction of seed viability and vigour in relation to stage of development in the silique has major commercial significance. Seed taken from several parts of broccoli (B. oleracea var. italica) pods at different times after pollination was dried and primed by Jett and Welbaum (1996). Rapid drying increased the viability of seed harvested between 35 and 42 days after pollination. Large seeds failed to germinate more quickly than small ones but produced seedlings of greater dry weight. Priming increased the percentage germination of seeds harvested 42 days after pollination. The germination rate of seed harvested before 42 days after pollination was also improved by priming. This overcame differences between mature and immature seed, and might offer a means of reducing the variation within batches of mechanically harvested seed batches.

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