Plant Growth Regulators (PGRs) – Class 11 | Chapter – 15 | Biology Short Notes Series PDF

Plant Growth Regulators (PGRs): Plants are an important part of our ecosystem which ensure the existence of life on earth. One of the most important sources of their growth is sunlight. The process of photosynthesis becomes possible only because of the presence of fresh air and light on earth. Apart from this, another most important source which is responsible for the growth of the plants is called as plant growth regulators. These are mainly composed of a group of hormones, also known as phytohormones. These hormones become an intrinsic element of the plants which consistently play an important role in regulating their growth and development. They are chemical substances produced naturally by plants.

Discovery of Plant Growth Regulators

The most remarkable discovery in history has been associated with the findings of the transmittable substance in the plants which in turn advances its growth and development. This invention was the work of the scientist Charles Darwin and his son Francis Darwin. In one of his experiments, he acknowledged the growth of the coleoptile of the canary grass, that too, in the direction of the light. This recognition made him believe in the presence of such regulators which help in the growth of these plants. Later, this chemical substance contributing to growth was termed as auxin, however, even further, various other experiments discovered multiple types of plant growth regulators. For example, Gibberellins or gibberellic acid.

Types of Plant Growth Regulators (PGRs)

There are the following types of plant growth.

  • Primary and Secondary Growth: The growth of a plant is termed primary when the same happens through the mitotic division of the meristematic cells which are present at the root and the shoot of the plants. Whereas, the secondary growth in a plant takes place through the division of the secondary meristem, which, in turn increases the diameter of the body of the plants.

Primary and Secondary Plant Growth

  • Unlimited Growth: The plant growth is termed as unlimited if there is a continuous increase in its length from the stage of germination to its entire life cycle until it dies.
  • Limited Growth: This refers to a type of plant growth which is limited in nature. Such types of plants can only attain a certain height, then eventually stop growing.
  • Vegetative Growth: This type of plant growth is not at all associated with the growth of flowers, however, only involves the growth and development of stem, leaves, etc.
  • Reproductive Growth: This type of plant growth mainly deals with the growth of the flowers in a plant.

What are the Factors affecting Plant Growth?

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

  • 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. 

  • 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. 

  • 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.

  • 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.

Characteristics of Plant Growth Regulators (PGRs)

Along with some chemical substances, plants also require water, oxygen, sunlight and nutrition to grow and develop well. These chemicals are called Plant Growth Regulators and are produced naturally by the plants themselves. They are also described as phytohormones, plant growth substances, or plant growth hormones as they can accelerate the growth of plants. 

Plant growth hormones exhibit the following characteristics:

  • Differentiation and elongation of cells

  • Formation of leaves, flowers and stems

  • Wilting of leaves

  • Ripening of fruit

  • Seed dormancy, etc

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By Team Learning Mantras