Plant Growth and Hormones

In the course of research on auxin biology, many compounds with noticeable auxin activity were synthesized. Many of them had been found to have the economical potential for man-controlled growth and development of plants in agronomy. Auxins are toxic to plants in large concentrations. They are most toxic to dicots and less so to monocots. Because of this property, synthetic auxin herbicides, including 2,4-D and 2,4,5-T, have been developed and used for weed control. However, synthetic auxins, especially 1-naphthaleneacetic acid (NAA) and indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants or for different agricultural purposes such as the prevention of fruit drop in orchards. Used in high doses, auxin stimulates the production of ethylene. Excess ethylene (also native plant hormone) can inhibit elongation growth, cause leaves to fall (abscission), and even kill the plant. 

• Auxins are a group of hormones that positively influence cell enlargement, bud formation, and root initiation. 
• In conjunction with cytokinins, they control the growth of stems, roots, and fruits, and convert stems into flowers. Auxins were the first class of growth regulators discovered. 
• They affect cell elongation by altering cell wall plasticity. Auxins act to inhibit the growth of buds lower down the stems, and also to promote lateral and adventitious root development and growth. 
• Leaf abscission is initiated by the growing point of a plant ceasing to produce auxins. Auxins in seeds regulate specific protein synthesis, as they develop within the flower after pollination, causing the flower to develop a fruit to contain the developing seeds. 
• Auxins are toxic to plants in large concentrations, they are most toxic to dicots and less so to monocots. 

• Abscisic acid is one of the most important plant growth regulators. This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. In general, it acts as an inhibitory chemical compound that affects bud growth, and seed and bud dormancy. 
• It mediates changes within the apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers. 
  

There are five major types of plant hormones. Abscisic Acid also called ABA is one of the most important plant's growth regulators and under water stress, this hormone plays a role in closing the stomata. Auxins are hormones that  positively influence cell enlargement, bud formation and root initiation. Cytokinins tend to positively influence cell enlargement, bud formation and root initiation. Others are ethylene regulates the fruit ripening and Gibberellins stimulates the flower production, cell elongation. Whereas 2, 4-D is not a plant hormone but a chemical weedicide. 

Cytokinin is a plant hormone that produces the Richmond Lang effect. It inhibits ageing process of the plant. Auxin is a plant hormone that induces apical dominance. It promotes the growth of stem tip and inhibits the growth of lateral buds. Abscisic acid is a plant hormone. It is also called as stress hormone as under stress environmental condition, its concentration increases. It induces seed dormancy. Ethylene is a plant hormone. It is also called as ripening hormone as it induces ripening of fruits. 

Gibberellic acid is a plant hormone. It stimulates stem elongation, germination, flowering. It also causes fruit elongation and delays senescence. Hence in brewery industry, it is used to speed up malting process. Auxin is a plant hormone which is used to induce flowering in pineapple as it induces synthesis of ethylene. Cytokinin is a plant hormone that stimulates cell division. It delays the senescence in leaves. 

Avena curvature test (Auxin/rooting), Chlorophyll preservation (Cytokinin/senescence), Triple response (Ethylene/female flowers), Cotton test (Auxin/antagonist).

• The plant growth regulators (PGRs) or phytohormones are small, simple molecules of diverse chemical composition. For example, auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
• NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic)are synthetic auxins.
• 6-Benzylaminopurine, benzyl adenine, BAP  is a first-generation synthetic cytokinin that elicits plant growth and development responses, setting blossoms and stimulating fruit richness by stimulating cell division.
  
• Hence 2,4-D, NAA, BAP  are synthetic phytohormone.
• So, the correct answer is '2,4-D, NAA, BAP '.

Pytohormone which causes mobalization of nutrients is cytokinin and  it also helps in the formation of new leaves and chloroplast. 

A- Phytohormones are plant hormones. These are organic substances which are produced in the plants. They bring about total control over growth and other physiological functions in the organism.

•  Plant hormones/phytohormones are numerous organic chemical substances that control the growth and differentiation of plant cells, tissues, and organs.
  
• Plant enzymes are both organic and inorganic nature. They are not involved in the regulation of growth directly.
  
• The plasma membrane regulates the transport of materials entering and exiting the cell. so option A is the correct answer.
  

Phytohormones are produced by plants, that regulate germination, growth, metabolism, or other physiological activities.