Plant Propogation and Nursery Management

Hormonal Treatment

viii) Hormonal treatment

• Among various hormones, GA3 is commercially used for breaking seed dormancy in different types of seeds. The concentration of GA3 depends upon the kind of seed but generally a concentration of 200-500 ppm is most widely used.
• Cytokinin is another group of hormones used for breaking physiological dormancy and stimulating germination in seeds of many species. Kinetin and BA(6-benzyle aminopurine) are commercial preparations of cytokinin used for breaking seed dormancy. Soaking seeds in 100 ppm solution of kinetin for 3-5 minutes is highly effective concentration for overcoming seed dormancy of many species. Etheral also stimulates germination in seeds of some species.
Table 3.2. Recommended concentrations of growth hormones in temperate fruits for increasing seed germination

Crop	Chemical/hormone	Concentration
Apple	Thiourea	5000 ppm
	Kinetin	25 ppm
	GA	50 ppm
	Ethrel	100-200 ppm
Pear	GA	150 ppm
	Thiourea	5000 ppm
Peach	Thiourea	5000 ppm
	GA	400 ppm
	BA	400 ppm
Walnut	GA	250 ppm
	Ethrel	1000 ppm

Hormonal changes during stratification:
A triphasic change in endogenous hormones is many seeds is depicted in Fig.3.1.

• A reduction of ABA
• Increased synthesis of cytokinin and gibberellins
• Reduction in hormone synthesis in preparation for germination.



Fig.3.1: Triphasic changes in hormone production during chilling stratification



• In general, gibberellins promote germination in dormant seeds, while ABA inhibits germination.
• Pre-sowing treatments with certain seeds not only reduce the stratification requirement and improve the seed germination but also enhances seedling growth in a number of temperate fruits.
Role of hormones in seed dormancy:
Plant hormones affect seed germinations and dormancy by affecting different parts of the seed. Embryo dormancy is characterized by a high ABA/GA ratio, whereas the seed has a high ABA sensitivity and low GA sensitivity. To release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation towards a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity needs to occur.

• Plant regulators can be used to break or prolong the dormancy. Sprouting of potato tubers and onion bulbs is a common phenomenon in storage.
• Pre-harvest spray of maleic hydrazide (MH) at 2000 ppm applied 15 days before actual date of harvest prolongs dormancy in the above storage organs by inhibiting the sprouting.
• In fruit trees of apple, plums and figs, early flowering is induced by spraying Dinitro orthocresol at 0.1 % in oil emulsion.
• Seed treatment of tomato with GA at 1 00 ppm breaks the dormancy and increases the percentage of germination.
• ABA controls embryo dormancy, and GA enhances embryo germination. Seed coat dormancy involves the mechanical restriction of the seed coat, this along with a low embryo growth potential, effectively produces seed dormancy.
• GA releases this dormancy by increasing the embryo growth potential, and/or weakening the seed coat so the radical of the seedling can break through the seed coat. Different types of seed coats can be made up of living or dead cells and both types can be influenced by hormones; those composed of living cells are acted upon after seed formation while the seed coats composed of dead cells can be influenced by hormones during the formation of the seed coat.
• ABA affects testa or seed coat growth characteristics, including thickness, and effects the GA-mediated embryo growth potential. These conditions and effects occur during the formation of the seed, often in response to environmental conditions. Hormones also mediate endosperm dormancy.
• Endosperm in most seeds is composed of living tissue that can actively respond to hormones generated by the embryo. The endosperm often acts as a barrier to seed germination, playing a part in seed coat dormancy or in the germination process.
• Living cells respond to and also affect the ABA/GA ratio, and mediate cellular sensitivity; GA thus increases the embryo growth potential and can promote endosperm weakening. GA also affects both ABA-independent and ABA-inhibiting processes within the endosperm.