Phases of Plant Growth and Plant Growth Rate

Plant growth is a fundamental biological process, characterised by an irreversible increase in a plant's size, mass, and volume. It's a complex phenomenon that's influenced by both internal (hormonal) and external (environmental) factors. We can break this process down into distinct phases, each with its own unique characteristics. Understanding these phases is crucial for comprehending how plants develop from a tiny seed into a mature organism.

Characteristics of Plant Growth

1. Plant growth generally is indeterminate :

• Plant growth is unique because plants retain the capacity for unlimited growth throughout their life.
• This ability of plants is due to the presence of meristems at specific locations in their bodies. The cells of such meristems have the capacity to divide and self-perpetuate.
• The product, however, soon loses the capacity to divide, and such cells make up the plant body.

2. This form of growth, in which new cells are continually added to the plant body by meristem activity, is called the open form of growth.
3. Plant growth is of two types. 

Primary growth: Root apical meristem and shoot apical meristem are responsible for the primary development of the plants and principally contribute to the elongation of the plants along their axis.

Secondary growth: In dicotyledonous plants and gymnosperms, the lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth and contribute to the increase in the girth of the organs (root, stem).

1.0Growth is Measurable

• At the cellular level, growth can be quantified by the increase in protoplasmic mass. Still, it is tough to measure directly, so growth is measured by a variety of parameters, which are:

(a) Increase in fresh weight

(b) Increase in dry weight

(c) Increase in surface area/volume

(d) Increase in the number or size of cells.

• 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 an increase in cell number; in the latter, growth is expressed as an increase in cell size. While the development of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth of a dorsiventral leaf.
• The Auxanometer measures growth.

2.0The Three Phases of Plant Growth

The period of growth is generally divided into three phases, namely :

(i) Meristematic phase

(ii) Elongation phase

(iii) Maturation phase

1. Meristematic or Cell Division Phase

• The constantly dividing cells at both the root and shoot apices represent the meristematic phase of growth. 
• The cells in this region are small and characterised by abundant plasmodesmal connections.
• Intercellular spaces are absent; if present, then they are tiny.
• Cell walls are primary, thin, and cellulosic.
• Cells are rich in protoplasm and possess large, conspicuous nuclei.

2. Elongation or Cell Enlargement Phase

• The cells proximal to the meristematic zone are in the elongation phase. 
• Cells in this region are characterised by :
• Increased vacuolation
• Cell enlargement
• New cell wall deposition

3. Maturation or Cell Differentiation Phase

• The cells more proximal to the elongation phase represent the maturation phase. 
• Cells of this zone attain their maximal size in terms of wall thickening and protoplasm modifications.

Measuring Plant Growth: The Growth Rate

• Increased growth per unit time is termed the growth rate.
• The growth rate shows an increase that may be arithmetic or geometric.

(i) Arithmetic growth

• In arithmetic growth, only one daughter cell among the two further divides while the other differentiates and becomes mature (stops dividing).
• Ex. Root & Shoot elongation at a constant Rate.
• It is mathematically expressed as
• L_t = L₀ + rt
• Where L_t: length at time 't' 
• L₀: length at time 'zero'
• r: growth rate/elongation per unit time.
• Its curve is linear.

(ii) Geometric Growth

• Here, both progeny cells following mitotic divisions retain the ability to divide and continue dividing.
• Ex: Early embryonic development/division in the zygote, division in a unicellular organism.
• It is mathematically represented as

W₁ = W0ert

Where W₁ - final size (Weight, height, number, etc.)

W₀ - initial size at the beginning of the period.

r - growth rate

e - base of natural logarithms.

• In most systems, the initial growth is slow (lag phase) and it increases rapidly thereafter at an exponential rate (log or exponential phase), which is also called the "grand phase of growth". However, with a limited nutrient supply, growth slows, leading to a stationary phase or steady state. 
• If we plot the parameter of growth against time, we get a typical sigmoid or S-curve. A sigmoid curve is a characteristic of living organisms growing in a natural environment. It is typical for all cells, tissues and organs of a plant.
• Here, r = relative growth rate, which is also the measure of the ability of the plant to produce new plant material, referred to as the efficiency index.
• Quantitative comparisons between the growth of living systems can also be made in two way

• Absolute growth rate: The Measurement and comparison of total growth per unit time is called the absolute growth rate.
• Relative growth rate: The growth of the given system per unit time, expressed on a common scale, e.g., per unit of the initial parameter, is called the relative growth rate.

Conditions For Growth

• Water, oxygen and nutrients are essential for growth. Plant cells grow in size through cell enlargement, which in turn requires water. Cell turgidity helps in extension growth. Thus, plant growth and further development are closely linked to the plant's water status. Water also provides the medium for enzymatic activities needed for growth.
• Oxygen helps release metabolic energy, which is essential for growth. Nutrients (macro and microessential elements) are required by plants for the synthesis of protoplasm and as a source of energy.
• In addition, every plant organism has an optimal temperature range for its growth. Any deviation from this range could be detrimental to its survival. Environmental signals such as light and gravity also affect certain phases/stages of development.