Plant Propogation and Nursery Management

Stratification

2. Stratification:

• Stratification is a method of handling dormant seed in which the imbibed seeds are subjected to a period of chilling to after ripen the embryo in alternate layers of sand or soil for a specific period. It is also known as moist chilling.
• However, temperate species displaying epicotyl dormancy (like fringed tree) or under developed embryo (like hollies) a warm stratification of several months followed by a moist chilling stratification is required.
• Several tropical and subtropical species (like palms) require a period of warm stratification prior to germination to allow the embryo to continue development after fruit drop.
• The seeds can be sown after fruit drop. The seeds can be sown immediately after stratification in the field.
• Seeds with a hard endocarp, such as Prunus spp. (the stone fruit including cherry, plum, apricot and peaches) show increased germination if planted early in the summer or fall to provide one to two months of warm temperature prior to the onset of chilling.
i) Outdoor stratification

• If refrigerated storage facilities are not available, outdoor stratification may be done either by storing seeds in open field conditions in deep pits or in raised beds enclosed on wooden frames.
• However it is likely that seeds are destroyed in outdoors by excessive rains, freezing, drying, or by rodents. Seeds are placed in alternate layers of sand to provide and low temperature and proper aeration in the stratification pit. The top is covered with Sphagnum moss to maintain moisture level.
• The pit or tray is irrigated at regular intervals to maintain appropriate moisture status (Plate 3.2).



Plate 3.1: Unstratified seed of apple Plate 3.2: Open field stratification tray

ii) Refrigerated stratification

• An alternative to outdoor field stratification is refrigerated stratification.
• It is useful for small seed lots or valuable seeds that require special handling.
• Dry seeds should be fully imbibed with water prior to refrigerated stratification. Twelve to twenty four hours of soaking at warm temperature may be sufficient for seeds without hard seed coats.
• After soaking, seeds are usually placed in a convenient size box in alternate layers of well washed sand, peat moss or vermiculite(Plate 3.3).
• A good medium is a mixture of one part of coarse sand to one part of peat, moistened and allowed to stand for 24 hours before use. Seeds are placed in alternate layers of sand or medium.
• The usual stratification temperature is 4-7^oC. At higher temperature seeds sprout prematurity and low temperature delays sprouting.



Plate 3.3: Refrigerated stratification

• The medium should be re-moistened. The stratified seed is separated from the medium prior to sowing in nursery beds.
• The stratification of seeds results in quick and uniform germination and therefore the seed should be subjected to stratification invariably under all conditions.



Plate 3.4: Stratified seed of apple (swollen ) Plate 3.5: Stratified seed of apple (inside view)


Table 3.1. Effect of seed stratification period on per cent germination of important temperate fruits

Kind of fruit	Stratification period (days)	% germination
Apple	70-75	70-75
Kainth (Pyrus pashia)	30-35	90-95
Peach	60-70	55-60
Apricot	45-50	75-80
Almond	45-50	85-90
Walnut	95-100	80-85
Pecan	70-75	75-80


iii) Leaching of inhibitors: It is established fact that some inhibitors and phenolic compounds are present in seed coverings of many species, which inhibit germination. Therefore, soaking of seeds in the running water for 12-24 hours or placing them in water for few hours help in leaching off the inhibitors and phenolic compounds, which help in easy seed germination.
iv) Pre-chilling: In seeds of certain plant species, dormancy can be overcome by pre-chilling treatment. In this treatment, the imbibed or soaked seeds are kept at a temperature of 5-10⁰C for 5-7 days before sowing. After that seed can be sown in the field immediately.
v) Pre-drying: This is also a useful practice in some seeds to overcome seed dormancy. In this treatment, the dry seeds are subjected to a temperature of 37-40⁰C for 5-7 days prior to sowing. After this, seed can be sown in the field.
vi) Seed priming: Seed priming refers to the procedures followed to overcome dormancy in freshly harvested fruits. Most widely used seed priming procedures are osmo- conditioning, infusion and fluid drilling.

• In osmo-conditioning, the seeds are placed in shallow layer in a container having 20-30 per cent solution of polyglycol (PEG). The seeds are then incubated at 15-20⁰C for 7-21 days, depending upon seed size and plant species.
• Different hormones and fungicides can also be added to protect the seeds from pathogens. After this, the seeds are washed and dried at 25⁰C and are stored until use.
• In infusion, the hormones, fungicides or insecticides and antidotes are infused into dormant seeds through organic solutions. In this process the seeds are placed in acetone or dichloromethane solution containing chemicals to be used for 1-4 hours.
• Afterwards, the solvent is allowed to evaporate and seeds are dried slowly in vacuum desiccators for 1-2 hours. The seeds absorb the infused chemical directly into the embryo when soaked in water.
• In fluid drilling, the seeds are suspended in a special type of gel before sowing. Now-a- days different types of gels are available in the market but sodium alginate, guar gum and synthetic clay are most widely used in fluid drilling.
vii) Treatment with chemicals: Some compounds other than hormones are also used to break dormancy but their role is not clear. Thiourea is one example known to stimulate germination in some kinds of dormant seeds. The seeds are soaked in 0.5 – 3 per cent solution of thiourea for 3-5 minutes. Afterwords seeds are rinsed with water and are sown in the field. Similarly, potassium nitrate and sodium hypochlorite also stimulate seed germination in many plant species.