Growth : Growth is an irreversible process for all living beings which is commonly accompanied by increase in size, weight and volume of the organism till maturity. In plants generally it takes place by the activity of apical, intercallary and lateral meristems.
Cellular growth of every living organism involves three main aspects called phases of growth.
(i) Cell division : This is called phase of cell formation. It results from the division of pre-existing cells. It is the first step of growth of a plant. The type of cell division responsible for growth is mitosis which results into the formation of many genetically similar cells.
(ii) Cell enlargement : This has most important role in the contribution of the size of the tissue and organs. In the beginning cell enlargement occurs in all directions, but in later stages it becomes confined to a specific direction.
(iii) Cell differentiation : It being the last phase of growth in which newly formed cells after getting enlarged mature into a particular type of tissue. Cells undergo structural and physiological changes, depending upon the function which they have to perform.
Grand period of growth is the total period in which the course of growth in completed. When it is represented by a graph it is called grand period curve, which is always sigmoid in nature so also known as sigmoid growth curve. This curve has three main parts :
(i) Lag phase : This phase is known as preparatory phase.
(ii) Log phase : This is the phase of exponential growth.
(iii) Senescence phase : This is called stationary phase.
Horizontal microscope and auxanometers are used for measurement of growth. It can also be measured directly with the help of a scale.
Measurement of growth : The measurement of growth is carried either in terms of increase of weight or in terms of increase in volume, area, length etc. The simplest method of the measurement of growth is taken as direct method in which growth is measured directly by a scale at regular intervals from beginning to end. In other methods for the measurement of growth, horizontal microscope, auxanometers, Bose’s crescograph and space marker disc are used.
Factors affecting growth : Growth is affected by the factors which affect the activity of protoplasm. Growth is affected by environmental and physiological factors such as absorption of water and minerals, photosynthesis, respiration etc. and environmental factors including climatic and edaphic factors both.
(i) Food supply : The rate of growth is directly proportional to the supply of food. If the food supply is deficient to the growing region the rate of growth decreases and ultimately stops.
(ii) Water supply : Water supply increases growth and has a direct relationship with the rate of growth because water is essential for all the metabolic activities of protoplasm and for increasing the turgidity of the cell for cell enlargement.
(iii) Oxygen supply : Oxygen increases growth rate because it helps in respiration to convert potential energy into kinetic energy needed for vital activities of the plant.
(iv) Temperature : Temperature also affects growth pattern directly or indirectly. Though growth occurs between the temperature 4OC to 45OC, optimum growth takes place between 28OC to 33OC.
(v) Light : Light affects growth rate under four heads namely,
(a) Intensity of light : In general, more light retards growth in plants. High light intensities induce dwarfing of the plant. Plants at the mountain tops are dwarf—whereas those of a valley are quite tall. Low intensity of light reduces the rate of overall growth and also photosynthesis.
(b) Quality of light : The growth pattern is affected by different colours (different wavelengths). With lower-wavelengths of light, internodal growth is pronounced, while green colour light reduces the expansion of leaves as compared to complete spectrum of visible light. The red colour of light favours elongation.
(c) Duration of light : There is pronounced effect of the periodicity of light on the growth of vegetative as well as reproductive structures. The induction and suppression of flowering is dependent on duration.
Plant growth hormones : Inside the plant body there are several types of chemicals called hormones which control growth as well as other physiological processes at a site remote from its place of production and are required in minute quantities called plant hormones or phytohormones or growth promoting substances.
All phytohormones are growth regulators but all growth regulators are not phytohormones. Growth regulators may be either growth promoters or growth inhibitors. For balanced and controlled development of plant, growth accelerators (promoters) and growth inhibitors both are essential.
Growth promoters are—
(1) Auxins.
(2) Gibberellins.
(3) Cytokinins.
Growth inhibitors are—
1. Abscisic acid.
2. Ethylene.
Gibberellins : Although this hormone was discoved in 1950, the same period when auxins were discovered, but their established role as growth hormone came into prominence only after 1950.
Yabuta and Hayashi (1939) gave name gibberellin to an active heat labile material extracted from seedlings suffering from backanae disease. They were able to isolate and active substance from fungus (Gibberella fujikuroi) and called it Gibberellin A.
Later on the discovery of Gibberellin was confirmed by Mitchell, Stodoia, Brian etc. and its structure was proposed by Cross et al (1961).
All gibberellins have a common skeleton the gibbane ring. By now about 56 types of gibberellins are discovered by different workers.
Role of gibberellins :
1. Apical bud dormancy.
2. Role in sub-apical meristem.
3. Cell elongation.
4. Fruit growth.
5. Flowering.
6. Mobilisation of foods in storage cells of seed.
Practical application of gibberellins :
1. Germination.
2. Rooting
3. Leaf expansion.
4. Hyponasty of leaves.
5. Flowering.
6. Parthenocarpy.
7. Fruit setting.
8. Fruit drop.
9. Stem elongation.
10. Pollen germination.
11. Breaking of dormancy.
12. Commercial use of GA.
Cytokinins
Applications of cytokinins :
1. Cell division.
2. Cell enlargement.
3. Morphogenesis.
4. Dormancy.
5. Apical dominance.
6. Initiation of interfascicular cambium.
7. Richmond-land effect.
8. Mobility.
9. Nucleic acid metabolis.
10. Protein synthesis.
11. Incorportation in RNA.
Abscisic acid (ABA) : In 1965 Cairns and Addicott Abscisin-l From old cotton balls and Abscisin-ll from young cotton balls which accelerated abscission of leaves.
Role of abscisic acid
1. Bud dormancy.
2. Senescence.
3. Abscission.
4. Flower initiation.
5. Stomatal physiology.
6. Release of ethylene.
7. Counteract GA.
Ethylene Role of ethylene
1. Ethylene is a chemical substance which inhibits elongation of stem, causes swelling of node and nullifies geotropism.
2. Ethylene reduces flowering in most plants but accelerates in pineapple.
3. Ethylene increases the number of female flowers but reduces the number of male flowers.
4. Ethylene causes faster abscission of leaves and flowers.
5. It is a ripening agent. It is formed in large quantities in ripening fruits.
Photoperiodism : The effect of the duration of daily hours of light and darkness on the phenomenon of flowering is called photoperiodism. Photoperiodism was demonstrated for the first time by Garner and Allard in 1920 in soyabean. On the basis of the period of day length required for the induction of flowering in different plants were classified by them into different groups—
(i) Short day plants (SDP) : These plants need light for short periods (usually 8 hours) and continuous dark period of about 14-16 hours for flowering. e.g., Xanthium pensylvanicum, Glycine max etc.
(ii) Long day plants (LDP) : This type of plants need for long light period (usually 14-16 hours) for flowering. e.g., Spinacea, Oleracea, Beta vulgaris etc.
(iii) Day neutral plants : These plant need continuous exposure of light for periods ranging from 5 hours to 24 hours e.g., Mirabilis, Lycopersicum Esculentum etc.
(iv) Intermediate plants : These plants need light for day lengths within a certain range usually 12-16 hours but fail to flower under either longer or shorter photoperiod. e.g., Andropogon furctus, Phaseolus polystachyus etc.
(v) Amphiphotoperiodic plants : These plants remain vegetative on intermediate day lengths and flower only on shorter or longer day lengths. e.g., Media elegans.
(vi) Short-long day plants : These plants flower when short photoperiods are followed by long photoperiods. e.g., Some variety of Triticum vulgre, Secale cereale etc.
(vii) Long-short day plant : These plants flower when long photoperiods are followed by short photoperiods. e.g., Cestrum nocturnum, Bryophyllum etc.
Most short-day and long-day plants have a definite critical photoperiod and specific influence of photoperiodic induction. For short day plants a definite continuous dark period is critical so they are also called long night plants.
It has been found from several experiments carried by different workers that perception of light stimulus is mainly performed by leaves and buds and for this purpose only a few leaves are sufficient.
Perception of light stimulus can be translocated from one branch to another and from one type of plant to another type of plant (if they are grafted together).
Chailakhyan (1936) suggested that florigen is a substance whose formation gives necessary stimulus for the act of flowering and the nature of this flowering hormone is almost similar in all types of plants.
Phytochrome : Borthwick and Hendrick were the first to suggest that in plant tissues there occurs a recepton pigment to which they named phytochrome. The pigment was later on isolated by Butler (1954). Phytochrome is a bright blue or bluish green proteinaceous pigment found in the two forms—
(i) Red light absorbing form designated as P8 form.
(ii) Far red light absorbing form desingnated as p58 form.
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