📈 Growth

Growth

• Growth may be defined as a dynamic vital process which brings about a permanent and irreversible change in any plant or its parts with respect to size, form, weight and volume.
• The permanent change maybe either in positive direction or negative direction. For example, the dry weight in the sprouting potato tubers decreases during the early phase of the growth.
• Growth is diffused in animals but in plants, growth localized & irregular is (nail in plant stem, occupies same height till several year of growth in plant).
• Growth is primarily affected by two climatic factors which are light and temperature.
• Types of Growth:
  • Primary Growth: It is due to apical meristems or temporal meristems called intercalary meristems.
  • Secondary Growth: Lateral meristems responsible for the thickness.

Growth is Measurable

• Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it. Growth is, therefore, measured by a variety of parameters some of which are increase in fresh weight, dry weight, length, area, volume and cell number.
• You may find it amazing to know that one single maize root apical meristem can give rise to more than 17,500 new cells per hour, whereas cells in a watermelon may increase in size by upto 3,50,000 times. In the former, growth is expressed as increase on cell number, the latter expresses growth as increase in size of the cell. While the growth of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth in a dorsivental leaf.

Methods of growth measurment

• By direct observation
• By horizontal microscope
• By Crescograph (J.C. Bose) – It magnifies growth as 10,000 times.
• By Auxanometers

Phases of Growth

• The period of growth is generally divided into three phases, namely, meristematic, elongation and maturation.
• The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.
• The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.
• Further away from the apex i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications. Most of the tissues and cell types represent this phase.

Growth Rate

A. Arithmetic Growth

B. Geometric Growth

• The pioneering work on growth was done by Von Sachs. He plotted growth curve between time and growth which is known as sigmoid curve or S-curve or GP (Grand Period) – Curve.
• Growth pattern of cell, organisms is uniform under favorable conditions. Thus following phases of growth are recognized:
  • Lag Phase: It is the initial lag period where internal changes in the cell occur which are the preparatory to growth. Here the increase in size or weight is very slow or negligible.
  • Log Phase: It is the grand period of growth. Here growth is very fast.
  • Third Phase: Here Growth rate gradually decreases.
  • Fourth Phase: It is the phase where organism reaches to maturity and growth ceases.
  • Final Phase: It is phase of sentence where death of organism sets in.

👉🏻 On the basis of the different phases of growth, a sigmoid growth curve (i.e. ‘S’ shaped) is obtained.

• Growth curve is usually sigmoid. ^{IBPS AFO 2012}

Absolute and Relative growth rate

• Absolute growth rate: Total growth rate occurs in unit time in plant or plant parts.
• Relative growth rate: Total growth occurs in unit time with respect to initial parameter in plant or plant parts. Relative growth rate is generally high in young developing plant parts.

  

• Both possess same absolute growth rate i.e. 5 cm² in 7 days.
• But high relative growth rate is in leaf ‘A’ 100 % while in leaf ‘B’ it is 10%.

Development

• Development is the process of growth and differentiation of individual cells into tissues, organs and organisms. It is the resultant of growth.
• Growth includes both intracellular (genetic factors) or intercellular factors (chemicals such as plant growth regulators) while development includes light, temperature, water, oxygen, nutrition, etc.

Difference between Growth and Development

Tropisms

• Tropisms are movements caused by external stimulus.
• Phototropism - tropism in response to ligh
• Geotropism - tropism in response to the earth gravity
• Thigmotropism - tropism in response to mechanical stimulus.
• Skototropism - growth of vines toward a darkened region of the environment.

Factors affecting Growth & Development:

• Growth and Development is affected by physiological processes and environmental conditions.
• Absorption of water and minerals, photosynthesis, respiration etc. are the physiological processes which govern the growth and development to a very large extent.
• The environmental factors include the climatic factors and edaphic factors.
• The major two climatic factors viz. temp. and light are discussed here:

Temperature

• There is a pronounced effect of temperature on the growth of the plant. Growth occurs in the range of 4°C to 45°C but the cardinal temperature range is 28-33°C. The low temperature at night reduces the rate of respiration but high temp. during the day time increases photosynthesis and accumulation of synthases which in turn increases growth. That is why potato tubers growing on hills are much larger than those of the plains.
• The very high temperature generally stops the growth of plant by affecting many physiological processes. At high temperature, the protein component of the protoplasm is also coagulated and the protoplasm is killed. This effect of high temperature is called heat injury. But there are some plants which have some heat resistance mechanism like high sugar content, thick bark etc.
• The very low temperature also generally stops the growth. There are three types of injuries caused by low temperature:
  • Dessication: The plant tissues become desiccated and injured when the rate of absorption is very slow due to low temperature but transpiration rate is rapid.
  • Chilling Injury: The plant tissue in hot climate when exposed to low temperature (above the freezing point) for some time is either killed or severely injured. This injury is called chilling injury. The ripe banana becomes black when is kept in refrigerator is only due to chilling injury.
  • Freezing Injury: When the plants is exposed to very low temperature (below the freezing point), the protoplasm of the plant cell is dehydrated resulting in its coagulation due to the formation of ice crystals of water. The high concentration of the cell sap aggravates the precipitation of protein and thus resulting into the death of the cell. But there are many perennial plants which withstand the freezing injury because of the high osmotic concentration of the cell sap. Such frost resistance (or hardiness) nature of the plant lower the freezing point and reduces the amount of water.

Light

• The intensity, quality and duration of light affect the growth. Light controlled morphogenesis of plant is called photomorphogenesis.
• The weak intensity of light promotes shortening of intenodes and expansion of leaves. Very weak intensity reduces the rate of overall growth. Very high intensity reduces the growth rate indirectly, increasing the water loss.
• Blue violet light enhances internodal growth whereas green light reduces expansion of leaves.
• Red colour is the most favorable light quality for growth. Beyond the visible spectrum i.e. infrared and uv-rays are detrimental for growth. The duration of light has pronounced effect on the vegetative and reproductive growth of plant. This phenomenon is called Photoperiodism.
• Longer periods of light causes luxuriant vegetative growth in most of the plants. Garner and his co-workers found that the amount of vegetative growth was proportional to the duration of day light.

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