Kinetics of plant growth

Kinetics of growth

It is an analysis of the motion of cells or expansion.

1. Stages in Growth rate

The total period from initial to the final stage of growth is called the grand period of growth. The total growth is plotted against time and ‘S’ shaped sigmoid curve (Grand period curve) is obtained. It consists of four phases (Figure 15.2). They are: 

i.                  Lag phase

ii.                  Log phase

iii.                  Decelerating phase

iv.                  Maturation phase

i. Lag phase

In this phase new cells are formed from pre­-existing cells slowly. It is found in the tip of the stem, root and branches. It is the initial stage of growth. In other words, growth starts from this period (Figure 15.2).

ii. Log phase or exponential growth

Here, the newly formed cell increases in size rapidly by deposition of cell wall material. Growth rate is maximum and reaches top because of cell division and physiological processes are quite fast. The volume of protoplasm also increases. It results in rapid growth and causes elongation of internode in the stem.

i.        Decelerating phase or Decline phase or slow growth phase

The rate of growth decreases and becomes limited owing to internal and external or both the factors because the metabolic process becomes slow.

ii.    Steady state period or maturation phase

In this phase cell wall thickening due to new particle deposition on the inner surface of the cell wall takes place. The overall growth ceases and becomes constant. The growth rate becomes zero.

2. Types of growth rate

The increased growth per unit time is termed as growth rate. An organism or part of an organism can produce more cells through arithmetic growth or geometric growth or both.

i. Arithmetic Growth Rate

If the length of a plant organ is plotted against time, it shows a linear curve and this growth is called arithmetic growth.

·              The rate of growth is constant and it increases in an arithmetic manner.

·              Only one cell is allowed to divide between the two-resulting progeny cell.

·              One continues to divide but the other undergoes cell cycle arrest and begins to develop, differentiate and mature.

·              After each round of cell division, only a single cell remains capable of division and one new body cell forms.

For example, starting with a single cell after round 1 of cell division there is one dividing cell and one body cell. After round 2 there are two body cells, after round 3 there are three and so on (Figure 15.3).

The plants single dividing cell would undergo one million rounds of nuclear and cellular division. If each round requires one day, this type of arithmetic increase would require one million days or 2739.7 years. This arithmetic rate is capable of producing small number of cells present in very small parts of plants. For example the hair on many leaves and stems consists of just a single row of cells produced by the division of the basal cell, the cell at the bottom of the hair next to other epidermal cells. 

Hair may contain 5 to 10 cells by the division of the basal cell. So, all its cells could be produced in just five to ten days. In the figure 15.4, on plotting the hight of the plant against time a linear curve is obtained. Mathematically it is expressed as:

L_t = L_o + rt

L_t = length at time ‘t’

L_o = length at time zero

r = growth rate of elongation per unit 

ii. Geometric growth rate:

This growth occurs in many higher plants and plant organs and is measured in size or weight. In plant growth, geometric cell division results if all cells of an organism or tissue are active mitotically. Example: Round three in the given figure 15.5, produces 8 cells as 23 = 8 and after round 20 there are 2²⁰ = 1,048,576 cells.

The large plant or animal parts are produced this way. In fact, it is common in animals but rare in plants except when they are young and small. Exponential growth curve can be expressed as,

Here ‘r’ is the relative growth rate and also a 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 W_0.

iii. Arithmetic and Geometric Growth of Embryo

Plants often grow by a combination of arithmetic and geometric growth patterns. A young embryonic plant grows geometrically and cell division becomes restricted to certain cells at the tips of roots and shoots. After this point, growth is of the slower arithmetic type, but some of the new cells that are produced can develop into their mature condition and begin carrying out specialized types of metabolism (Figure 15. 6). Plants are thus a mixture of older, mature cells and young, dividing cells.

Quantitative comparisons between the growth of living system can also be made in two ways and is explained in the table 1.

In figure 15.7, two leaves A and B are drawn at a particular time. Then A¹and B¹ are drawn after a given time. A and B 5 Area of leaves at a particular time. A¹ and B¹ 5 Area of leaves after a given time. (A¹-A) and (B¹- B) represents an absolute increase in area in the given time.

 Leaf A increases from 5 cm² to 10 cm2; 5 cm² in a given time. Leaf B increases from 50 cm² to 55 cm² ; 5 cm² in a given time. Hence, both leaves A and B increase their area by 5 cm² in a given time. This is absolute growth. Relative growth is faster in leaf A because of initial small size. It decreases with time (Figure 15.7).

3.Conditions of growth

Plant growth is influenced by a variety of external and internal factors. A brief account of these factors is given below:

I. External Factors

a. Water

Water is essential for cell enlargement as well as growth in the size of the cell. Turgidity of cells helps in growth extension. Water provides the medium for enzymatic activities needed for growth.

b. Nutrition

Nutrition plays an important role in the formation of protoplasm. Macro and micro elements are very important as sources of energy. For example, carbon and oxygen in carbon-di-oxide and hydrogen in water are assimilated in photosynthesis.

c. Temperature

Temperature plays a significant role in the growth of the plant. Proper growth of a plant occurs at a about 28o C to 30o C temperature and above 45o C will damage the protoplasm and hinders the growth.

d. Oxygen

Oxygen has a vital role in the growth of the plant. It helps in releasing metabolic energy essential for growth activities. It is necessary for respiration.

e. Light

Light has its own contribution in the growth of the plant. Light is important for growth and photosynthesis. Light stimulates healthy growth. Absence of light may lead to yellowish in colour. This is called etiolation.

II. Internal Factors

a.     Genes are intracellular factors for growth.

b.     Phytohormones are intracellular factors for growth. Example: auxin, gibberellin, cytokinin.

c.      C/N ratio.

The ratio of carbohydrates and nitrogenous compounds regulate the specific pattern of growth in plants. For example, if a plant contains more nitrogenous compounds as compared to carbohydrates it produces more protoplasm less mechanical tissues and vigorous vegetative growth. On the other hand, less nitrogenous compounds and more carbohydrates favour the synthesis of more wall material, less protoplasm, and more mechanical tissues.

4. Measurement of growth

Activity

Measurement of growth by direct method.

Step 1: Take ordinary scale. 

Step 2: Measure ground stem up to the growing point of the plant. 

Step 3: Use Indian ink and mark at regular intervals to measure the length of root, stem, and girth of the trunk.

Experiment: 1. Arc auxanometer:

The increase in the length of the stem tip can easily be measured by an arc auxanometer which consists of a small pulley to the axis of which is attached a long pointer sliding over a graduated arc. A thread one end of which is tied to the stem tip and another end to a weight passes over the pulley tightly. As soon as the stem tip increases in length, the pulley moves and the pointer slide over the graduated arc (Figure 15.8). The reading is taken. The actual increase in the length of the stem is then calculated by knowing the length of the pointer and the radius of the pulley. If the radius of the pulley is 4 inches and the length of pointer 20 inches the actual growth is measured as follows:

5. Sequence of developmental process in a plant cell

Development is a term that includes all the changes that an organism goes through during life cyle from germination of a seed to senescence. Diagrammatic representation of the sequence of processes which constitute the development of a cell of a higher plant is given in the figure. It is also applicable to tissues/organ. 

1.  Differentiation

The process of maturation of meristematic cells to specific types of cells performing specific functions is called differentiation.

2. Dedifferentiation

The living differentiated cells which had lost capacity to divide, regain the capacity to divide under certain conditions. Hence, dedifferentiation is the regaining of the ability of cell division by the differentiated cells. Example: Interfascicular cambium and Vascular cambium.

3. Redifferentiation

Differentiated cells, after multiplication again lose the ability to divide and mature to perform specific functions. This is called redifferentiation (Figure 15.9). Example: Secondary xylem and Secondary phloem.

4. Plasticity

Plants follow different pathways in response to environment or phases of life to form different kinds of structures.

This ability is called plasticity. Example: Heterophylly in cotton and coriander. In such plants, the leaves of the juvenile plant are different in shape from those in mature plants. On the other hand, the difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to the environment. This phenomenon of heterophylly is an example of plasticity.