Plant Growth and Development

Chapter 13

Plant Growth and Development

Phases and Rate of Plant Growth

• Growth is common in human being and also in plants, but it does not take place throughout the life of plants. Growth may be apical, lateral or intercalary. Growth is for restricted period and in special regions only.

• Growth in length is due to gradual enlargement and elongation of the cells of apical meristems (Root apex or stem apex).

• The growth in thickness is due the activity of lateral merisperms that is due to fascicular.

• Three phases of growth can be marked in the growth of any organ of plant body which are as under:

1.   Formative phase: It is restricted to apical meristem of the root or stem. The cells of this region are continuously dividing and multiplying. Abundant protoplasm, large nucleus and this cellulose wall are characteristic to the cells of this region.
2. Elongation phase: This phase lies immediately behind the formative phase, wherein, cells no longer divide but increase in size until the cells reach their maximum dimensions.
3. Maturation phase: This phase lies further back. The cells reach their permanent size which results in the thickening of the cell wall

Conditions of Plant Growth

• Growth is the outcome protoplasm and conditions for growth are the conditions which maintain the activity of the protoplasm. Growth of plant depends on the following essential requirements.

• Food material or nutritive material supplied to growing region is of prime importance for building the body of the plant.

• Water availability is another important factor for growth, which enables the plant to maintain the turgidity, which also make loss caused by transpiration.

Introduction to Plant Growth Regulator

• Plant growth regulators are the organic compounds, other than nutrients which affect the morphological structure and/or physiological processes of plants in low concentrations. Phytohormones or plant hormones are naturally occurring growth regulators which in low concentrations control physiological processes in plants. More commonly, the term plant growth regulators are used, because it includes both the native (endogenous) and the synthetic (exogenous) substances, which modify the plant growth.

• As the native plant growth regulators, five major kinds of substances are reported, viz. auxins, gibberellins, cytokinin, abscisic acid and ethylene. All of these, except ethylene and abscisic acid, are multiple forms of endogenous plant growth regulators. In general, the plant growth regulators or substances regulate cell enlargement, cell division, cell differentiation, organogenesis, senescence and dormancy.

Auxins

Auxins is a general term used to indicate substances that promote elongation of coleoptiles tissues. Indole acetic acid (IAA) is an auxin that occurs naturally in plants. They are either natural auxins which are produced by plants themselves or synthetic auxins, which have the same action as natural auxins. IAA is the principal a Uxin and other natural auxins are indoIe-3-acetonitriIe (IAN) 4-chIoroindoIe 3-acetic acid and phenyl acetic acid. The synthetic auxins are indoIe-3-butyric acid (IBA), 2-napthyIoxyacetic acid (NOA), a-naphthyls acetic acid (NAA), 1-napthyI acetamide (NAD), 2, 4-dichIorophenoxyacetic acid (2, 4-D), 2, 4, 5 trichloro phenoxy acetic acid and 5-carboxymethyI-N, N-dimethyl dithiocarbamate.

• AUxins are involved in different growth processes in plants like internodes' elongation, leaf growth, initiation of vascular tissues, cambial activity, fruit setting in absence of pollination, fruit growth, apical dominance, inhibition of root growth, influencing physical and chemical properties in leaf abscission and inhibition of lateral buds.

• The proposed mechanism of action of IAA is its interaction with one or more components of biochemical systems involved in the synthesis of proteins. The other hypothesis suggested is the role of IAA to alter the active osmotic contents of cell vacuole dUring cell expansion or cell wall extension.

• IBA and NAA in combination are used in rooting of cuttings. NOA is used as a fruit setting spray 2, 4-D and 2, 4, 5-T are used both as plant growth regulators and in higher concentrations, as selective weed killers, especially for dicot plants. IBA has shown promising results to induce rooting in cuttings for cinchona, Pinus, papaya and coffee. The addition of different auxins like IAA, NAA and 2, 4-D in tissue cultures of ergot has led to increase in Indole alkaloids. Treatment of derivatives of NAA given to seedlings and young plants of Mentha piperita has shown about 40 per cent increase in volatile oil content.

Gibberellins

• They are a class of endogenous plant prowth regulators and at present over 50 gibberellins are known. About 40 of these occur in green plants, while others are present in some fungi. They are present in different organs and tissues like roots, shoots, buds, leaves, floral apices, root nodules, fruits and callus tissues. The commercial formulations of gibberellins are used currently for promoting vegetative and fruit growth, breaking dormancy, flower initiation and induction of parthenocarpy.
• Kurosawa, a Japanese physiologist, is credited for initiating the discovery of gibberellins from fungus Gibberella [ujikuroi (previously known as Fusarium heterospermic) grown on rice. According to Paleg, gibberellins are compounds having gibbane skeleton and biological activity in stimulating cell division or cell elongation, or both.

• All of these are the derivatives of gibbane ring skeleton. In addition to free gibberellins, they are also present in conjugated forms. GA has not yet been synthesized but can be produced by large scale fermentation on commercial scale. The angiosperms, gymnosperms, ferns, algae, fungi and bacteria contain several forms of gibberellins, but no single plant contains all of them together. Many activities attributed to gibberellins are promotion of rapid expansion of plant cells, stimulation of seed germination, breaking dormancy of overwintering plants, induction of flowering under non-inductive conditions, marked increase in stem elongation, increase in the size of leaves and induction of parthenocarpy leading to seedless fruit sets.

Cytokinins

• These are either natural (zeatin) or synthetic (kinetin) compounds with significant growth regulating activity. Zeatin has effect on cell division and leaf senescence and synthetic cytokinins are useful in promoting lateral bud development and inhibition of senescence.

• Cytokinins influence a broad spectrum of physiological processes in plants like promotion of cell division. The other activities exerted are participation in orderly development of embryos during seed development, influencing the expansion of cells in leaf discs and cotyledons, delaying breakdown of chlorophyll and degradation of proteins in ageing leaves.

• Miller isolated the crystalline substance from autoclaved herring sperm DNA capable of inducing cell division in tobacco cultures and named it as kinetin, which was found to be 6-furfuryIadenine. Further, some other adenine derivatives were also found having similar biological activity and were called 'kinins' collectively known as cytokinins. These substances are found in young and actively dividing tissues like embryos, seedlings and apical meristems.

• The naturally occurring cytokinin's are zeatin, N6 dimethyl amino purine, and N6-A2 - isopentenyl aminopurine. The synthetic cytokinin's are kinetin, adenine, 6-benzyl adenine benzimidazole and N, N'-diphenyl urea.

• Cytokinins are reported to increase marginally sennoside content in Tinnevelley senna leaves and also enhance the dry weight of shoots. In opium, they cause formation of elongated capsule and reduce alkaloid content. In Duboisia hybrids, the cytokinins activity present in extract of seaweed shows marked increase in both leaf content and also hyoscine content.

• Kinetin is reported to play the role in nucleic acid metabolism and protein synthesis. In plant metabolism, it is proposed that some I-RNA contain cytokinins like activity. They have an action on some enzymes responsible for formation of certain amino acids.

Ethylene

• It is a simple organic molecule present in the form of volatile gas and shows profound physiological effects. It is present in ripening fruits, flowers, stems, roots, tubers and seeds. It is present in very small quantity in plant, say about 0.1 ppm (part per million) possibly, its quantity increases in local areas during the time of growth and development.

• Ethylene is produced by incomplete burning of carbon rich substances like natural gas, coal and petroleum. Denny (1924) showed the yellowing of lemons due to stove gas. Also, the plant damage was noticed after the introduction of illuminating gas. Gane (1935) found that a gas evolved from ripe apples can also effect the ripening of green apples and showed that it was ethylene gas which had a role in ripening of other fruits.

• Ethylene shows a broad array of growth responses in plants, which include fruit ripening, leaf abscission, stem swelling, leaf bending, flower petal discoloration, and inhibition of stem and root growth. It is commercially used for promotion of flowering and fruit ripening, induction of fruit abscission, breaking dormancy and stimulation of latex flow in rubber trees.

Abscisic Acid (ABA)

• The physiological activities in plants like retaining or shedding of different organs such as leaves, stems, flowers and fruits have led to finding of natural growth inhibitor. A diffusible abscission - accelerating substance was found by Osborne (1955) in senescent leaves. Cams et a/. isolated several abscission accelerating substances from cotton plants and named them as abscission I and abscission.

• In an inhibitory way, ABA interacts with other plant growth substances. It inhibits the GA-induced synthesis of a-amylase and other hydrolytic enzymes. During maturation, ABA accumulates in many seeds and helps in seed dormancy. ABA concentrations are found to be enhanced in stress conditions, like mineral deficiency, injury, drought and flooding. ABA serves an important role as potential antitranspirant by closing the stomata, when applied to leaves.

• A number of other synthetic growth inhibitors and retardants reported are maleic hydrazide, daminozide, glyphosine, chromequat chloride, S, S, S-tributyl phosphorotrithioate, ancymidol, chlorophonium chloride, piproctanyl bromide, etc. However, commercial use of these compounds is yet to be reported. A group of synthetic substances called morphactins is a potent inhibitor of auxin transport causing tropic responses, reduction of apical dominance and promoting lateral growth. Morphactins include chloroflurecol methyl, flurecol-butyl and TIBA (2, 3, 5-tri-iodobenzoic acid).