Secondary dormancy

• Secondary dormancy occurs in some non-dormant and post dormant seeds that are exposed to conditions that are not favorable for germination, like high temperatures.
• It is caused by conditions that occur after the seed has been dispersed.
• The mechanisms of secondary dormancy are not yet fully understood but might involve the loss of sensitivity in receptors in the plasma membrane.

Reasons for seed dormancy and methods to break them

(1) Coat (pericarp) barrier: water cannot penetrate coat of hard seeds in legumes etc. O2 cannot penetrate coat of cocklebur etc. a strong coat, such as peach or plum nuclei (seeds) prevents embryo sprout out.

• Methods to break removing or wounding coats by mechanic means, high concentration of acids, hot water. (2) After-ripening limited: Not well-developed embryo : ginkgo, panax, Fraxinus excelsior etc. Not ripening in physiology Wheat seed, apple seed and pine seed etc.

• Methods: Dry under full sun light, GA treatment and scarification. (3) Inhibitors in seeds • ABA， alkaloid, phenols and quinones. • Methods： Washing， GA treatment

Seed Scarification

• Seed scarification involves breaking, scratching, or softening the seed coat so that water can enter and begin the germination process.
• In acid scarification, seeds are put in a glass container and covered with concentrated sulfuric acid.
• The seeds are gently stirred and allowed to soak from 10 minutes to several hours, depending on the hardness of the seed coat. When the seed coat has become thin, the seeds can be removed, washed, and planted.
• Another scarification method is mechanical. Seeds are filed with a metal file, rubbed with sandpaper, or cracked with a hammer to weaken the seed coat.
• Hot water scarification involves putting the seed into hot water (170 to 212˚F). The seeds are allowed to soak in the water, as it cools, for 12 to 24 hours before being planted.
• A fourth method is one of warm, moist scarification. In this case, seeds are stored in nonsterile, warm, damp containers where the seed coat will be broken down by decay over several months.

Seed Stratification

• The following procedure is usually successful. Put damp sand or vermiculite in a clay pot to about 1 inch from the top.
• Remove the fleshy outer coating (fruit) from the seed, if present.
• Place the seeds on top of the medium and cover with 1/2 inch of damp sand or vermiculite.
• Place the pot containing the moist medium and seeds in a plastic bag and seal. Place the bag in a refrigerator.
• Periodically check to see that the medium is moist, but not wet. Additional water will probably not be necessary.
• After 10 to 12 weeks, remove the bag from the refrigerator. Take the pot out and set it in a warm place in the house.
• Water often enough to keep the medium moist. Soon the seedlings should emerge.
• When the young plants are about 3 inches tall, transplant them into pots to grow until they are ready to be set outside.

Other method are:

• Chilling treatment; low temperature(0-5ºC) for some time e.g. apple, peach, plum
• Alternate high and low temperature: 30-40º C to 5-10º C. Care to be taken that difference
• should not be more than 20º C. e.g. Rumex, Lycopus
• Impaction: by vigorous shaking for seeds having plug for easy breaking.
• Light requirement: red light hasten germination in positive photoblastic seeds
• Pressure treatment; high hydraulic pressure about 2000 atm at temperature of 18-20 ºC for 5-20 minutes. E.g. sweet clover, alfa alfa
• Chemical treatment: thio urea, KNO3, 2- chlorohydrin, purine, gibberlin, cytokinine
• hasten the germination in several plants.