Plant Growth and Development Test - 35

Auxins are an important class of phytohormones. They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering, e.g., in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits. Less auxin in fruit than in stem specially in early stages will lead to premature fruit fall.

Auxins (from Greek auxein: to grow) was first isolated from human urine. The term auxin is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Most of the auxins in plants are believed to be produced by apical shoot meristem. This auxin then suppresses the growth of lateral buds, the phenomenon is called as apical dominance. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. 

Indole-3-acetic acid (IAA) is the most common, naturally-occurring, plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. Chemically, IAA is a carboxylic acid in which the carboxyl group is attached through a methylene group to the C-3 position of an indole ring. In appearance, IAA is a colorless solid.Indole acetic acid is an auxin. Auxins are growth promoting phytohormones. They are also called as rooting hormone because they stimulate growth of roots in plants specially in tissue culture experiments. Auxins inhibit the growth of axillary buds. Hence, they have an inhibitory effect on growth of stem. The inhibition of growth of axillary buds by auxins is called as apical dominance.

In this question, growth hormone refers to plant growth growth regulators or phytohormones. The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They could be indole compounds (indole-3-acetic acid, IAA); adenine derivatives ( N6-furfuryl amino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA3) or gases (ethylene, C$$_2$$H$$_4$$). Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones in literature.The PGRs can be broadly divided into two groups based on their functions in a living plant body. One group of PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called as plant growth promoters, e.g., auxins, gibberellins and cytokinins. The PGRs of the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. The PGR; abscisic acid belongs to this group. The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.

The term auxin is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. As most important site of auxin synthesis is shoot tips, they frequently migrate in the downward direction.

Auxins are natural plant growth regulators. They are growth promoting  phytohormones. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. 

Auxanometer is a simple instrument to measure growth of a plant. Most common type of auxanometer is arc auxanometer, which measures growth as the increase in length of axis. This increase can be caused due to increase in mass of protoplasm (cell division) or simply by absorption of water (increase in length or surface of existing cells).

The period of growth is generally divided into three phases, namely, meristematic, elongation and maturation. We can understand this by looking at the root tips. The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmata connections. The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase. Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

Auxin is a group of several natural (and artificial) plant hormones; the most common and physiologically important auxin is indoleacetic acid (IAA). Some manufactured, or synthetic, auxins have structures similar to that of IAA. The synthetic auxin naphthalene acetic acid stimulates root development on stem cuttings and is used for asexual propagation, particularly of woody plants with horticultural importance.