As the plants require oxygen, water, sunlight, and nutrition to grow and develop, they do require certain chemical substances to manage their growth and development. These chemicals substances are called Plant Growth Regulators and are produced naturally by the plants itself.
The plant growth regulators are simple organic molecules having several chemical compositions. They are also described as phytohormones, plant growth substances, or plant hormones.
Based on their action, plant growth regulators are broadly classified into two major groups: Plant growth promoters and Plant growth inhibitors.
Auxins, Gibberellins, and Cytokinins are grouped into Plant growth promoters and Abscisic acid and Ethylene are grouped into Plant growth inhibitors.
Note: Ethylene can be grouped either into the promoters or into the plant inhibitors.
Let us study more in detail about the Plant Growth Regulators.
Plant Growth Promoters
The first phytohormones to be discovered is the Auxin and it was discovered by the biologist Charles Darwin.
Auxins are one of the most important plant hormones and the chief naturally occurring auxin is indole-3 acetic acid – IAA and other related compounds. The term Auxins is derived from the Greek language meaning to grow.
These plant growth regulators are generally produced at the points of stems and roots from where they are transported other parts of the plants. These plant hormones include both natural and synthetic sources. Indole-3-acetic acid and indole butyric acid are obtained from natural plants sources, whereas naphthalene acetic acid and 2, 4-dichlorophenoxyacetic acid are obtained from synthetic sources.
Functions of Auxins
- Promote flowering in plants
- Used in the process of plant propagation.
- Used by gardeners to keep lawns free from weeds.
- Involved in the initiation of roots in stem cuttings.
- Prevent dropping of fruits and leaves at early stages.
- Control xylem differentiation and help in cell division.
- Auxins are widely used as herbicides to kill dicot weeds.
- Used for the production of fruit without preceding fertilization.
- Promote natural detachment (abscission) of older leaves and fruits.
- Apical dominance may occur in which the growth of lateral buds is inhibited by the growth of apical buds. In such cases, the shoot caps may be removed.
Gibberellins are an extensive chemical family based on the ent-gibberellane structure. The first gibberellin to be discovered was gibberellic acid. Now there are more than 100 types of gibberellins and are mainly obtained from a variety of organisms from fungi to higher plants.
They are acidic and are denoted as follows – GA1, GA2, GA3etc. GA3
Functions of Gibberellins
- Delay senescence in fruits.
- Involved in leaf expansion.
- Break bud and seed dormancy.
- Promote bolting in cabbages and beet.
- Help fruits like apples to elongate and improve their shape.
- Used by the brewing industry for increasing the speed on the malting process.
- Used as the spraying agent to increase sugarcane yield by lengthening of the stem.
- Used to fasten the maturity period in young conifers and promote early seed production
- Helps in increasing the crop yield by increasing the height in plants such as sugarcane and increase the axis length in plants such as grape stalks.
These are produced in the regions where cell division occurs; mostly in the roots and shoots. They help in the production of new leaves, lateral shoot growth, chloroplasts in leaves etc. They help in overcoming apical dominance and delay aging of leaves.
Functions of Cytokinins
- Break bud and seed dormancy.
- Promotes the growth of the lateral bud.
- Promotes cell division and apical dominance.
- They are used to keep flowers fresh for a longer time.
- Used in tissue culture to induce cell division in mature tissues.
- Promote lateral shoot growth and adventitious shoot formation.
- Promote nutrient mobilization which in turn helps delay leaf senescence.
- Helps in delaying the process of aging (senescence) in fresh leaf crops like cabbage and lettuce.
- Involved in the formation of new leaves and chloroplast organelles within the plant cell.
- Used to induce development of shoot and roots along with auxin, depending on the ratio.
Plant Growth Inhibitors
It is a growth inhibitor which was discovered in the 1960s. It was initially called dormant. Later another compound abscisin-II was discovered and are commonly called as abscisic acid. This growth inhibitor is synthesized within the stem, leaves, fruits, and seeds of the plant. Abscisic acid mostly acts as an antagonist to Gibberellic acid. It is also known as the stress hormone as it helps by increasing the tolerance of plants to different kinds of stress.
Functions of Abscisic acid
- Stimulates closing of stomata in the epidermis.
- Helps in development and maturation of seeds.
- Inhibits plant metabolism and seed germination.
- It is involved in regulating abscission and dormancy.
- It is widely used as a spraying agent on trees to regulate dropping of fruits.
- Induces dormancy in seeds and helps in withstanding desiccation and other unfavorable growth factors.
Ethylene is a simple, gaseous plant growth regulator, synthesis by most of the plant organs including ripening fruits and aging tissues. It is an unsaturated hydrocarbon having double covalent bonds between and adjacent to carbon atoms.
Ethylene is used as both plant growth promoters and plant growth inhibitors. Ethylene is synthesized by the ripening fruits and aging tissues.
Functions of Ethylene
Ethylene is the most widely used plant growth regulator as it helps in regulating many physiological processes.
- Induce flowering in the mango tree.
- Promotes sprouting of potato tubers.
- Breaks the dormancy of seeds and buds.
- Enhances respiration rate during ripening of fruits.
- Applied to rubber trees to stimulate the flow of latex.
- Promotes abscission and senescence of both leaves and flowers.
- Used to stimulate the ripening of fruits. For example, tomatoes and citrus fruits.
- Affects horizontal growth of seedlings and swelling of the axis in dicot seedlings.
- Increases root growth and root hair formation, therefore helping plants to increase their absorption surface area.
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