PLANT GROWTH AND DEVELOPMENT
Growth
is regarded as one of the most fundamental and conspicuous characteristics of a
living being. 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. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability of the plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate.
The product, however, soon loses the capacity to divide and such cells make up the plant body. This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth
Plant growth is of two types:
Primary growth:
Root apical meristem and shoot apical meristem are responsible for the primary
growth of the plants and principally contribute to the elongation of the plants
along their axis.
Secondary growth:
In dicotyledonous plants and gymnosperms, the lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth and contribute to the increase in the girth of the organs (root, stem).
GROWTH IS MEASURABLE
At
cellular level growth can be measured by measuring the increase in the amount
of protoplasm but it is very difficult to measure directly, so growth is
measured by a variety of parameters, some of which are :-
Increase in fresh weight
Increase in dry weight
Increase in surface
area/volume
Increase in number or
size of cells.
One single maize root apical meristem can give rise to more than 17,500 new cells per hour, whereas cells in a watermelon may increase in size by upto 3,50,000 times. In the former, growth is expressed as increase in cell number; the latter expresses growth as increase in size of the cell. While the growth of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth in a dorsiventral leaf.
PHASES OF GROWTH
(A)
Meristematic phase :
The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth.
(B) Elongation phase :
The cells proximal to the meristematic zone represent the phase of elongation. Cells in this region are characterised by:
Increased vacuolation
Cell enlargement
New cell wall deposition
(C) Maturation phase :
The cells more proximal to the phase of elongation represent the phase of maturation. Cells of this zone, attain their maximal size in terms of wall thickening and protoplasm modifications.
CONDITIONS FOR GROWTH
Water, oxygen and nutrients are very essential for growth. The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth. Thus, plant growth and further development is intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities needed for growth.
Oxygen helps in releasing metabolic energy essential for growth activities. Nutrients (macro and micro essential elements) are required by plants for the synthesis of protoplasm and act as source of energy.
In addition, every plant organism has an optimum temperature range best suited for its growth. Any deviation from this range could be detrimental to its survival. Environmental signals such as light and gravity also affect certain phases/stages of growth.
Plant Growth Regulators
PGRs
are also called plant growth substances or plant hormones or phytohormones. The
PGRs can be broadly divided into two groups based on their functions in a
living plant body. One group of PGRs are involved in growth promoting
activities, such as cell division, cell enlargement, pattern formation, tropic
growth, flowering, fruiting and seed formation. These are also called plant
growth promoters, e.g., auxins, gibberellins and cytokinins
The PGRS of the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group. The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.
THE
DISCOVERY OF PLANT GROWTH REGULATORS
Interestingly, the discovery of each of the five major groups of PGRs have been accidental. All this started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile. Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.
The 'bakanae' (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.
F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Miller et al. (1955) later identified and crystallised the cytokinesis promoting active substance that they termed kinetin.
During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).
Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.
VERNALISATION
In some plants flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed as vernalisation.
"Vernalisation refers specially to the promotion of flowering by a period of low
temperature". After perception
of suitable low temperature period, a hypothetical hormonal substance
(vernalin) is produced.
of suitable low temperature period, a hypothetical hormonal substance
(vernalin) is produced.
Some important food
plants, wheat, barley, rye have two kinds of varieties:
(i) Spring varieties
(ii) Winter varieties
Spring varieties are normally planted in the spring and come to flower and produce mature grain before the end of growing season.
Winter varieties, however, if planted in spring would normally fail to flower or produce mature grains within a span of flowering season so they are planted in autumn. They germinate and over winter come out as small seedlings, resume growth in spring and usually harvested around mid-summer.
Vernalisation prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity. Another example of vernalisation is seen in biennial plants (e.g. sugarbeet, cabbages, carrots
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