**2. Importance of plant hormones**

The small amounts of plant hormones promote, control, influence and develop the growth from embryo to reproductive development, also, stress tolerance and pathogen defense. According to the importance of plant hormones in this chapter will be divided into two main points: first: the effect of plant hormones on

germination and growth of plants under internal or external suitable conditions, second: the effect of plant hormones on the germination and growth of plants under internal or external unsuitable environmental conditions.

### **2.1 The effect of plant hormones on germination and growth of plants under internal or external suitable conditions**

Seed germination is attracted to the effective growth of the embryo when appropriate environmental conditions are present, leading to seed rupture and the appearance of a small plant. There are five basic steps to germination: water imbibition, enzyme activation, initiation of embryo growth, rupture of the seed coat and emergence of seedling, seedling establishment [4, 5].

In the second step stage of germination (enzyme activation), after the absorption of water through the natural openings in the casing of the seed and spread through the tissues of the seed, gibberellins which activate the formation of the hydrolytic enzymes, mainly α- amylase in the aleurone cells, which are responsible for hydrolysis of storage macro-molecules such as starch and proteins and convert them into available forms to the embryo, usage to increase in size, and raise the osmotic content of the seed, to increase water potential [6, 7].

In addition, plant hormones have an important role in plant growth not only germination such as cytokinins (CKs) which influence cell division, the formation of shoot and helping in delay tissues senescence [8, 9]. Also, the ratios of Cytokinins and auxins affect most major growth periods during a plant's lifetime [10]. Also, Peptide hormones, control of cell division, expansion, and play crucial roles in plant growth and development [11]. Furthermore, gibberellins (Gas) strongly promote cell elongation in seedlings [12, 13]. It can also affect cell cycling in plant [14]. Meanwhile, the responses of **n**itric oxide (NO) are in germination, cell death [15] and regulate plant cell organelle functions (e.g. mitochondria and ATP synthesis in chloroplasts) [16].

For enhances and increases plant hormones production in the plants, many studies have proved the need to add plant hormones either directly (GA3, kinetin and cytokinins) [17, 18] or indirectly (humic substances, manures, magnetite, natural zeolites, *Moringa* extract and bio-fertilization) to increase or accelerate the productivity of plant hormones in the plant [19] indicated that, the presence of organic matter represented in compost which a source of hormones like substances as auxin-like activity and gibberellin-like activity. Similar results were obtained from [20]. Ref. [21] concluded that it is also possible that the production of plant hormones influences symbiotic bacteria, such as nodule N2 fixing bacteria. During the establishment of the soybean (*Glycine max* L.) and *Bradyrhizobium japonicum* N2-fixing symbiosis, the production of plant hormones can determine the bacterial population in the nodules by, for example affecting the available substrate for the use of rhizobium. The other significant and interesting view of the effects of soil bacteria on the production of plant hormones is the alteration they may reason in plant signaling pathways, resulting in the output of plant hormones from the host plant [22, 23]. Ref. [24] concluded that the magnetic treatments have the same affected for phytohormone production. Ref. [25] reported that the highest mean values of IAA, GA, and CK (12.70, 13.71, 11.06 μg/g FW), respectively were recorded with compost and zeolite mixture in comparison with control. Ref. [26] concluded that the addition of a mixture of organic fertilizers and soil amendments led to significant increment in indigenous hormones characterized in **i**ndole **a**cetic **a**cid (IAA), gibberellic acid (GA3), and cytokinins (CK), which led to a significant increase in morphological growth, floral characteristics and chemical composition of *Oenothera biennis*. In contrast [27] found that the application of HA inhibits

**3**

*Plant Growth Hormones*

growth hormone.

*DOI: http://dx.doi.org/10.5772/intechopen.84350*

indoleacetic acid (IAA) oxidase, thereby hindering the destruction of this plant

**internal or external unsuitable environmental conditions**

**2.2 The effect of plant hormones on the germination and growth of plants under** 

Sometimes even under favorable germination conditions (an adequate water supply, a suitable temperature and the normal composition of the atmosphere) seeds do not germinate. In this case, seeds are considered dormant. Seed dormancy is defined as an inactive phase in which the growth and development are deferred and the respiration is greatly reduced [28, 29]. Seed coat dormancy involves the mechanical restriction of the seed coat. 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. ABA affects the testa or seed coat growth characteristics, including thickness, and affects the GA-mediated embryo growth potential [5]. Hormones also can 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 process within the endosperm [30]. In addition, [33] concluded that the prevented germination of some seeds of tomato [31], iris [32], and some varieties of cabbage was due to the present of inhibitors (ABA, parasorbic acid, and coumarin) which cases distributed in plants and to possess the property of inhibiting seed germination and other growth phenomena [5, 34].

Plant hormones affect seed germination and dormancy by acting on different parts of the seed such as [35] found that the inhibitors in seeds of peach were at least one of the factors controlling in germination by preventing or retarding cell division of the radical. In *Lupinus angustifolius*, the contents of auxins increased through the 5th day of germination and started to decrease on the 7th day. Oppositely, gibberellins contents were decreased first then increased later, so it was clear that there was inversely related between auxins and gibberellins [36]. The germination percentage and germination rate of four studied Acacias (*A. saligna*, *A. sophorae*, *A. cyclopis,* and *A. melanoxylon*) were correlated positively with endogenous promoting and negative

with endogenous inhibiting substances in their cotyledons plus embryo [37].

with GA3 at 100 and 200 ppm and found that the higher GA3 concentration

500 ppm was the best in the case of *Acacia catechu* [41].

The promotion of germination by gibberellin and cytokinins has been demonstrated in many seed species [38, 39]. Ref. [40] treated the seeds of *Acacia longifolia*,

(200 ppm) was more effective in increasing germination while the concentration of

Ref. [42] found that fresh seed of *Acacia nilotica* and *Acacia albida* were fully germinated when soaked in a solution of GA3 at 200 ppm for 12 h. While soaking seeds of *Acacia nilotica* in gibberellic acid (100 or 300 ppm for 16 h) was the best [43]. Ref. [44] studied the effect of GA3 at a concentration of (50 ppm) on 16 species (four Acacia species), and found a high germination percentage for all species. The effect of gibberellic acid and cytokinins were also recognized on the germination of other plant species seeds. Ref. [45] studied the effect of Kinetin at different concentrations on the seed germination of *Acer tataricum*, and found the highest germination percentage at the concentration of 500 ppm. Ref. [46] found the best germination percentage on soaking the seeds of *Trifolium pratense* in 50 ppm 6-benzylaminopurine (6-BAP). Ref. [47] studied the effect of Kinetin at different conc. (10, 25, 50 and 100 ppm) in the seeds of *Cassia sophera*, and found the highest

*Cell Growth*

chloroplasts) [16].

germination and growth of plants under internal or external suitable conditions, second: the effect of plant hormones on the germination and growth of plants under

**2.1 The effect of plant hormones on germination and growth of plants under** 

In the second step stage of germination (enzyme activation), after the absorption of water through the natural openings in the casing of the seed and spread through the tissues of the seed, gibberellins which activate the formation of the hydrolytic enzymes, mainly α- amylase in the aleurone cells, which are responsible for hydrolysis of storage macro-molecules such as starch and proteins and convert them into available forms to the embryo, usage to increase in size, and raise the

In addition, plant hormones have an important role in plant growth not only germination such as cytokinins (CKs) which influence cell division, the formation of shoot and helping in delay tissues senescence [8, 9]. Also, the ratios of Cytokinins and auxins affect most major growth periods during a plant's lifetime [10]. Also, Peptide hormones, control of cell division, expansion, and play crucial roles in plant growth and development [11]. Furthermore, gibberellins (Gas) strongly promote cell elongation in seedlings [12, 13]. It can also affect cell cycling in plant [14]. Meanwhile, the responses of **n**itric oxide (NO) are in germination, cell death [15] and regulate plant cell organelle functions (e.g. mitochondria and ATP synthesis in

For enhances and increases plant hormones production in the plants, many studies have proved the need to add plant hormones either directly (GA3, kinetin and cytokinins) [17, 18] or indirectly (humic substances, manures, magnetite, natural zeolites, *Moringa* extract and bio-fertilization) to increase or accelerate the productivity of plant hormones in the plant [19] indicated that, the presence of organic matter represented in compost which a source of hormones like substances as auxin-like activity and gibberellin-like activity. Similar results were obtained from [20]. Ref. [21] concluded that it is also possible that the production of plant hormones influences symbiotic bacteria, such as nodule N2 fixing bacteria. During the establishment of the soybean (*Glycine max* L.) and *Bradyrhizobium japonicum* N2-fixing symbiosis, the production of plant hormones can determine the bacterial population in the nodules by, for example affecting the available substrate for the use of rhizobium. The other significant and interesting view of the effects of soil bacteria on the production of plant hormones is the alteration they may reason in plant signaling pathways, resulting in the output of plant hormones from the host plant [22, 23]. Ref. [24] concluded that the magnetic treatments have the same affected for phytohormone production. Ref. [25] reported that the highest mean values of IAA, GA, and CK (12.70, 13.71, 11.06 μg/g FW), respectively were recorded with compost and zeolite mixture in comparison with control. Ref. [26] concluded that the addition of a mixture of organic fertilizers and soil amendments led to significant increment in indigenous hormones characterized in **i**ndole **a**cetic **a**cid (IAA), gibberellic acid (GA3), and cytokinins (CK), which led to a significant increase in morphological growth, floral characteristics and chemical composition of *Oenothera biennis*. In contrast [27] found that the application of HA inhibits

Seed germination is attracted to the effective growth of the embryo when appropriate environmental conditions are present, leading to seed rupture and the appearance of a small plant. There are five basic steps to germination: water imbibition, enzyme activation, initiation of embryo growth, rupture of the seed coat and

internal or external unsuitable environmental conditions.

**internal or external suitable conditions**

emergence of seedling, seedling establishment [4, 5].

osmotic content of the seed, to increase water potential [6, 7].

**2**

indoleacetic acid (IAA) oxidase, thereby hindering the destruction of this plant growth hormone.
