Biology GROWTH RATE AND CONDITIONS

KEY TOPICS

star Growth rates
star Growth Conditions

GROWTH RATES

● The color{violet}"increased growth" per unit time is termed as color{Brown}"growth rate".

● Thus, color{violet}"rate of growth" can be color{violet}"expressed mathematically".

● An organism, or a color{violet}"part of the organism" can produce more cells in a color{violet}"variety of ways".

● The color{violet}"growth rate" shows an increase that may be color{vBrown}"arithmetic or geometrical".

● In color{Brown}"arithmetic growth", following color{violet}"mitotic cell division", only color{violet}"one daughter cell" continues to divide while the color{violet}"other differentiates" and matures.

● The color{violet}"simplest expression" of arithmetic growth is exemplified by a color{violet}"root elongating" at a color{violet}"constant rate".

● On color{violet}"plotting" the color{violet}"length of the organ" against color{violet}"time," a color{Brown}"linear curve" is obtained.

● color{violet}"Mathematically", it is expressed as

L_t = L_0 + rt
L_t = length at time t
L_0 = length at time ‘zero’
r = growth rate / elongation per unit time.

● Let us now see what happens in color{Brown"geometrical growth".

● In color{violet}"most systems," thecolor{violet}" initial growth is slow" (color{Brown}"lag phase"), and it color{violet}"increases rapidly" thereafter – at an color{violet}"exponential rate" (color{violet}"log or exponential phase").

● Here, both the color{violet}"progeny cells" following mitotic cell division retain the color{violet}"ability to divide" and continue to do so.

● However, color{violet}"with limited nutrient supply", the growth slows down leading to a color{Brown}"stationary phase".

● If we plot the color{violet}"parameter of growth" against time, we get a tcolor{violet}"ypical" color{Brown}"sigmoid or S-curve"

● A color{violet}"sigmoid curve" is a characteristic of color{violet}"living organism" growing in a color{violet}"natural environment".

● It is typical for all color{violet}"cells, tissues" and color{violet}"organs of a plant".

● The color{violet}"exponential growth" can be color{violet}"expressed" as

W_1 = W_0e^(rt)
W_1 = final size (weight, height, number etc.)
W_0 = initial size at the beginning of the period
r = growth rate
t = time of growth
e = base of natural logarithms

● Here, color{Brown}"r" is the color{violet}"relative growth rate" and is also the measure of the color{violet}"ability of the plant" to produce new plant material, referred to as color{Brown}"efficiency index".

● Hence, the color{violet}"final size" of W_1 depends on the icolor{violet}"nitial size", W_0.

● color{violet}"Quantitative comparisons" between the color{violet}"growth of living system" can also be made in two ways :

(i) color{violet}"Measurement" and the color{violet}"comparison" of total growth per unit time is called the color{Brown}"absolute growth rate".

(ii) The color{violet}"growth" of the color{violet}"given system" per color{violet}"unit time" expressed on a common basis, e.g., per unit initial parameter is called the color{Brown}"relative growth rate".

● In Figure color{violet}"two leaves," color{vBrown}"A and B," are drawn that are of color{violet}"different sizes" but shows absolute color{violet}"increase in area" in the given time to give color{violet}"leaves", A_1 and B_1.

● However, one of them shows much color{violet}"higher relative growth rate".

CONDITIONS FOR GROWTH

● There are color{violet}"many necessary conditions" for growth.

● This list may have color{violet}"water, oxygen" and color{violet}"nutrients" as very color{Brown}"essential elements" for growth.

● The plant cells color{violet}"grow in size" by color{violet}"cell enlargement" which in turn requires color{Brown}"water."

● color{violet}"Turgidity of cells" helps in color{violet}"extension growth".

● Thus, color{violet}"plant growth" and color{violet}"further development" is intimately linked to the color{violet}"water status" of the plant.

● color{violet}"Water" also provides the color{violet}"medium for enzymatic activities" needed for growth.

● color{Brown}"Oxygen" helps in releasing color{violet}"metabolic energy essential" for growth activities.

● color{Brown}"Nutrients" (macro and micro essential elements) are color{violet}"required by plants" for the synthesis of color{violet}"protoplasm" and act as color{violet}"source of energy".

● In addition, color{violet}"every plant organism" has an color{vBrown}"optimum temperature" range best suited for its growth.

● Any color{violet}"deviation" from this range could be color{violet}"detrimental to its survival."