In this article we will discuss about:- 1. Introduction to Plant Growth and Development 2. Factors affecting Plant Growth & Development 3. Growth Hormones & Growth Regulators.
Introduction to Plant Growth:
Plant growth may be defined as a dynamic vital process which brings about a permanent change in any plant or its parts with respect to size, form, weight and volume. The permanent change may be either in positive direction or negative direction. For example, the dry weight in the sprouting potato tubers decreases during the early phase of the growth. Development is the process of growth and differentiation of individual cells into tissues, organs and organisms. It is the resultant of growth.
Therefore the main points of growth are:
a) It is dynamic vital process.
b) It brings about a permanent change.
Phases of Growth:
Broadly there are three phases of growth:
i. The phase of Cell Division/Cell Formation. It is also called Logarithmic or Exponential phase.
ii. The phase of Cell Enlargement. It is called linear phase.
iii. The phase of cellular differentiation or cell maturation. It is also called senescence phase.
But there are five distinct phases of growth:
a) Lag Phase:
It is the initial lag period where internal changes in the cell occur which are the preparatory to growth. Here the increase in size or weight is very slow or negligible.
b) Log Phase:
It is the grand period of growth. Here growth is very fast.
c) Third Phase:
Here Growth rate gradually decreases.
d) Fourth Phase:
It is the phase where organism reaches to maturity and growth ceases.
e) Final Phase:
It is phase of senescence where death of organism sets in.
On the basis of the different phases of growth, a sigmoid growth curve (i.e. ‘S’ – shaped) is obtained.
The growth rate is measured in different ways by Auxanometer and Crescograph (J.C. Bose).
Factors Affecting Plant Growth & Development:
Growth and Development is affected by physiological processes and environmental conditions. Absorption of water and minerals, photosynthesis, respiration etc. are the physiological processes which govern the growth and development to a very large extent. The environmental factors include the climatic factors and edaphic factors.
The major two climatic factors viz., are:
1. Temperature, and
2. Light:
1. Temperature:
There is a pronounced effect of temperature on the growth of the plant. Growth occurs in the range of 4°C to 45°C but the cardinal temperature range is 28-33°C. The low temperature at night reduces the rate of respiration but high temp, during the day time increases photosynthesis and accumulation of synthates which in turn increases growth. That is why potato tubers growing on hills are much larger than those of the plains.
The very high temperature generally stops the growth of plant by affecting many physiological processes. At high temperature, the protein component of the protoplasm is also coagulated and the protoplasm is killed. This effect of high temperature is called heat injury. But there are some plants which have some heat resistance mechanism like high sugar content, thick bark etc.
The very low temperature also generally stops the growth.
There are three types of injuries caused by low temperature:
i) Desiccation:
The plant tissues become desiccated and injured when the rate of absorption is very slow due to low temperature but transpiration rate is rapid.
ii) Chilling Injury:
The plant of the hot climate when – exposed to low temperature (above the freezing point) for some time is either killed or severely injured. This injury is called chilling injury. The ripe banana becomes black when is kept in refrigerator is only due to chilling injury.
iii) Freezing Injury:
When the plant is exposed to very low temperature (below the freezing point), the protoplasm of the plant cell is dehydrated resulting in its coagulation due to the formation of ice crystals of water.
The high concentration of the cell sap aggravates the precipitation of protein and thus resulting into the death of the cell. But there are many perennial plants which withstand the freezing injury because of the high osmotic concentration of the cell sap. Such frost resistance (or hardiness) nature of the plant lowers the freezing point and reduces the amount of water.
2. Light:
The intensity, quality and duration of light affect the growth. The weak intensity of light promotes shortening of internodes and expansion of leaves. Very weak intensity reduces the rate of overall growth. Very high intensity reduces the growth rate indirectly, increasing the water loss. Blue violet light enhances internodal growth whereas green light reduces expansion of leaves. Red colour is the most favourable light quality for growth.
Beyond the visible spectrum i.e., infrared and uv-rays are detrimental for growth. The duration of light has pronounced effect on the vegetative and reproductive growth of plant. This phenomenon is called Photoperiodism. Longer periods of light cause luxuriant vegetative growth in most of the plants. Garner and his co-workers found that the amount of vegetative growth was proportional to the duration of day light.
Growth Hormones & Growth Regulators:
Growth Hormones:
Growth Hormones are the organic substances which are produced generally in meristematic tissues of the plant and translocated towards the site of action inducing a physiological process or response and can work in extremely minute quantities. Thimann (1948) suggested the term Phytoharmone for hormones of plants.
Plant Growth Regulators (PGR):
Such organic compounds occurring naturally in plants as well as synthetic other than nutrients which in small amounts promote, inhibit or modify any physiological process are called PGR.
The PGR are of two types:
i) Growth promotor e.g., auxins, gibberellins & cytokinin.
ii) Growth inhibitors e.g., abscisic acid and ethylene.
I. Auxins:
1) F.W. Went (1928) isolated the growth substance which he named Auxin.
2) The plant Auena satiua (i.e. Oat) was used by Went for the bioassay hence the test is known as Avena Curvature test or Avena Coleoptile test. It was found that Auxin was responsible for curvature in Avena Coleoptile.
3) Thimann (1934) found that the highest concentration of auxin was occurred in the coleoptile tip and a gradual decrease from the tip to the base of the coleoptile. He also noticed that the concentration of auxin was much less in the root tip than that of the coleoptile tip.
4) Auxin was a general term used to denote for such substance which promote the elongation of the coleoptile tissues.
5) Indole acetic acid (IAA) is an endogenous auxin occurring naturally in plants.
6) Synthetic auxins.
Examples are:
a) Indole – 3 – butyric acid (IBA)
b) Indole – 3 – Propionic acid (IPA)
c) Napthalene acetic acid (NAA)
d) Dichlorophenoxy acetic acid (2, 4-D)
e) Malic Hydrazide (MH).
7) MH and Acid paracoumaric have the property of anti- auxins.
8) Precursor of IAA is Tryptophane (produced from SKIMMIC pathway of respiration)
9) Two types of endogenous Auxin:
a) Free Auxins:
Such are utilised in various metabolism.
b) Bound Auxins:
Such auxins are attached with enzyme and/or antiauxins and therefore such are not utilised in the various metabolism. In Mango, there is no rooting even after the use of NAA. It means it is due to the presence of bound auxin.
10) Non-Indole Auxin:
Example is Phenyl acetic acid found in tomato.
11) Polar Transport of Auxin:
Auxin is known for polar transport:
a) Poar transport means the movement of auxins from the morphological apex towards the base of the plant.
b) Polar transport of IAA is strongly developed in monocot coleoptiles.
c) Polar transport is negated if anaerobic condition is maintained or treated with respiratory inhibitors. It means the movement of IAA is not polar and becomes free to move irrespective of morphological apex.
12) Apical Dominance of Auxin:
The growth of the apical bud suppresses the growth of the lower axillary buds in many plants. It means the terminal (apical) bud dominants over the lateral buds by inhibiting their development. Such dominancy is called apical dominance. The development of lateral shoots or buds is inhibited by a substance which arises from the apex. When the tip of the main shoot is removed, the side shoots or buds start to develop. Thimann and Skoog (1934) found that the dominance of the terminal bud was due to the auxin.
13) Physiological Effects (Practical Application):
i) Cell Division:
Auxin is responsible for promoting cell division in certain tissues like cambium. The cambial activity and callus formation at the wounded site is stimulated by auxin. The formation of callus has practical use in grafting which strengthens the union of stock and scion. The cell division of tissue culture is entirely dependent on auxin.
ii) Cell Elongation:
The primary physiological effect of auxin on growth of a plant is the elongation of cells.
The cell elongation is activated by auxin in three ways:
a) By increasing osmotic solutes.
b) By decreasing wall pressure.
c) By increasing permeability of cytoplasm to water.
The avena curvature test was the bioassay for cell elongation test. But Auxin has inhibitory effect on root elongation due to the auxin-induced production of ethylene.
iii) Inhibition of Lateral Buds:
The sprouting of lateral buds i.e., eyes in potato tuber is checked by applying synthetic auxins. Therefore the dormancy period of tubers is increased by using IBA, NAA and MH. The opening of flower bud on fruit trees is also delayed by using synthetic auxins to avoid the damage caused by late frost.
iv) Shortening of Internodes:
High concentration of α-NAA prevents the elongation of inter nodes and the plant becomes dwarf.
v) Root Initiation:
Due to the polar transport of auxin, rooting starts at the morphologically lower end. Thimann & Went (1930) found that the indole acetic acid and outer growth substances were essential for initiating adventitious root formation in cuttings. For commercial use a-IBA and NAA are markedly superior to IAA.
vi) Prevention of Abscission Layer:
The formation of abscission layers at the bases of petiole, pedicel or peduncle results into the separation of leaves, flowers and fruits from the plant. The premature drop of fruits may be stopped by spraying 2, 4-D; IAA, NAA etc.
vii) Flower Initiation:
Auxin generally inhibits flowering and thus is helpful in delaying the flowering in lettuce.
viii) Production of Parthenocarpic Fruits:
Seedless fruits are being developed by horticulturists by spraying synthetic auxins.
ix) Weed Control:
The roots are extremely sensitive to auxins. Auxin distorts the roots, blocks the sieve tubes and disturbs the cell division of roots. 2, 4-D is used for weed control.
1) The name ‘gibberellin’ was used by Yabuta and Sumiki (1938) for a pure crystalline chemical which was isolated from ‘Bakanae or Foolish seedling’ diseased rice plants. Kurosawa of Japan in 1926 confirmed that the disease was caused by a fungus ‘Gibberella fujikoroi’ (Fusarium heterosporum). Due to this disease, rice plant grows abnormally thin and tall.
2) 6 gibberellins viz. GA1, GA2, GA3, GA4, GA7, & GA9 were isolated from the fungus Gibberella by Cross et al (1961). 3 gibberellins viz. GA5, GA6 & GA8 were isolated from bean seeds by Mac Millan et al (1961). Chemically gibberellins are known as gibberellic acid.
3) Most commonly available gibberellic acid is GA3.
4) Gibberellins are common in higher plants but restricted to the only certain species of Fungi & bacteria. The conc. is higher in stem apex, young leaves and seeds.
5) Gibberellins are synthesised through the normal isoprenoid pathway of terpene biosynthesis.
6) Gibberellin promotes shoot growth by accelerating the cell elongation & cell division in the sub-apical meristem region which increases the length of internodes. Gibberellin regulates the mitotic activity of the sub apical meristem.
7) In certain cases it protects the apical meristem from the inhibitory effect of dormin (endogenous growth inhibitor)
8) Gibberellin induces the synthesis of hydrolytic enzymes especially protease and a-amylase which triggers seed germination. Gibberellin is released by the seed embryo and is transported to the aleurone layer of endosperm where such enzymes are synthesized under its influence. This is the example of hormonal control of enzyme synthesis.
9) Gibberellin has no effect on root growth and the activity of apical meristem of stem apex.
10) Physiological Effects:
i) Stem Elongation:
It increases the length of internodes. It speeds up RNA-synthesis.
ii) It converts the dwarf plant into a plant of normal height. When ‘Rosette’ plant of sugarbeet (example of extreme dwarfism) is treated with gibberellins, it undergoes a rapid growth or bolting.
iii) Substituting Cold Treatment:
Many biennials complete their life cycle within a single year by treatment with G A.
iv) Partthenocarpic fruits: GA induces parthenocarpic development of fruits in tomato, apple & pear more effectively than auxin.
v) Breaking Dormancy:
It is effective in breaking of dormancy in potato tubers and in tree buds in winter.
vi) It promotes flowering in long day plants and induces maleness. GA introduces male flowers whereas Ethrel/ Ethephon increases femaleness.
vii) It increases the size of leaves and fruits.
viii) It prevents senescence.
ix) It increases the cell division and cell size.
1) Jablonski and Skoog (1954) reported that the cell division in the pith cells was due to a substance present in vascular tissues. Miller et al (1956) showed that this substance was very effective in cell division. Such cell-division inducing substance is known as Kinetin and Letham (1963) used the term cytokinin (specific effect on cytokinesis) for kinetin like substances viz. Kinetenoid, Phytokinin, Phytocytomine.
2) Cytokinin is a derivative of the purine base adenine which has furfuryl substituent at the 9th position which changes to 6th position of the adenine ring during autoclaving of DNA.
3) At present it is clear that cytokinins are a part of t RNA (transfer RNA).
4) The chemical name of kinetin is N6 furfuryl adenine or 6- furfurylamino purine.
5) Kinins promote cytokinesis in cells of various plant organs.
6) Kinetin alongwith auxin increases mitotic activity tremendously because division is promoted mainly by kinetin and auxin induces cell enlargement.
7) The endosperm of coconut (coconut milk) also contains endogenous (naturally occurring) cytokinin. Zeatin is endogenous cytokinin of Maize.
8) Physiological Effects:
i) It promotes cell division and the related DNA and RNA synthesis.
ii) It has morphogenesis effect, that’s why it is used for organ formation in a variety of tissue cultures.
iii) It counteracts the apical dominance of auxin.
iv) It is used in the breaking of dormancy. It also promotes the seed germination.
v) It delays the phase of senescence. Senescence means the disappearance of chlorophyll and the degradation of protein. Richmond and Lang (1957) reported that the senescence was delayed in the detached xanthium leaves for several days when they were treated with kinetin. Such effect of Kinetin in retarding the senescence (ageing) is called Richmond-Lang Effect.
IV. Abscisic Acid (ABA):
1) ABA is a common growth Inhibitor.
2) Robinson and P.F. Weiring (1963-64) extracted the inhibitory substance and called it ‘dormin’ because it caused dormancy.
3) Okhuma et al (1963, 65) isolated the very active inhibitor from young cotton fruits and called it abscisin II. Abscisin I was isolated from the burrs of matrre cotton fruits. Later on in 1967 it was realised that the dormin and abscisin II were the same and was named Abscisic acid (ABA).
4) Physiological Effects:
i) It accelerates the senescence phase of growth.
ii) It regulates the buds and seeds dormancy by inhibiting the growth processes.
iii) It inhibits GA-induced a-amylase synthesis thus inhibiting germination of seeds.
iv) It inhibits gibberllin – stimulated growth hence called antigibberellin.
v) It causes abscission of leaves.
vi) It inhibits RNA and Protein synthesis.
vii) It causes the closure of stomata by interfering with the uptake of K+ (Na+) in guard cells.
V. Ethylene:
1) Ethylene (CH2 = CH2) is a volatile gas which is included under Hormones in 1971.
2) It is synthesized in plant from the amino acid Methionine.
3) The most important effect is fruit ripening (climacteric rise of respiration). The climacteric rise indicates the beginning of senescence and death.
4) Ethylene increase the cell permeability due of which the fruit becomes soft.
5) The inhibitory effect of auxin on root elongation and buds growth is due to auxin-induced production of ethylene.
6) High concentration of CO2 i.e. 5-10% inhibits the effect of ethylene. Ag+ is also the inhibitor of ethylene action.
7) According to D.N. Neljubow, ethylene caused triple response on Pea seedling:
i) It inhibits stem elongation.
ii) It increased stem thickening.
iii) It stimulated horizontal growth habit.
8) Ethrel/Ethephon:
The chemical which releases ethylene.
9) Physiological Effects:
i) It induces climacteric rise and fruit ripening.
ii) Induction of epinasty (leaf bending), leaf abscission and stem swelling.
iii) Inhibition of stem and root growth.
iv) Induction of flower petal discolouration.
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