Plant Growth and Development

Growth is an irreversible permanent increase in size of an organ, its parts or an individual cell.

Differentiation is the process in which cells become specialised and mature to perform specific functions.

Development is the sum of growth and differentiation.

Dedifferentiation is the regain of the capacity to divide by living differentiated cells.

Redifferentiation is the process in which these cells again lose the capacity to divide and mature into specialised cells.

Determinate growth is growth that stops after a particular size or stage is reached.

A meristem is a region of actively dividing cells that can self-perpetuate and produce new cells.

Growth rate is the increase in growth per unit time.

No single parameter can show growth throughout the life of a flowering plant because different parts and stages of the plant grow in different ways. Sometimes growth is seen as an increase in length, sometimes area, volume, fresh weight, dry weight, or cell number. For example, a root may elongate, a leaf may increase in area, and a fruit may increase in weight. Also, plant growth is indeterminate, so the kind of measurement suitable at one stage or for one organ may not suit another stage or organ. Therefore, growth has to be measured using different parameters depending on the plant part and its stage of development.

In arithmetic growth, after mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. Because the increase is constant, the growth is linear.

It is expressed as:

$L_t = L_0 + rt$

where $L_t$ is length at time $t$, $L_0$ is initial length, and $r$ is growth rate.

In geometric growth, both daughter cells formed after mitotic division retain the capacity to divide and keep dividing.

The growth is usually:
- slow at first: lag phase
- then rapid: log/exponential phase
- then slows down because of limited nutrients: stationary phase

So, it produces a typical sigmoid or S-shaped pattern.

A sigmoid growth curve is an S-shaped curve.

It shows three phases:
1. Lag phase — initial slow growth
2. Log/exponential phase — rapid increase
3. Stationary phase — growth slows down

It is typical of living organisms growing in a natural environment.

Absolute growth rate is the measurement and comparison of the total growth per unit time.

Relative growth rate is the growth per unit time expressed on a common basis, such as per unit initial parameter.

The five main groups of natural plant growth regulators are:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic acid (ABA)
5. Ethylene

### Note on auxins
Discovery:
- Charles Darwin and Francis Darwin observed phototropism in coleoptiles.
- Later, F.W. Went isolated auxin from oat coleoptile tips.

Physiological functions:
- Promote root initiation in stem cuttings
- Promote apical dominance
- Help in fruit and leaf abscission regulation
- Induce parthenocarpy
- Control xylem differentiation and help in cell division

Agricultural/horticultural applications:
- Used for rooting of cuttings
- Used to produce seedless fruits
- Used as herbicides such as 2,4-D for dicot weeds
- Used in tea plantations and hedge-making by removing apical buds