What is Growth Rate?
Growth is the fundamental characteristics of any organism which is irreversible, progressive and exponential.
Growth is a gradual phenomenon taking place at fixed interval of time and specific for a given species.
The growth mechanism of plant is significant than other organisms where the cells divide throughout their life having an unlimited growth.
Plants have a specialized region called meristem where cells get dedifferentiated to divide and increase the biomass of the plant.
The growth is a quantitative measure over time.
The growth in plants is measured by growth rate which is a measure of increase in growth per unit time.
Types of Growth Rate
Growth rate is of two types and are classified based on cell division
(i) Arithmetic Growth Rate
(ii) Geometric Growth Rate
(i) Arithmetic Growth Rate
In arithmetic growth, following a cell division only a single cell attains the capacity for further division, another cell gets differentiated and matured.
This follows for further division. The division is along one side increasing the length of the plant.
For example: Elongation of root. When plotting a graph for root elongation against time, a linear curve is obtained.
The linear plot indicates the growth rate was arithmetic in nature.
In Arithmetic growth is long one direction.
The Arithmetic Growth is expressed as:
Lt = L0 + rt
Lt : Length at time t
L0 : Initial length at time zero
r : Elongation per unit time
(ii) Geometric Growth Rate
Geometric growth type involves cell division were both daughter cell retains the capability for further division.
The growth is exponential and rapid for a particular period of time and when subjected to external and internal factors, their growth varies.
They represent the overall growth of a plant or a system at a particular period of time.
When plotted against time a sigmoid curve is obtained.
The sigmoid shape represents the rate od growth over a different period of time indicating different phases.
The four phases are: Lag phase, Log Phase, Diminishing Phase and Stationary Phase
1. Lag Phase: the initial growth period is referred as lag phase. In this phase each cell starts to divide continuously and make itself easily available to uptake of nutrients and increase cell mass. The phase involves gradual increase in cell growth.
2. Log Phase: the rapid cell growth period is the log phase. Under Favorable environmental condition the cell growth increases exponentially in large scale by the multiplication of cell division. Simultaneous nutrient input and maturation takes place in this stage. However, the cell division exceeds the maturation
3. Diminishing Phase: the cells start maturation providing a higher yield of cellular metabolites. The growth or new cell formation is confined to certain region of meristems which divides but to keep up with overall plant growth. Reduces the rate of formation of new cells
4. Stationary Phase: A final stage of plant growth where the meristematic regions constantly produce new cells and old cells are removed. This constant maintenance of cell cycle is the Stationary Phase.
This exponential growth can be represented as;
Wl = W0 ert
Wl = Final Size
W0 = Initial size
e = base of natural logarithm
r = growth rate
t = time
r = relative growth rate indicating the efficiency index.
Relative growth rate is the measure of given system per unit time.
This measure is compared with Absolute growth rate, a total measure of plant growth per unit time.
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