Plant Growth, Growth Curve and Growth Regulators
Plant Growth:-
- All cells of a plant are descendents of the zygote (fertilized egg).
- The zygote develops into a mature plant through growth and differentiation resulting in the formation of roots, leaves, branches, flowers, fruits, and seeds. Then they eventually die.
Growth:-
- Growth is an irreversible permanent increase in size of an organ or its parts or an individual cell.
- It involves metabolic processes that consume energy.
Plant Growth Generally is Indeterminate:-
- Plant growth continues throughout the life due to the presence of meristems at certain locations in their body.
- Meristematic cells have the capacity to divide and self-perpetuate.
- The growth where new cells are always added to the plant body by the activity of the meristem is called the open form of growth.
- Primary growth of the plants:- It occurs due to Root apical meristem and shoot apical meristem. It causes the elongation of the plants along their axis.
- Secondary growth of the plants (In gymnosperms & dicots):- It occurs due to the lateral meristems, vascular cambium and cork-cambium. These meristems cause the increase in the girth of the organs.
Growth is Measurable:-
- At cellular level, growth occurs due to increase in the amount of protoplasm.
- Increase in protoplasm is difficult to measure directly. So growth is measured by parameters like increase in fresh weight, dry weight, length, area, volume & cell number. E.g.
o In cell number:- One single maize root apical meristem can give rise to more than 17,500 new cells per hour.
o In cell size:- Cells in a watermelon may increase in size by up to 3,50,000 times.
o In length:- Growth of a pollen tube.
o In surface area:- Growth in a dorsi-ventral leaf.
Phases of Growth:- 3 phases: meristematic, elongation and maturation.
i. Meristematic phase:- It occurs in the meristems at the root apex & the shoot apex. Cells in this region have rich protoplasm and large conspicuous nuclei. Cell walls are primary, thin & cellulosic with abundant plasmodesmata.
ii. Elongation phase:- It occurs in the cells proximal (just next, away from the tip) to the meristematic zone. The cells have increased vacuolation, cell enlargement and new cell wall deposition.
iii. Maturation phase:- It occurs in the cells further away from the apex, i.e., more proximal to the phase of elongation. The cells attain their maximal size in terms of wall thickening and protoplasmic modifications.
Growth Rates and Growth Curves:-
- It is the increased growth per unit time.
- The growth rate shows an increase that may be arithmetic or geometrical.
i. Arithmetic growth:-
- In this, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. It is represented as below:
On plotting the length of the organ against time, a linear curve is obtained.
Mathematically, it is expressed as
Lt = L0 + rt
Lt = length at time ‘t’
L0 = length at time ‘zero’
r = growth rate / elongation per unit time.
ii. Geometrical growth:-
- In most systems, the initial growth is slow (lag phase), and it increases rapidly thereafter – at an exponential rate (log or exponential phase).
- Here, both the daughter cells continue and retain the ability of mitotic cell division.
- However, with limited nutrient supply, the growth slows down leading to a stationary phase.
- If we plot the parameter of growth against time, we get a typical sigmoid (S) curve.
- A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.
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The exponential growth can be expressed as
W1 = W0 ert
W1 = final size (weight, height, number etc.)
W0 = initial size at the beginning of the period
e = base of natural logarithms
r = relative growth rate
t = time of growth
- Relative growth rate (r) is the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the final size of W1 depends on the initial size, W0.
- Quantitative comparisons between the growth of living system can also be made in two ways:
(i) Measurement and the comparison of total growth per unit time. It is called absolute growth rate.
(ii) Measurement of growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter. It is called relative growth rate.
Conditions (essential elements) for Growth:-
1. Water:- It is essential for cell enlargement. Turgidity of cells helps in extension growth. Water also provides the medium for enzymatic activities needed for growth.
2. Oxygen:- It helps to release metabolic energy for growth.
3. Nutrients:- Macro & micro elements are needed for the synthesis of protoplasm and act as source of energy.
4. Temperature:- Plants have an optimum temperature at which growth is maximum. Deviation from this range could be detrimental to its survival.
5. Light & gravity:- Affect certain phases/stages of growth.
Growth analysis:- Growth analysis is a mathematical expression of environmental effects on growth and development of crop plants. This is a useful tool in studying the complex interactions between the plant growth and the environment.
Mathematical Formulae:-
i. Leaf Area Index (LAI):- Green leaf area per unit ground surface area is called leaf area index. It was given by Williams in 1946.
ii. Leaf Area Ratio (LAR):- Green leaf area per unit dry weight of plant is called leaf area ratio. It was given by Radford in 1967.
iii. Absolute Growth Rate (AGR):- Increase of plant dry weight in per unit of time is called absolute growth rate.
iv. Crop Growth Rate (CGR):- Increase of plant dry weight per unit ground area in per unit of time is called crop growth rate. It was given by Watson in 1956.
v. Ralative Growth Rate (RGR):- Increase of plant dry weight per unit initial dry weight in per unit of time is called relative growth rate. It was given by Blackman.
vi. Harvest Index (HI):- The ratio of grain to total shoot dry weight is called harvest index. It was given by Nichiporovich in 1951.
vii. Crop Index (CI):- The ratio of grain to straw weight is called crop index.
Growth Regulators:- Plant growth regulators (PGRs) are small, simple molecules that regulate growth of plants. Based on the functions, PGRs are divided into 2 groups.
o Plant growth promoters:- For growth promoting activities like cell division & enlargement, tropic growth, pattern formation, flowering, fruiting & seed formation. E.g. auxins, gibberellins and cytokinins.
o Plant growth inhibiters:- For growth inhibiting activities like dormancy & abscission. Take part in plant responses to wounds & stresses of biotic and abiotic origin.
E.g. PGR abscisic acid & ethylene. (Ethylene fits either of the groups, but it is largely a growth inhibitor).