Plant growth regulators are the chemical substances that control the aspects of growth and development within the plants completely. They are also known as plant growth hormones or phytohormones.

Plant growth hormones are those organic compounds that are either produced naturally within the plants or are synthesized in laboratories. They control and modify physiological processes like the growth, development and movement of plants.

Plant growth regulators can be broadly classified into two categories namely-

• Plant growth promoters

• Plant growth inhibitors

Auxins, Gibberellins and Cytokinins are grouped into Plant growth promoters while Abscisic acid and Ethylene are grouped into Plant growth inhibitors. Ethylene can be grouped either into the promoters or into the plant inhibitors.

Types of Plant Growth

• Primary and Secondary Growth

The meristematic cells present at the root and shoot apiece segregate mitotically and increase the length of the plant body. This is known as primary growth. Secondary growth is referred to as the increase in the diameter of the plant body by the division of the secondary meristem.

• Unlimited Growth

When the plant continually grows from the germination stage to death, it is called unlimited growth.

• Limited Growth

In this stage, the plant parts stop growing after acquiring a certain size.

actors affecting Plant Growth

Four major factors affect the growth of plants. They are:

1. Light

Plants are autotrophs which means they require light for manufacturing their food. Limited light or the absence of it greatly affects the growth of the plant. The intensity of light, quality of light and light duration influence the movement of stomata, chlorophyll synthesis, photosynthesis, and various other physiological factors. Light also helps in the process of flowering and fruiting. During winters when the days are short, the growth of the plants is retarded.

1. Water

We know that plants cannot survive without water and around 90% of the plant body comprises water. Plants become deadly in the absence of water and die. Water present in the soil is absorbed by the plant which absorbs and transports the nutrients along with it to keep it hydrated.

1. Temperature

Plant growth is greatly influenced by temperatures. High temperature helps speed up transpiration, photosynthesis and germination processes. Low temperature is considered harmful for the plants as it slows down their growth.

1. Nutrients

Plants need proper nourishment for their growth and development. Soil nutrients are divided into macronutrients and micronutrients. Nitrogen, potassium, calcium, magnesium, sulphur and phosphorus are some of the most important macronutrients that are required by plants. The deficiency of these nutrients in plants makes them prone to several diseases. Even if a single nutrient is lacking, it results in stunted growth of the plant.

Plant Growth Promoters

Auxins

The first phytohormone that was discovered is the Auxin discovered by the biologist Charles Darwin. Auxins play a very important role as a plant hormone. The chief naturally occurring auxin is indole-3 acetic acid – IAA and other related compounds. The term Auxin is derived from the Greek language which means ‘To grow’.

Gibberellins

Gibberellins are an extensive chemical family based on the ent-gibberellin 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.

Cytokinins

Cytokinins 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 the ageing of leaves.