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Plant growth regulators

Plant hormones, or phytohormones (Greek: hormone – inducing, causing), are low-molecular-weight organic compounds that participate in the interaction of cells, tissues and organs. They are needed in small amounts to initiate and regulate physiological and morphological processes of plant ontogenesis.

Plant hormones

Hormones mediate physiological processes, converting specific environmental signals into biochemical information. Hormones formed in plants are called endogenous, those used by humans to treat plants are exogenous.

The plant’s need for hormones is 10-13⋅10-5 mol/L, in most cases synthesized in sufficient amounts by the plant itself. They are synthesized in individual parts of the plant, but spread throughout the body. Under their action, metabolism is regulated. Hormones exhibit physiological effects on:

  • enzymes and enzyme systems;
  • metabolism of proteins, lipids, nucleic acids;
  • informational and transport ribonucleic acids;
  • deoxyribonucleic acid.

The effect of the action of hormones in some cases is reduced to a temporary change in the intensity of biochemical reactions, in others – manifested in a steady deviation of processes, in the third – in morphological changes affecting the somatic sphere of the body, in the fourth – in hereditary morphological changes.

Among the most active and studied compounds of hormonal action of plant origin are auxins, gibberellins, cytokinins, abscisic acid and ethylene.

Unlike animals, plants do not have glands that secrete hormones.

The action of hormones on plant metabolism is specific: gibberellins are involved in transcription, that is, the transfer of information about the nucleotide sequence of DNA to informational RNA during protein synthesis, cytokinins – in translation, that is, the process of translation of the nucleotide sequence of informational RNA into the sequence of amino acids of the synthesized polypeptide, auxins – in changes in membrane permeability, abscisins inhibit ion transport and related cell growth processes, ethylene acts as a “permissive” growth factor, controlling the balance in the stimulant-inhibitor system.

Auxins

Auxins, or indolylacetic acid (IAA) compounds, are formed in areas with high meristematic activity: in the apexes of stems, in forming seeds, from where they move in the basipetal direction, reaching lateral shoots and leaves.

Auxins initiate cell division and influence the rate of cell stretching, regulate the formation of conductive bundles, and determine the phenomena of plant photo- and geotropism related to the asymmetry of their distribution. Activation of cell stretching occurs when auxin stimulates proton secretion into the cell wall. The resulting increased concentration of hydrogen ions leads to a more active enzymatic cleavage of transverse bonds connecting cellulose microfibrils.

Other properties of auxins are the ability to induce parthenocarpy, delay leaf and ovary fall, and activate root formation in cuttings. Tissues enriched with auxin have an attraction effect, i.e., they can attract nutrients. Auxin provides correlation between the organs of the growing plant.

Gibberellins

Gibberellins are phytohormones derived from the fluorene series. They stimulate cell division and stretching of apical and intercalary meristem cells. Under the influence of gibberellins, leaves, flowers and inflorescences elongate. Gibberellins enhance stem growth more strongly than auxins. At the same time, gibberellins have almost no effect on root growth. They participate in the processes of seed germination and transition of long-day plants to flowering. They promote the formation of parthenocarpic fruits.

Gibberellins can shift the sex of plants to the male side. Their influence on plant metabolism is associated with their participation in nucleic metabolism: their action induces synthesis of matrix RNA, which encodes the formation of hydrolytic enzymes, primarily amylases.

Gibberellins are synthesized mainly in the leaves and from there move up and down the stem.

Cytokinins

Cytokinins are phytohormones, derivatives of purines, stimulate cytogenesis, seed germination, promote bud differentiation. They have the ability to inhibit the aging process of plant organisms and maintain normal metabolism of yellowed leaves, causing their secondary greening.

Cytokinins are involved in mobilization-attraction of nutrients to localized sites: fruits, seeds, tubers. They release lateral buds from apical dominance caused by auxin, stimulate their growth. At the molecular level, cytokinins in complex with a specific protein receptor increase RNA polymerase activity and chromatin matrix activity, with an increase in polyribosomes and protein synthesis. Cytokinins are involved in the synthesis of the enzyme nitrate reductase and the transport of H+, K+, Ca2+ ions.

They are formed in roots, from where they move up the stem in acropetal direction.

Abscisins

Abscisins are natural inhibitors of terpenoid nature. They inhibit growth in the phase of cell division and stretching, do not exhibit toxic effect even in high concentrations. They induce plant dormancy, accelerate leaf and fruit drop (abscission), inhibit coleoptile growth, inhibit seed germination.

By inhibiting the excessive growth of the stem, abscisins direct the metabolites to form the photosynthetic apparatus, i.e. coordinate the growth process. They participate in stress mechanisms by regulating stomatal movements.

Abscisic acid accumulates rapidly in tissues when plants are exposed to unfavorable environmental factors, first of all water deficiency, causing closure of stomata, reducing transpiration and reducing energy consumption. At molecular level abscisins inhibit DNA, RNA and protein synthesis. They can decrease the functional activity of the H+-pump.

Abscisic acid is synthesized in the leaves, transported up and down the stem. It is also formed in the root sheath.

Ethylene

Ethylene is a specific hormone synthesized in all plant organs from methionine. It contributes to the regulation of plant growth and development. It participates in maintenance of apical curvature in seedlings grown in darkness, causes epinasty, i.e. rapid growth of the upper side of the organ, resulting in downward curvature of the leaf or petal. For this reason it is used to accelerate flower opening. Leaf lowering under the action of ethylene reduces transpiration.

Ethylene is responsible for auxin-controlled suppression of growth of lateral buds showing apical dominance. It inhibits cell division and seedling elongation, changes the direction of cell growth from longitudinal to transverse, reducing the length and thickening the stem. By promoting tissue aging, ethylene accelerates leaf fall, flower wilting, and accelerates fruit ripening.

In most cases, it increases dormancy period of seeds and tubers, promotes shift of plant sex to female side, plays role of mediator of hormonal complex in processes of correlation interactions in plant. Inhibits polar transport of auxin and promotes formation of its conjugates. Ethylene regulates stress response in plants. On molecular level increases cell membrane permeability and protein synthesis rate.

Brassinosteroids

Brassinosteroids are hormones that support the immune system of the plant, primarily in stressful situations. Steroids, like gibberellins and abscisic acid, belong to the class of terpenoids.

Brassinosteroids are contained in every plant cell, but their natural level in the changed ecological situation is insufficient to maintain immunity and normal development throughout the growing season.

Preparations - plant growth stimulants

Sodium humate

Main article: Humates

Campozan M

Campozan M is used to prevent lodging of fiber flax, winter rye, winter barley.

Rosaline

Rosaline is used on cotton to prevent boll drop and increase raw cotton yields.

Fospinol

Fospinol increases potato yield by 15-20%, reduces the incidence of fungal and viral diseases, improves the potato tubers’ storability.

Tur

Tur, or chlormequat chloride, and chlorcholine chloride is used in crops, especially in winter crops. It prevents lodging of high-yielding crops by thickening straw, strengthening mechanical tissues and reducing stem length.

Immunocytophyte

Immunocytophyte is a mixture of polyunsaturated fatty acids with high content of archidonic acid. It is used on cereals, legumes, root and tuber crops, vegetables, technical and fruit crops as a multipurpose stimulator of protective responses, growth and development of plants.

Stimulates natural immunity to diseases such as phytophthora, various types of scabs, blackleg, powdery mildew, rot, bacterioses. It accelerates seed germination, fruit ripening, cork layer formation on tubers and root crops; increases flower size, green mass and bushiness; provides yield increase by 20-30%, reduces yield losses during storage.

Application of plant growth regulators

For effective application of plant growth regulators, the conditions must be met:

  • positive effects can be achieved only if endogenous phytohormones are lacking in the plant or in individual organs;
  • cells, tissues and organs must be susceptible to phytohormones;
  • the effect of all growth regulators depends on the concentration, overdose leads to an inhibitory effect;
  • optimal supply of water and nutrients to plants.

Growth regulators do not replace plant nutrition. According to M.H. Chailahan (1976), they increase “appetite” and therefore stimulate growth processes.

Plant growth regulators are used to:

  • stimulating rooting of cuttings;
  • obtaining parthenocarpic (seedless) fruits;
  • increased production of seedless grape varieties;
  • thinning of flowers and ovaries of fruit crops;
  • weed control;
  • inhibition of stem elongation;
  • regulation of dormancy;
  • acceleration of fruit ripening.

Among the growth regulators of auxin nature, 1-naphthylacetic acid (1-NAA), indomethyl-3-oilic acid (IMA), 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2-naphthoxyacetic acid (2-NAUC), 4-chlorophenoxyacetic acid (4X), maleic acid hydrazide (MAH), 2-methyl-4-chlorophenoacetic acid (2M 4X) and 2,4-dichlorophenoxyoilic acid (2,4-DO). 1-NAA and IMA are successfully used in horticulture for rooting cuttings, improvement of seedlings survival and restoration of the root system of transplanted shrubs and trees.

Gibberellins have practical application. Spraying of vine plants during flowering with the aqueous solution containing 30-35 g/ha of gibberellic acid increases the yield of seedless (sultanas) varieties by 10-15%. It is also used in the cultivation of citrus.

Cytokinins have found application in tissue culture. They are a factor necessary for obtaining a culture of dedifferentiated callus tissue as well as for the induction of then organogenesis and somatic embryogenesis. Cytokinin is also necessary for maintaining the functional activity of isolated tissues and organs.

Ethylene is used as a stimulant of fruit and vegetable ripening.

Retardants

Retardants are synthetic substances that inhibit the synthesis of gibberellins, inhibit the growth of stem and vegetative shoots, giving the plant resistance to lodging.

Retardants selectively inhibit stem growth without having a negative effect on physiological and biochemical processes. The action is based on the inhibition of cell division in the medial and subaerial zones of the meristem of the growing cone, which forms the stem. Retardants have no effect on the apical meristem zone, from which leaves and generative organs develop. These regulators inhibit the stem cells growth in length and enhance their division in transverse direction, due to which the stem becomes shorter and thicker. At the same time, the development of mechanical tissues increases: cell walls thicken, the number of vascular-fiber bundles increases. At the same time, retardants promote root growth, increase the assimilative surface area of leaves and plastid pigment content, and increase plant resistance to adverse environmental factors.

More than a thousand chemical compounds with retardant properties have been studied. Most belong to four groups of substances:

  1. quaternary onium compounds;
  2. hydrazine derivatives;
  3. triazole derivatives;
  4. ethylene-producing.

Among the retardants on the basis of quaternary onium salts, chlorcholine chloride (CCC), morphol, and pike are common. The characteristic retardant effect of these drugs is due to their ability to interrupt gibberellin biosynthesis. Their administration blocks the formation of geranylgeranyl pyrophosphate and its subsequent cyclization into entkauren, which is an intermediate in the synthesis of gibberellins.

Triazole derivatives block gibberellin biosynthesis by preventing the oxidation of entkauren into caurenic acid.

Ethylene-producing drugs do not interrupt gibberellin biosynthesis, their action is associated with an antigibberellin effect that occurs during the formation of the hormone-receptor complex or in the subsequent stages of realization of the hormonal activity of gibberellins.

The mechanism of action of hydrazine derivatives is also not associated with the inhibition of gibberellin synthesis, but is due to the suppression of their hormonal activity.

Of all known retardants, chlorocholine chloride (CCC), better known as Tur, has the greatest practical value. This retardant gives good results in cereal crops. In order to increase resistance to lodging chloroquine chloride is applied in the period of tillering – the beginning of piping at the rate of 3-12 kg/ha. It does not reduce the quality of grain, increases the yield, and reduces the economic costs of harvesting.

Retardants show high efficiency also on rice crops.

Table. Rice yield of Krasnodar 86 variety when using retardants (Sheudzhen A.Kh., 2005)

Retardant
Plant height, cm
Length of panicle, cm
Coverability, point
Grain yield, 100 kg/ha
Control
118,5
17,1
2
52,6
CCC, 10 g/ha
112,3
17,3
4
56,9
Oriz, 30 kg/ha
100,5
16,9
5
59,0
Sumadik, 30 kg/ha
98,3
17,0
5
60,8

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. – Moscow: Bylina, 2000. – 555 с.