Plant growth is a process characterized by the irreversible change in the size of cells and organs that is a result of cell division and enlargement. Plant development is the process of progression from seed germination to maturation.
Growth and Development are often used to mean the same thing in discussions in colloquial language. However scientifically speaking, growth and development are two distinctly different events in the organization and formation of a mature plant and its body structures. Plant growth and development are very lengthy and very complicated processes. Both begin at germination and last all through the lifetime of the plant.
Development is defined as the process of progressing from an earlier to a later stage in the process of maturation. For example, development occurs when from a fertilized egg of a single cell a fully matured plant is formed. In the process of plant development processes like growth, morphogenesis and differentiation (formation of tissues with specialised and/or specific functions) take place. How a plant develops is determined by the interaction of the inherited genetic material and the environment.
On the other hand growth is defined as the irreversible change in cell size due to cell elongation. It is also used to explain the process of increase in plant organs' size due to cell division and elongation. Elongation in cells requires change in the elasticity of cell walls along with an increase in the size of vacuoles and/or water content of the vacuole. Growth can be of two types:
Growth is called determinate when an organ or a part of or the whole of the organism stops growing after reaching a certain size. For example, plant leaves and flowers mostly show determinate growth.
Growth is said to be indeterminate when the cells of the organ, part or organism continues to divide indefinitely. Plants as a whole have indeterminate growth.
Two important processes involved in the growth and development of plants are:
Differentiation is the process by which undifferentiated cells transform into cells with specialised functions and having distinct morphological and physiological characteristics. All cells in the plant body have the same genetic makeup. The only thing separating the cells of different parts or organs from each other in morphology and physiology is the way the cells undergo differentiation. Differentiation happens by overexpressing or expressing or repressing certain genes. The differentiation of the cells is dependent on the location of the cells. For example, cells are placed towards the ground or water source form the root, while the cells exposed to the sun form shoot cells. Root cells do not differentiate into flower cells, and nor do shoot cells transform into root cells.
Dedifferentiation is the exact opposite of differentiation. In this process, the mature and differentiated cells of the plant are stimulated under specific conditions to divide and become undifferentiated and lose all the specific characteristics it acquired previously. These dedifferentiated cells again differentiate. This is noted when the plant tissues undergo damage.
Unlike animals, plants show indeterminate growth. As animals grow, when they turn into a fully matured animal they stop growing. When they are growing the different parts of the animal grow until they reach a size which is determined by their genetic make-up. Plants on the other hand never stop growing. They continue to grow all through their lifespan. As young plants grow older, the growth gets restricted to their meristematic tissues. Meristematic tissues are young, actively growing/dividing tissues found at the apexes of the plant.
The continuous growing patterns of the plant result in the formation of two types of tissues, they are:
The Primary Tissues:
The primary tissues are involved with apical growth. Example: apical meristems
The Secondary Tissues:
The tissues involved in the lateral growth of the plant are called the secondary tissues. Example: lateral meristems.
Apical meristems are zones of cell division, made up of readily dividing cells, which are responsible for the increase in the height of the plant in both the shoot and root region. It is responsible for growth in the length of the primary plant body. This is because primary tissues develop from primary meristems. Apical meristems have cells rapidly dividing and elongating. Rapid cell elongation is possible due to rapid enlargement of the vacuoles in the primary tissue cells. This results in the stem and roots increase in girth till they reach a maximum size. The maximum size is determined by the elasticity of the cells. After reaching the maximum size, the primary cells do not grow any further. Herbaceous plants only have primary meristems. This is why they only grow in axial length, while their girth doesn't grow so much in proportion.
However in Woody plants we see that they grow enormous in size as well as in girth. This is because the secondary tissues formed by the lateral meristems provide strength and protection. Secondary tissues develop around the periphery of the roots and shoots of the plant.
The plant growth regulators are hormones which control the growth of plants. The plant hormones are small organic molecules with low molecular weight and a wide variety of chemical composition.
Depending upon the primary trigger or conditions of synthesis, plant growth regulators are classified to be of 2 types:
Healing plant growth promoters
Growth promoting plant growth regulators
Healing Plant Growth Promoters:
These plant growth promoters act on the plant when there is stress of some sort. These plant growth promoters are simple organic compounds which are produced in response to wounds and/or stresses. The stresses can be of the biotic origin of the abiotic origin. The biotic stressors include biological agents like animals, insects, pests and so on. The abiotic stressors include other factors like temperature, moisture and so on.
Growth Promoting Plant Hormones:
These are simple compounds called plant growth promoters. The plant growth regulators have diversified chemical properties and constitution. These PGRs help in the growth of plants. Examples of plant growth promoters include auxins and cytokines.
1. What are Auxins?
Auxins are a type of plant growth-promoting plant hormone. They were first isolated from human urine. Auxins refer to a group of hormones with similar chemical formula and functioning. The most commonly found Auxins is Indole Acetic Acid (IAA). Other naturally occurring Auxins include Indole Butyric Acid (IBA), Indole -3- Propionic Acid, phenylacetic acid and so on.
Auxins are also synthesized artificially; some artificial examples of Auxins include 2,4D also known as 2,4-Dichlorophenoxyacetic acid, 1-naphthalene acetic acid and so on.
Auxins are very important hormones involved in regulating plant growth and also tropic based development. Auxins are mainly produced in the meristematic region of the plant.
2. What are the Functions of Auxins?
The functions of Auxins involve:
They help in increasing cell division
They help in cell elongation
By the combination of cell elongation and cell division, Auxins promote axial growth of plants.
Auxins help in geotropic development of roots.
Auxins help in the process of root initiation. Root development from a branch when added to soil or from newly planted seeds are all encouraged by auxins.
Auxins are used for preventing ageing in plants.
Auxins are important for encouraging apical dominance. This means that in a healthy plant the apical growth is more than its secondary growth. However in plants without Auxins the secondary growth seems to increase while the apical growth decreases.
Auxins help phototrophic development of plants.
Auxins are used to get rid of weeds. Auxins make weeds grow too fast and this results in the weeds dying.
Auxins help in encouraging ethylene production.
3. What are Gibberellins or Gibberellic Acids?
Gibberellins are plant hormones which promote growth. Chemically all Gibberellins are tetracyclic diterpene acids. They are the largest molecules of the gibberellin acids group. They are produced in the plastids and later modified in the endoplasmic reticulum and the cytosol. More than 126 Gibberellins have been identified to date. Gibberellins can have either 19 or 20 carbon atoms as a part of their structure.
4. What are the Functions of Gibberellins?
The functions of gibberellins are:
Gibberellins help in breaking seed dormancy. This means that Gibberellins help in initiating germination.
Gibberellins help in cell division.
Gibberellins help encourage cell elongation.
Gibberellins help in stem elongation.
It helps in the breakdown of starch through the production of alpha-amylase enzyme.
Gibberellins encourage flowering.
They help in elongation of internodes.
5. What are the Names of Some Other Plant Hormones?
Some major important hormones of plant growth and development are Cytokines, Ethylene, and Abscisic acid.