Plant Growth

Growth can be defined as a vital process which brings about irreversible permanent change in any plant or its part with respect to its size, form, weight and volume.

Regions of growth : In unicellular plants there is overall growth and not confined to any specific region but in multicellular plants growth is restricted to specific regions having meristematic cells. On the basis of their position in the plant body (higher plants) meristems are divided into three main categories.

(1) Apical meristems : These meristems are found at shoot and root apex. As a result of activity of these meristems plant increases in length. In angiosperms and gymnosperms there is a group of meristematic cells but in bryophytes and pteridophytes there is a single tetrahedral cell found at the shoot apex.

(2) Intercalary meristems : These meristems are found above the nodes. As a result of the activity of these meristems increase in length takes place. e.g., Bambusa.

(3) Lateral meristems. : These meristems are made up of cells which divide in radial direction only. They form laterally placed new cells towards the centre and periphery. Cork cambium (phellogen) and vascular cambium are the examples of lateral meristems. Increase in girth of shoots and roots take place because of the activity of this cambium.

Phases of growth

(1) Cell division (Formative phase) : Growth is based on mitotic cell division.

(2) Cell enlargement : Cell division is followed by cell enlargement. The cell increases in size due to vacuolation (by absorption of water). The cell enlargement has been explained in two different ways. According to the first view, the turgor of the cell is responsible for cell enlargement. The other view considers that as a result of growth of the cell wall the volume of the cell increases.

(3) Cell maturation (Differentiation) : Cell differentiation following cell division and cell enlargement leads to the development of specialized mature tissue cell. e.g., xylem tracheids and trachea, sieve tubes and companion cells.

Growth curve : The rate of growth varies in different species and different organs. The young leaf sheath of banana grows for a time at the rate of almost three inches per hour. Growth begins slowly, then enters a period of rapid enlargement, following which it gradually decreases till no further enlargement occurs. The mathematical curve which represents this variation in growth rate is some what flattened S-shaped curve or sigmoid curve. Time in which growth takes place has been called grand period of growth. This term was coined by Sachs. The analysis of growth curve shows that it can be differentiated into three phases :

(1) Lag phase : The rate of growth is very slow in lag phase. More time is needed for little growth in this phase.

(2) Log phase (Exponential phase) : The growth rate becomes maximum and more rapid. Physiological activities of cells are at their maximum. The log phase is also referred to as grand period of growth.

(3) Final steady state (Stationary phase) or Adult phase : When the nutrients become limiting, growth slows down, so physiological activities of cells also slows down. This phase is indicates maturity of growth system. The rate of growth can be measured by an increase in size or area of an organ of plant like leaf, flower, fruit etc. The rate of growth is called efficiency index.

In many plants another phase is also evident in their growth curve. This is called linear phase or phase of maximum growth rate. Sachs called it as grand phase.

Measurement of growth

The following methods are designed to measure growth in length :

(1) Direct method : Measurement is done between two marked points by a scale at regular intervals.

(2) Horizontal microscope (Travelling microscope)  

(3) Auxanometer : Several kinds of auxanometers have been devised to measure the growth in length of a plant. Two of them are :

(i) Arch auxanometer

(ii) Pfeffer’s auxanometer (Automatic auxanometer)

(4) Bose’s crescograph : The crescograph invented by Sir Jagdish Chandra Bose is a more delicate instrument and gives magnification upto 10,000 times. The rate of growth of root can be measured by the use of a root auxanometer.

Factors influencing rate of growth : Growth is affected by the factor which affect the activity of protoplasm. It is affected by a large number of factors both environmental and physiological. Physiological factors such as absorption of water, minerals, photosynthesis, respiration etc, and environmental factors including climatic and edaphic both. The effect on these factors on one region of plant are also transmitted to other region of the plant.

Since growth is a resultant of many metabolic processes, it is affected by many external and internal factors, which are as follows :

External factors

(1) Light : Light affects variously e.g., light intensity, quality and periodicity.

(i) Intensity of light : In general, light retards growth in plants. High light intensities induce dwarfing of the plant. Plants at hill tops are short whereas those of a valley are quite tall. Very weak light induces the rate of overall growth and also photosynthesis. Development of chlorophyll is dependent on light and in its absence etiolin compounds in formed which gives yellow colour to the plant. The phenomenon is called etiolation. Similarly high light intensity affecting indirectly increases the rate of water lose and reduces the rate of water growth.

(ii) Quality of light : The different colours (different wavelengths) affect the growth of plant. In blue-violet colour light internodal growth is pronounced while green colour light reduces the expansion of leaves as compared to complete spectrum of visible light. The red colour light favours elongation but they resemble etiolated plants. Infrared and ultraviolet are detrimental to growth. However, ultraviolet rays are necessary for the development of anthocyanin pigments in the flowers. Blue and violet colours increase size of lamina of leaf.

(iii) Duration of light : There is remarkable effect on duration of light on the growth of vegetative as well as reproductive structures. The induction and suppresion of flowering are dependent on duration. The phenomenon is termed photoperiodism.

(2) Temperature : Temperature has pronounced effect on the growth of plant. The temperature cardinals for growth vary according to temperature zones. The minimum, optimum and maximum temperatures are usually 5^oC (arctic), 20 - 30^oC (temperate) and 35 - 40^oC (tropical). The optimum temperature needed for the growth of a plant is much dependent on the stages of development.

(3) Water : As water is an essential constituent of the living cell, a deficiency of water causes stunted growth.

(4) Oxygen : In poorly aerated soil there is low concentration of oxygen and a high concentration of $C{O}_2$. Under such conditions plants usually show stunted growth. Normal growth of most plants occurs only when abundant oxygen is present since $O_2$ is important for respiration. It has been reported that oxygen plays some important role during GI stage of cell division.

(5) Mineral salt : Absence of essential mineral salts results in abnormal growth. For example, the absence of nitrogen prevents protein-synthesis, while the absence of iron prevents chlorophyll formation and thus leads to pale and sickly growth of plants, known as chlorotic condition.

(6) Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticites etc have detrimental effect on plant growth. Citrus and Gladiolus are very sensitive to fluorides. Poor growth of tobacco is observed in regions where ozone concentration is high. White pine cannot survive under high $O_3$ concentration. Cotton plants are, similarly very sensitive to ethylene.

(7) Carbon dioxide : CO₂ is essential for photosynthesis and hence nutrition. Due to change in photosynthetic rate, with the increase or decrease in $C{O}_2$ concentration, the plant growth is also affected.

Internal factors

(1) Nutrition : It provides raw material for growth and differentiation as well as source of energy. C/N (Carbohydrate/Nitrogen) ratio determines the type of growth. High C/N ratio stimulates wall thickening. Less protoplasm is formed. Low C/N ratio favours more protoplasm producing thin walled soft cells. According to law of mass growth, the initial rate of growth depends upon the size of germinating structure (seed, tubes, rhizome, bulb, etc.)

(2) Growth regulators : These are manufactured by living protoplasm and are important internal growth regulators which are essential for growth and development. These growth regulators include several phytohormones and some synthetic substances.