Plant Growth & Development

Introduction

Growth is the most fundamental characteristic of any living being. Like animals, plants also grow and change their size over time. Plant growth and development depend on multiple intrinsic (internal) and extrinsic (external) factors. External factors include humidity, temperature, light, nutrients and internal factors include plant growth regulators.

Types of Plant Growth

There are two main types of growth – primary and secondary.

 1. Primary growth

Apical meristems (tips) increase the size of the root and shoot of plants, resulting in the subsequent expansion of the cell. In simple words, primary growth in plants begins at the roots and extends to the shoots giving rise to a primary plant.

 2. Secondary growth

Secondary growth happens due to cell division in the lateral meristems, increasing the diameter (girth) of the plant.

Other types of plant growth include:

• Unlimited growth: The period of plant growth from germination to death is known as unlimited growth
• Limited growth: Plants don’t grow further after acquiring a fixed size
• Vegetative growth: During this, plants grow stems, branches and leaves 
• Reproductive growth: It involves the flowering of a plant

Stages of Plant Growth

There are three stages of plant growth-

• Meristematic

It is also known as the formative phase, where plants grow by mitosis (cell division). It is carried out in two steps – nucleus division (karyokinesis) and cytoplasm division (cytokinesis).

• Elongation

It is also known as the enlargement phase. Formation of protoplasm, absorption of water, vacuoles development and formation of cell walls make the plant body thicker.

• Maturation

It is the final stage of plant growth when the cell wall thickens. During maturation, cells undergo protoplasmic modifications and acquire a definite shape.

Factors Affecting Plant Growth

The crucial factors affecting plant growth include:

• Temperature

Increased temperature accelerates various physiological processes like photosynthesis, transpiration, germination and spurring plant growth.

• Light

Plants are autotrophs which means they prepare their own food in the presence of light. Many physiological processes in a plant depend on light intensity and duration of exposure.

• Water

Plants require a sufficient amount of water to grow. Excess or scarcity of moisture may cause wilting and hinder plant growth.

• Nutrients

Nutritional deficiency may cause stunted growth or plant death in the worst case. Thus, soil nutrients play a crucial role in plant growth and development.

• Plant hormones

These are also known as plant growth regulators. These regulate late root formation, development of stems, leaves, flowering and facilitate fruiting. Let’s discuss in detail the types of plant hormones.

Plant Growth Regulators

Plant growth regulators are small, simple molecules of diverse chemical composition, which in low concentration regulate growth, differentiation and development by promoting or inhibiting the same. They are variously described as plant growth substances, plant hormones or phytohormones in literature. Plant growth regulators could be indole compounds (Indole-3 acetic acid or IAA), adenine derivatives (N6 furfurylamino purine or kinetin), derivatives of carotenoids and fatty acids (abscisic acid or ABA), terpenes (gibberellic acid or GA3) or gases (ethylene, C2H4).

Technically, a plant hormone is a chemical substance produced naturally in plants which is translocated to another region for regulating one or more physiological reactions, when present in Low concentration.

Based on function in the plant body, plant growth regulators (PGRs) can be broadly divided into two groups.

One Group of PGRs are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth. Flowering, fruiting and seed formation. These are called growth promoters. These include auxins, gibberellins and cytokinins.

The PGRs of other groups are involved in various growth inhibiting activities such as promotion of dormancy and abscission (shedding of leaves, flowers and fruits). They also play an important role in plant responses to wounding and stresses of biotic and abiotic origin. Abscisic acid (ABA) belongs to this group of PGRs.

The gaseous PGR, ethylene, could fit either of the groups, but it largely inhibits growth activities.

The Discovery of Plant Growth Regulators

1.     Discovery of auxins: The existence of the first plant growth hormone came from the observation of Charles Darwin and his son Francis Darwin (1880). They observed that the coleoptiles of canary grass (Phalaris canariensis) responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile. Later, the experiments conducted in different laboratories, resulted in the discovery of the first PGR called auxin (Gk. Auxin = to grow), by F.W. went (1928) in complete tip Avena sativa seedlings.
2.     Discovery of Cytokinins:  In the early part of the 20th century, ‘bakane’ (foolish seedling) disease of rice seedling, which resulted in weak, elongated stems and produced little or no grains. Swept across Japan. A Japanese plant pathologist E. Kurosawa (1926) found that the disease was caused by a fungus, Gibberella fujikori. He reported the appearance of symptoms of disease in uninfected rice seedlings, when they were treated with sterile filtrates of the fungus. The active substance was later identified as gibberellic.
3.     Discovery of cytokinins: The discovery of cytokinins is an off shot of the efforts of F. Skoog (USA) and his co-workers (1955) who were studying the nutritional requirements of tissues cultures derived from the internodal segments of tobacco stems. They observed that the tobacco callus (a mass of undifferentiated cells) proliferated only, if in addition to auxins nutrient medium is supplemented with the extract of vascular tissues or yeast or coconut milk (water of endosperm of coconut) or DNA. Skoog and Miller found that the active ingredient in these substances as Kirstein (6-furfurylamino-purine) because it promoted cell division (cytokinesis). Later many other compounds were discovered which showed similar effects. Skoog et.al (1965) gave the term cytokinins for those compounds that exhibited kinetin-like activities.
4.     Discovery of abscisic acid: When the research on plant growth regulators was progressing, it was found that certain substances of plant origin interfaced with auxin responses. The advent of analytical techniques called paper chromatography, helped to identify the PGR, abscisic acid. The hormone was first isolated by Addicot et.al (1967) from cotton balls.
5.     Discovery of Ethylene: The fruit merchants involved in shipping and storing fruit used kerosene lamps and hay to hasten colour development (ripening) in fruits. They have been aware that ripe and rotten fruits could accelerate the ripening of other fruits stored nearby. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of unripened bananas stored nearby. R.Gane (1934) provided undisputable gas chromatography.

Types of Plant Hormones/Plant Growth Regulators

Plant hormones control various functions related to plant growth and development. These are divided into two major categories, further divided into five sub-categories.

• Growth inhibitors

These induce seed dormancy and inhibit the growth of the plants.

• Abscisic acid (ABA)

• Stimulate closing of stomata
• Regulate dormancy
• Inhibit seed germination
• Used as spraying agent to regulate dropping of fruits
• Help in withstanding undesired growth factors
• Resist desiccation (removal of moisture)
• Hampers plant metabolism
• Ethylene
• Facilitate flowering
• Enhance respiration rate of plants
• Stimulate fruit ripening
• Affects horizontal growth of seedlings
• Increase root hair formation and growth
• Expand surface area for absorption in plants
• Facilitate senescence

Other plant growth inhibitors include salicylic acid, coumarin and more(artificial regulators).

• Growth promoters

These promote seed formation, cell division, flowering  promoting plant growth and more.

• Auxins

• Promote cell division
• Regulate xylem differentiation
• Facilitate flowering
• Prevents dropping of leaves and fruits at early stages
• Promotes natural detachment of older leaves and fruits
• Act as herbicides to kill weeds
• Promotes the growth of root and shoot apices

• Gibberellins

• Facilitate elongation of fruits to enhance their shapes
• Delays senescence (loss of cell’s power of division)
• Used as spraying agent to increase the yield
• Promote bolting
• Accelerate the malting process
• Facilitate early seed production
• Involved in leaf expansion

• Cytokinins

• Promote the growth of lateral bud, leaves
• Promote apical dominance (the phenomenon due to which central stem is dominant over side stems)
• Facilitate shoot formation and growth
• Mobilize nutrients to prevent decay
• Form chloroplast organelles (conduct photosynthesis) within the plant cell
• Delays senescence
• Keep flowers fresh for a long time

Some Important Terms to Remember

• Differentiation: It is a process in which cells from the root & shoot apical meristems , cambium separate from their precursor cell and mature to perform different functions
• Dedifferentiation: It is a process in which the previously differentiated cells (living) regain their ability to differentiate
• Redifferentiation: The process in which the dedifferentiated cells lose their ability to divide further. However, these cells are mature enough to perform specific functions

As mentioned earlier, temperature, light, humidity, nutrients and plant growth regulators affect plant growth and development. Let’s discuss the role of these factors, mainly light and temperature, in initiating flowering.

Conclusion

Growth is the most fundamental characteristic of any living being. Like animals, plants also grow and change their size over time. Plant growth regulators are small, simple molecules of diverse chemical composition, which in low concentration regulate growth, differentiation and development by promoting or inhibiting the same.

Growth is an inevitable event that changes the length, height, width of the plant. It also changes the cell volume and cell count of the plant. Different stages of growth have diverse physiological effects on plants. Plant growth regulators play the most significant role in inhibiting or promoting plant development. Apart from plant hormones, other factors that affect plant growth include light, temperature, humidity and soil nutrients.