PLANT GROWTH AND DEVELOPMENT

The life of a plant originates from a single cell called zygote. All the structures of plant such as roots, stems, leaves, flowers, fruits and seeds arises from a single cell in a very orderly sequence. Plant growth is generally indeterminate because growth in plants is unlimited and occurs continuously throughout their life by the activity of Meristematic cells. The new cell continually produced by cell division occur in Meristem. Different phases of plant growth are cell division, cell enlargement and cell maturation.

FACTORS RESPONSIBLE FOR GROWTH IN PLANTS

1. Water
2. Light
3. Oxygen
4. Temperature
5. Supply of nutrients
6. C/N Ratio

PLANT GROWTH HORMONES

Plant growth regulators are variously described as plant growth substances, plant hormones or phyto hormones. These are small, simple organic molecules of diverse chemical composition produced naturally in higher plants that control the growth and other physiological functions. These are required in very small amount by plants. For the regulation of every phase of growth in plants, two or more growth hormones are intimately related to each other. The plant hormones are prepared by the apical meristem of younger and mature leaves and are transported through the vascular tissue, phloem to other parts of the plant. Some plant hormones are given below:

1- AUXINS: It is chemically known as Indole 3-acetic acid (IAA). It is also known as natural auxin. Apart from this Napthalene acetic acid (NAA) and 2, 4 dichlorophenoxyacetic acid (2, 4 D) are known as synthetic auxins.

FUNCTIONS

• Cell elongation and longitudinal growth.
• Cell division in the cambium.
• Promotion of root growth and root initiation.
• Induces parthenocarpy, i.e., formation of seedless fruits without fertilisation.
• Prevents abscission layer, i.e., shedding of mature leaves from stem or ripe fruits from the stem.

2- GIBBERELLINS: It was discovered by Kurosawa in 1926. These are generally known as weakly acidic growth hormones. GA₃ is the most important gibberellic acid.

FUNCTIONS

• Elongation of stem.
• Reversal of dwarfism in plants.
• Breaking of dormancy (i.e,. making the seed to germinate).
• Promotes seed germination in lettuce, cereals.
• Cause parthenocarpy in apple and pear.
• Control flowering in long day plants.
• Control growth and development of fruits.
• Delay senesence(ageing) in fruits like citrus fruits, apples etc.

3- CYTOKININS: It was invented by Miller in 1955. These growth hormones are basic in nature and have specific effect on cytokineseis.

FUNCTIONS

• It acts on cell division but have little or no effect on growth.
• Breaks the dormancy and promote their germination.
• It stimulates the growth of lateral buds and thus suppress apical dominance.
• It also delays senescence of plant organs by controlling protein synthesis.
• Induce parthenocarpy in many plants.
• Increase self life of marketed vegetables, cut shoots and flowers to keep them fresh for several days.

4- ETHYLENE: It is a simple gaseous plant hormone synthesised from the amino acid methionine. It was invented by Burg in 1962 and is also known as ripening hormone.

FUNCTIONS

• Inhibits growth.
• Helps in fruit growth and ripening.
• Inhibits the growth of lateral buds and causes apical dominance.
• Stimutates root initiation, growth of lateral roots and root hair.
• Promotes the yellowing and senescence of leaves.
• Breaks the dormancy of buds and seeds.

5- ABSCISIC ACID: It was discovered by Adicote and Corsin in 1965. It is slightly acidic growth hormone  that functions as a growth inhibitor.

FUNCTIONS

• Inhibits growth and metabolism.
• Seed development and germination.
• Senescence by causing loss of chlorophyll pigment.
• Accelerates senescences in leaves.
• Maintains dormancy in many seeds.
• Promotes abscision of leaves, fruits and flowers.

POINTS TO REMEMBER

1. ZEATIN is the naturally occurring cytokinin isolated from maize grains.
2. FLORIGENS also known as flowering hormone is used in flower blossomimg.
3. The plant hormone produced in response to wound is called TRAUMATIN. Its formation occurs in damaged cells.
4. The hormone which is secreted from plant organs and is used in controlling the growth of plant organs is called CALINES.