Chapter 13: Plant Growth and Development

Growth – It is an irreversible, permanent increase in the size of an organ or its parts or even of an individual cell. Growth is supplemented by metabolic activities taking place due to the energy.

Differentiation – The cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed differentiation.

Development – All the changes that organisms go through in their lifetime are called development.

Dedifferentiation – Plants which has lost the capacity to divide can regain the capacity under certain conditions. This phenomenon is called dedifferentiation. Example: meristem formation.

Redifferentiation – Dedifferentiation produces cells that once again lose the capacity to divide but mature to perform specific functions are said to be redifferentiated.

Determinate growth – The ability of a cell, tissue or organism to grow for a certain period is called determinate growth. In most plants, growth is indefinite, where some plants grow to a certain level and then stop growing.

Meristem – Plant tissue containing undifferentiated cells (meristematic cells) is called meristem.

Growth rate – The increase in growth per unit of time is called a growth rate.

Growth is a consequence of an increase in the quantity of protoplasm. Measuring the protoplasmic growth includes several parameters, to name a few – increase in height, weight, number of cells, fresh tissue sample, length, area, volume, etc. Hence, it is difficult to demonstrate any one parameter of growth throughout the life of a flowering plant.

(a) In arithmetic growth, only one daughter cell continues to divide while the other differentiates and matures. The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate.

It can be mathematically expressed as follows:

L_t = L_O + rt

L_t = length of time ‘t’

L_O = length at the time ‘zero’

r= growth rate/elongation per unit time

(b) In geometric growth, the initial growth is slow (lag phase), and it increases rapidly after that at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. The growth slows down due to limited nutrient supply leading to a stationary phase. The number increases in a multiplicative pattern in geometric growth.

In geometric growth, if we plot the parameter of growth against time, we get a typical sigmoid or S-curve.

Exponential growth can be expressed as

W₁ = W₀ ert

W₁ = final size

W₀ = initial size of the period

r=growth rate

t=time growth

e=base of natural logarithms

‘r’ is the relative growth, as well as the measure of the ability of plants to generate new plant substances, which is termed as efficiency index. Thus, the final size of W₁ is dependent on the initial size W₀

(c) Sigmoid growth curve

It is an S-shaped graph that is generated by plotting growth against time and has four main components: a slow lag phase, exponential phase or rapid phase, stage of diminishing growth and stationary phase.

(d) Absolute and relative growth rates – Absolute growth rate is the net growth per unit of time. The relative growth rate is the growth rate per unit time per unit of initial growth.

Plant growth regulators are the intercellular intrinsic factors (chemical substances) that are responsible for the growth and development of plants. The five main groups of natural plant growth regulators (PGR) are as follows:

• Auxins
• Gibberellins
• Cytokinins
• Abscisic acid
• Ethylene

These PGRs are synthesised in different plant parts, governing various differentiation and developmental events that take place in the life cycle of a plant.

Gibberellins

Discovery:

• Denoted as GA₁, GA₂, GA₃, gibberellic acid, or GA₃, was one of the first gibberellins to be discovered; they are all acidic.
• There are more than 100 gibberellins reported from widely different entities such as fungi and other higher plants.

Physiological functions

• Gibberellins produce a wide range of physiological responses in plants.
• They can cause an increase in the length of grapes stalks as they are capable of bringing about an increase in the length of the axis.
• They cause fruits such as apples to elongate and improve their shape.
• They are responsible for delaying the process of senescence.

Agricultural/horticultural applications

• As the senescence process is delayed, the fruits can be left on the tree for longer in order to extend the market period.
• In the brewing industry, gibberellic acid, or GA₃, is used to speed up the malting process.
• As sugarcane stores carbohydrate as sugar in their stems, spraying the sugarcane crop with gibberellins causes the length of the stem to increase, hence causing an increase in the yield by as much as 20 tonnes per acre.
• The maturity period of juvenile conifers can be hastened by spraying with GAs resulting in early seed production.
• It also promotes the process of bolting in beetroots, cabbages, etc. Bolting is the internode elongation observed just before the flowering process.

Photoperiodism can be termed as the response of plants to periods of day/night. It is theorised that the hormonal substance that is responsible for flowering, is formed in the leaves, which subsequently migrates to the shoot apices and alters them into flowering apices. This process of photoperiodism helps in studying the response of flowering in different crop plants when the duration of exposure to light is considered.

Vernalisation is the phenomenon where the process of flowering in some plants is either quantitatively or qualitatively dependent on exposure to lower temperatures. In particular, it refers to promoting the flowering process through a period of lower temperatures. The process prevents precocious reproductive development late in the growing season, which thereby enables the plant to have sufficient time to attain maturity.

Abscisic acid is responsible for stimulating the closure of stomata in the epidermis and raises the tolerance of plants to different types of stresses. Hence, it is also called the stress hormone. Abscisic acid is responsible for promoting seed dormancy, thereby ensuring the germination of seeds during favourable conditions. This helps the seeds to withstand desiccation and to induce dormancy towards the end of the growing season in plants, thereby promoting abscission of the fruits, leaves and flowers.

The ability of higher plants to retain the capacity to have indefinite growth through their life span is because of the presence of meristems at specific locations of their body. The cells, as a result of these meristems, have the capacity to divide and perpetuate on their own. This is why the growth in higher plants is open. A few of these cells undergo differentiation, always subsequent to a few rounds of cell division. Thus, differentiation is open too.

In a few plants, flowering depends on relative durations of dark and light periods. Both the long-day plants and the short-day plants can flower at the same place, with the condition being that they are supplied with sufficient photoperiod.

Listed below are the plant growth regulators for the corresponding events:

(a) induce rooting in a twig. – Auxins

(b) quickly ripen a fruit. – Ethylene

(c) delay leaf senescence. – Cytokinins

(d) induce growth in axillary buds. – Cytokinins

(e) ‘bolt’ a rosette plant. – Gibberellins

(f) induce immediate stomatal closure in leaves. – Abscisic acid

No, a defoliated plant will not respond to the photoperiodic cycle. This is because the leaves are the sites of perception of dark or light duration. Hence, if leaves were not present, plants would not respond to light.