Plant Growth and Development

1. Chapter at a Glance

A tree that started as a tiny seed grows into a massive structure with leaves, branches, flowers and fruits. All of this happens through growth (an irreversible increase in size) and development (the whole series of changes an organism goes through during its life). In this chapter we study how growth happens in plants, how it is measured, the phases and kinds of growth rate, the processes of differentiation, dedifferentiation and redifferentiation, and the five major plant growth regulators (auxins, gibberellins, cytokinins, ethylene and abscisic acid) — their discovery and their many uses in agriculture and horticulture. NEET usually asks one or two questions, most often on the functions and discovery of growth regulators and on geometric growth.

2. Growth

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. It is an irreversible permanent increase in size of an organ, its parts or even an individual cell. It is accompanied by metabolic processes, both anabolic and catabolic, that use energy. Expansion of a leaf is growth; swelling of a piece of wood placed in water is not.

Plant growth is generally indeterminate

• Plants keep the capacity for unlimited growth throughout their life because they have meristems at certain locations. Meristem cells can divide and self-perpetuate. The products of division soon lose the capacity to divide and become part of the plant body.
• This form of growth, where new cells are always being added by meristem activity, is called the open form of growth.
• Root apical meristem and shoot apical meristem cause primary growth — elongation along the axis.
• In dicots and gymnosperms, the lateral meristems — vascular cambium and cork cambium — appear later and cause increase in girth. This is secondary growth.

Growth is measurable

At the cellular level, growth is mainly the result of an increase in the amount of protoplasm, which is hard to measure directly. So we measure quantities more or less proportional to it: increase in fresh weight, dry weight, length, area, volume or cell number.

• A single maize root apical meristem can produce more than 17,500 new cells per hour.
• Cells in a watermelon may increase in size by up to 3,50,000 times.
• Growth of a pollen tube is measured in terms of its length; growth of a dorsiventral leaf in terms of its surface area.

Phases of growth

The period of growth is generally divided into three phases: meristematic, elongation and maturation.

• Meristematic phase: the constantly dividing cells at the root apex and shoot apex. They are rich in protoplasm, have large conspicuous nuclei, and thin primary cellulose walls with abundant plasmodesmata.
• Elongation phase: cells just behind the meristem. They show increased vacuolation, cell enlargement and new cell wall deposition.
• Maturation phase: further from the apex. Cells attain their maximal size in terms of wall thickening and protoplasmic modifications.

The zones of growth can be found by marking a young root (or stem) with parallel lines at equal distances and watching which parts move apart most after some time. In roots, the region just behind the tip elongates most.

Growth rates

The increased growth per unit time is the growth rate. It can be arithmetic or geometric.

In the geometric equation, W₁ is the final size, W₀ the initial size, r the growth rate (relative growth rate, also a measure of the ability of the plant to produce new plant material, called the efficiency index), t the time of growth and e the base of natural logarithms.

• Absolute growth rate is the total growth per unit time. Relative growth rate is growth per unit time per unit initial size.
• Example: two leaves A and B, of 5 cm² and 50 cm², both increase by 5 cm² in a given time. Their absolute growth is the same, but the relative growth rate of A (5/5 = 100%) is ten times that of B (5/50 = 10%).

Conditions for growth

• Water: cell enlargement needs turgidity, and water is the medium for enzyme reactions.
• Oxygen: releases metabolic energy needed for growth.
• Nutrients: macro- and micronutrients for protoplasm and energy.
• Temperature: every plant has an optimum range; deviations can be harmful.
• Light and gravity: affect certain phases and stages of growth.

3. Differentiation, Dedifferentiation and Redifferentiation

• Differentiation: cells from the root and shoot apical meristems and cambium mature to perform specific functions. During it, cells undergo major structural changes in their walls and protoplasm. For example, to form a tracheary element, cells lose their protoplasm and develop a strong, elastic lignocellulosic secondary wall to carry water even under extreme tension.
• Dedifferentiation: living differentiated cells that had lost the capacity to divide regain it under certain conditions. Example: formation of meristems — interfascicular cambium and cork cambium — from fully differentiated parenchyma cells.
• Redifferentiation: the products of such dedifferentiated cells again lose the capacity to divide and mature to perform specific functions (e.g. secondary xylem from interfascicular cambium).
• Growth in plants is open, and differentiation is also open: cells derived from the same meristem have different structures at maturity, depending on their location. Cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.

4. Development and Plasticity

Development includes all changes an organism goes through during its life cycle, from germination of the seed to senescence. It is the sum of growth and differentiation, and is controlled by intrinsic factors (intracellular genetic factors and intercellular chemical factors such as growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition).

• Plasticity: plants follow different pathways in response to the environment or phases of life to form different kinds of structures.
• Heterophylly is an example. In cotton, coriander and larkspur, the leaves of the juvenile plant differ in shape from those of mature plants (developmental heterophylly). In buttercup, leaves produced in air differ from those produced in water (environmental heterophylly).

5. Plant Growth Regulators (PGRs)

PGRs are small, simple molecules of diverse chemical composition. They are also called plant hormones or phytohormones.

• Plant growth promoters: auxins, gibberellins and cytokinins — they promote cell division, enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.
• Plant growth inhibitors: abscisic acid — it acts in response to wounds and stresses, and promotes dormancy and abscission.
• Ethylene fits either group but is largely an inhibitor.
• Chemically: indole compounds (IAA), adenine derivatives (kinetin), derivatives of carotenoids (ABA), terpenes (gibberellic acid, GA₃) and gases (ethylene, C₂H₄).

Discovery of PGRs

Auxins

Auxins (Greek auxein = to grow) were first isolated from human urine. The term is applied to indole-3-acetic acid (IAA) and other natural and synthetic compounds with growth-regulating properties. They are generally produced by the growing apices of stems and roots, from where they move to regions of action.

• Natural: IAA and indole butyric acid (IBA). Synthetic: NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid).
• Help to initiate rooting in stem cuttings — widely used for plant propagation.
• Promote flowering (e.g. in pineapples); prevent fruit and leaf drop at early stages but promote abscission of older, mature leaves and fruits.
• Apical dominance: in most higher plants, the growing apical bud inhibits the growth of lateral (axillary) buds. Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plantations and hedge-making.
• Induce parthenocarpy (e.g. in tomatoes).
• Widely used as herbicides: 2,4-D kills dicotyledonous weeds but does not affect mature monocot plants. It is used to prepare weed-free lawns.
• Control xylem differentiation and help in cell division.
• Cause tropic responses: unequal distribution of auxin makes shoots bend towards light and against gravity, and roots bend towards gravity.

Gibberellins

Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants; they are denoted GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first discovered and remains the most intensively studied. All GAs are acidic.

• Increase the length of the axis, used to increase the length of grape stalks.
• Cause fruits such as apple to elongate and improve their shape.
• Delay senescence, so fruits can be left on the tree longer to extend the market period.
• GA₃ is used to speed up the malting process in the brewing industry.
• Sugarcane stores carbohydrate as sugar in its stems; spraying sugarcane with gibberellins increases stem length and the yield by as much as 20 tonnes per acre.
• Spraying juvenile conifers with GAs hastens maturity, leading to early seed production.
• Promote bolting (internode elongation just before flowering) in beet, cabbage and many plants with a rosette habit.

Cytokinins

Cytokinins have specific effects on cytokinesis. Kinetin, a modified form of adenine (a purine), was discovered from autoclaved herring sperm DNA; it does not occur naturally in plants. Zeatin was later isolated from corn kernels and coconut milk. Natural cytokinins are synthesised in regions of rapid cell division, such as root apices, developing shoot buds and young fruits.

• Help to produce new leaves and chloroplasts in leaves.
• Promote lateral shoot growth and adventitious shoot formation; they help overcome apical dominance.
• Promote nutrient mobilisation, which helps in the delay of leaf senescence.

Ethylene

Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits.

• Causes horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
• Promotes senescence and abscission of plant organs, especially leaves and flowers.
• Highly effective in fruit ripening; it enhances the respiration rate during ripening (the rise in respiration is called the respiratory climacteric).
• Breaks seed and bud dormancy, initiates germination in peanut seeds and sprouting of potato tubers.
• Promotes rapid internode/petiole elongation in deep-water rice plants, helping leaves and the upper shoot stay above water.
• Promotes root growth and root hair formation, increasing the absorption surface.
• Used to initiate flowering and synchronise fruit-set in pineapples, and induces flowering in mango.
• The most widely used compound as a source of ethylene is ethephon. In aqueous solution it is readily absorbed and slowly releases ethylene. It hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers, increasing yield.

Abscisic acid (ABA)

ABA was discovered for its role in regulating abscission and dormancy, but like other PGRs it has many effects. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.

• Inhibits seed germination.
• Stimulates the closure of stomata and increases tolerance of plants to various stresses — so it is called the stress hormone.
• Plays an important role in seed development, maturation and dormancy. By inducing dormancy, it helps seeds withstand desiccation and other unfavourable factors.
• In most situations ABA acts as an antagonist to GAs.

For any phase of growth, differentiation and development, one or another PGR plays a role. Such roles may be complementary or antagonistic, individualistic or synergistic. Many events are affected by more than one PGR: dormancy in seeds and buds, abscission, senescence and apical dominance. The role of a PGR is only one kind of intrinsic control; along with genomic control and extrinsic factors such as light and temperature, it regulates growth and development.

6. Master Summary Table

7. Mind Map

8. Tips and Tricks

9. Important One-liners

• Growth in plants is open and indeterminate because of meristems.
• A maize root apical meristem can give rise to more than 17,500 new cells per hour.
• Geometric growth is described by the equation W₁ = W₀e^rt.
• Formation of interfascicular cambium from parenchyma is an example of dedifferentiation.
• Heterophylly in buttercup is an example of environmental plasticity.
• Auxin was first isolated from human urine; Went isolated it from oat coleoptile tips.
• Bakanae disease of rice is caused by Gibberella fujikuroi.
• Kinetin was obtained from autoclaved herring sperm DNA.
• Ethylene is responsible for the respiratory climacteric during fruit ripening.
• Ethephon is the most widely used source of ethylene.
• Abscisic acid is called the stress hormone.

10. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

11. Quick Revision Summary