Plant Growth And Development
Plant Growth And Development: Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. Therefore, for example, expansion of a leaf is growth.
GROWTH CURVE OF Plant Growth And Development
The rate of growth of a plant or plant part is not always the same during its life
span. Sometimes it is slow and at other times rapid. If we plot the increase in cell
number (growth rate) against time, a typical S-shaped curve is obtained. This is
called growth curve or sigmoid growth curve. This curve has three phases of growth.
(i) Lag Phase – This is the initial phase of growth when the rate of growth is
very slow.
(ii) Log Phase – It shows rapid growth and is maximum during the entire life span.
(iii) Stationary Phase – Here the rate of growth starts decreasing and finally it
stops.
MEASUREMENT OF GROWTH(Plant Growth And Development)
After knowing the different phases of growth let us know how to measure growth
in plants. Growth in plants being a quantitative phenomenon can be measured in
relation to time. It can be measured in terms of
Increase in length or growth – in case of stem and root;
Increase in area or volume – in case of leaves and fruits;
Increase in the number of cells – in algae, yeast and bacteria.
Let us discuss some methods of measuring growth in length.
DEVELOPMENT(Plant Growth And Development)
seed germination; the elaboration of a mature vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many of the plant’s responses to its environment. Plant development encompasses the growth and differentiation of cells, tissues, organs, and organ systems.
FACTORS AFFECTING PLANT GROWTH(Plant Growth And Development)
Generally plant growth is influenced by a number of factors both external and
internal.
External growth factors(Plant Growth And Development)
External factors are those factors present in the environment that affect the growth
of the plants directly or indirectly. These factors are
(i) Light (ii) Temperature (iii) Water (iv) Mineral nutrients
(i) Light (Plant Growth And Development)
You have already learnt about the necessity of light for the process of photosynthesis.
Besides photosynthesis, light is also essential for seed germination, growth of
seedling, differentiation of various tissues and organs, and reproduction.
(ii) Temperature(Plant Growth And Development)
Some plants grow in cold climate and some in hot climate. The optimum
temperature required for growth of plants ranges between 28-30°C, but it may occur
in the temperature range of 4-45°C. All metabolic activities of plants are directly
affected by variation of temperature. A very low temperature causes injuries to the
plant due to chilling and freezing, and very high temperature stops its growth.
(iii) Water(Plant Growth And Development)
You have already learnt that a plant absorbs water by its roots, uses it in
photosynthesis and other biochemical processes and some of it is lost through
transpiration. For proper growth of plants a particular quantity of water is required.
Both deficiency and excess of water retards the growth of plants.
(iv) Mineral Nutrients(Plant Growth And Development)
“Plant nutrition” we have already discussed the importance of mineral
nutrients for plant growth and development. All metabolic processes require
inorganic nutrients. Plant growth is adversely affected by the deficiency of nutrients
Internal Growth Factors(Plant Growth And Development)
In addition to the external factors as discussed above, there are some substances
produced in the plant body itself, which affects the growth of the plant. These are
called plant hormones or phytohormones or growth hormones
Auxins
Auxins (from Greek ‘auxein’ : to grow) was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices. They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
Gibberellins
Gibberellins are another kind of promotery PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. However, Gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic. They produce a wide range of physiological responses in the plants. Their ability to cause an increase in length of axis is used to increase the length of grapes stalks. Gibberellins, cause fruits like apple to elongate and improve its shape. They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period. GA3 is used to speed up the malting process in brewing industry.
Cytokinins
Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk. Since the discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds with cell division promoting activity, have been identified. Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.
Ethylene
Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic. Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers. Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/ upper parts of the shoot to remain above water.
Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples. It also induces flowering in mango. Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture. The most widely used compound as source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission
in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers thereby increasing the yield.
Abscisic acid
As mentioned earlier, abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. But like other PGRs, it also has other wide ranging effects on plant growth and development. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibits seed germination. ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone. ABA plays an important role in seed development, maturation and dormancy. By inducing
dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations, ABA acts as an antagonist to GAs.
