Introduction

Plants grow by following a development process. All the plants start growing from one cell (the zygote). Still, some cells grow and form the roots while the others grow as leaves, flowers and stems. How? The answer is plant growth hormones or regulators.

These growth regulators called Phytohormones are a group of organic chemicals secreted in trace amounts to carry out the physiological activities in the plant. Phytohormones can alter plant growth based on the stage of development of the plant.

Before diving into the growth regulators, let us look into the process of plant growth.

Types of Plant Growth

The presence of meristematic tissue at the apex of shoot and root gives plants the ability to grow forever. Meristematic cells keep undergoing the cell division cycle and new cells are constantly being added to the plant.

Such a mode of growth is called an open growth system. Meristems help the plants to grow by constantly adding new cells.

Secondary growth involves the increase in girth by growing vascular and cork cambium (which appeared in later stages).

Phases of Plant Growth

Based on the state of development, plant growth is divided into 3 different phases.

1. Meristematic-In this the cells are continuously dividing. 
2. Elongation-In this stage the already divided cells show elongation and thus increase the length of the plant.
3. Maturation-This is the last  stage in which the maturation of the plant organ takes place.  

Factors Affecting Plant Growth

Since growth is a metabolic process, it is affected by several factors.

Note that only the factors affecting the activity of protoplasm can alter the growth of a plant.

These factors are broadly classified as:

• External 
• Internal

External Factors 

• Light: 

The intensity, duration and quality of incident light play an important role in plant growth. 

• Temperature: 

A plant requires optimum temperature for its stage of development. 

Also, the optimum temperature depends on the temperature zone of the plant 

The minimum temperature for plant growth based on temperature zone are:

• 5 degrees celsius for the Arctic zone 
• 20 degrees celsius for the temperate zone
• 35 degrees celsius for the tropical zone

• Water: 

If not watered regularly, the plant may exhibit a stunted appearance.

• Oxygen: 

Oxygen plays a direct role in the initial stage of cell division. Oxygen deficiency may result in stunted growth of a plant. Optimum oxygen concentration in the soil is essential for respiration.

• Mineral Salts:

Just like oxygen, mineral salts like Nitrogen are essential to avoid abnormal growth in plants.

Internal Factors 

1. Nutrition: 

Plants being autotrophs require raw material for their growth. Nutrients like Carbon and Nitrogen directly affect their growth. 

2. Growth Regulators:

Growth regulators include phytohormones and some synthetic substances which are crucial for the growth and development of plants.

Characteristics of Plant Growth Regulators

Plants require some chemical substances for their growth and development. They are called plant growth hormones or growth regulators. These chemical substances are produced by the plants themselves.

Let us look at some of the characteristics of these chemical substances.

• They lead to the differentiation and elongation of cells
• They lead to the development of leaves, stems, and flowers
• They are responsible for the wilting of leaves
• Fruits ripen due to plant growth hormones
• Growth regulators can also lead to seed dormancy

Plant hormones are divided into 2 major groups:

1. 1. Plants growth promoters 

• Plant Growth inhibitors 

Promoters: As the name suggests, this group of hormones ‘promote’ plant growth by their action.

• Auxins
• Gibberellins 
• Cytokinins 

They fall under the category of Promoters. 

Inhibitors: These phytohormones ‘inhibit’ the growth of plants by inducing the phase of dormancy. 

• Ethylene 
• Abscisic Acid

They fall under the inhibitors group.

Note: Ethylene is considered both, a promoter as well as an inhibitor.

Ethylene readily promotes transverse growth but restricts longitudinal growth. 

Plants growth promoters

 Auxins 

• They lead to flowering in plants
• Gardeners use Auxins to keep lawns free of weeds
• It prevents the dropping of leaves and fruits at early stages
• It helps in the natural detachment of older fruits and leaves
• They help in the development of roots in stem cuttings
• Auxins are very useful in the process of plant propagation

Gibberellins

• They help in the expansion of leaves
• They delay the senescence in fruits
• These hormones lead to bolting in cabbages and beet
• They promote the elongation of some fruits and enhance their shape
• It is sprayed on sugarcane to increase its yield by elongating the stem
• It helps to fasten the maturity period in some plants, leading to early seed production

Cytokinins 

• They lead to the growth of the buds and seeds by breaking their dormancy
• They help in keeping flowers fresh for a long time
• The help in inducing cell division in mature tissues
• They assist in nutrient mobilization, leading to the delay in leaf senescence
• They help in the development of new leaves and chloroplast organelles in the plant cell
• They are used to develop the roots and shoots of the plant

Plants Growth Inhibitors

Ethylene 

• It helps in the sprouting of potato tubers
• It leads to flowering in a mango tree
• It boosts the respiration rate during the ripening of fruits
• It helps to increase the flow of latex in rubber trees
• It leads to the senescence of flowers and leaves
• It helps in the growth of root hair. That increases the surface area for better absorption of water and minerals

Abscisic Acid 

• It helps in closing the stomata in the epidermis
• It leads to the maturation of seeds
• It inhibits seed germination and metabolism in plants
• It is sprayed on trees to regulate the dropping of fruits.
• It helps in inducing seed dormancy 

Conclusion

Just like sunlight, air, water, and nutrients, plants require growth hormones for their growth and development. Though the plant naturally produces these chemical substances, they are also applied externally to promote or inhibit the growth of some plants.