Plant Growth and Development Class 11 Notes

Growth in Plants — Phases and Parameters

Three Phases of Growth

• Meristematic phase — cells are small, thin-walled, dense cytoplasm, large nuclei, rich in protoplasm. They divide actively. Found at root and shoot apices (apical meristems) and cambium.
• Elongation phase — cells behind the meristematic zone enlarge rapidly, vacuoles form and enlarge, cell wall loosens. This is the zone of maximum water uptake and is responsible for increase in length.
• Maturation phase — cells differentiate and attain their final form and function. Vacuoles occupy most of the cell volume; specialised cell walls form. Also called the zone of permanent cell.

Growth Rates — Arithmetic and Geometric

Growth can be measured by increase in cell number, cell size or fresh/dry weight. The rate of growth varies and can follow arithmetic or geometric patterns.

Arithmetic Growth

If only one daughter cell continues to divide while the other differentiates, the number of cells increases by a constant amount per unit time. A straight line is obtained when length is plotted against time.

Geometric Growth

When both daughter cells from each division continue to divide, the cell number doubles at each division, giving a characteristic S-shaped (sigmoid) curve when plotted on a linear scale. On a log scale the initial phase appears linear.

The sigmoid growth curve has three phases: lag (slow), log or exponential (rapid cell division), and stationary or senescence (rate slows as nutrients limit growth).

Conditions for Plant Growth

• Water — essential for cell elongation (turgidity), enzyme activity, and as a solvent for nutrients. Water deficit reduces growth markedly.
• Oxygen — required for aerobic respiration to generate ATP needed for growth. Anaerobic conditions severely retard root growth.
• Temperature — enzymatic reactions of growth have a Q₁₀ of about 2; most plants grow best at 25–35 °C. Extreme heat denatures enzymes.
• Light — not required by all cells for growth per se, but photomorphogenesis, photosynthesis, and photoperiodism depend on light quality, intensity and duration.
• Nutrients — mineral elements such as N, P, K, Ca, Mg and trace elements are needed for building cell walls, membranes, nucleic acids and enzymes.

Differentiation, Dedifferentiation and Redifferentiation

Plant Growth Regulators — Phytohormones

Plant growth regulators (PGRs) are small organic molecules that act as signalling compounds to regulate growth and development. They are grouped into five major families based on their physiological effects.

Auxins (e.g. IAA — Indole-3-Acetic Acid)

• Synthesised in shoot apices, young leaves and developing seeds.
• Promote cell elongation (acid-growth hypothesis: proton pumps lower wall pH, expansins loosen cross-links).
• Apical dominance — high auxin from the tip suppresses lateral bud growth.
• Root initiation on stems, vascular differentiation, phototropism (lateral redistribution under unilateral light).
• Applications: rooting of cuttings, weed control (2,4-D as selective herbicide), parthenocarpy.

Gibberellins (GA)

• Discovered from the fungus Gibberella fujikuroi causing 'foolish seedling' disease in rice.
• Promote stem elongation by stimulating both cell division and elongation — can restore dwarf varieties to normal height.
• Break dormancy of seeds and buds; induce germination by stimulating α-amylase synthesis in the aleurone layer.
• Promote bolting (internode elongation) before flowering in rosette plants.
• Applications: malt production in brewing, thinning of fruit, increasing grape berry size.

Cytokinins

• First isolated from coconut milk; the natural form is zeatin (from maize endosperm).
• Promote cell division (cytokinesis) — often used together with auxin in tissue culture.
• Delay senescence (anti-ageing effect) — keep leaves green by mobilising nutrients toward them (Richmond–Lang effect).
• Stimulate lateral bud growth — antagonistic to auxin-mediated apical dominance.
• Applications: tissue culture media, preservation of cut flowers and vegetables.

Ethylene (C₂H₄)

• The only gaseous PGR; produced in all higher plant tissues, especially during ripening and senescence.
• Promotes fruit ripening — stimulates autocatalytic ethylene production and respiration climacteric.
• Triple response in seedlings: inhibition of stem elongation, radial swelling, horizontal growth (useful for navigating around obstacles in soil).
• Promotes abscission of leaves, flowers and fruits; induces flowering in pineapple (with auxin interaction).
• Applications: commercial ripening of bananas, mangoes and tomatoes in ripening chambers.

Abscisic Acid (ABA)

• A sesquiterpenoid; functions mainly as a growth inhibitor and stress hormone.
• Induces and maintains bud and seed dormancy — prevents precocious germination.
• Promotes stomatal closure during water stress by increasing cytosolic Ca²⁺ in guard cells.
• Inhibits growth, accelerates senescence and leaf abscission — sometimes called the 'stress hormone'.
• Promotes desiccation tolerance in developing seeds (seed maturation).

Photoperiodism and Vernalisation

• Short-day plants (SDP) flower when the night length exceeds a critical duration (long nights, short days). E.g. rice, soybean, chrysanthemum, cocklebur, potato.
• Long-day plants (LDP) flower when the night length is shorter than a critical duration (short nights, long days). E.g. wheat, barley, spinach, lettuce, clover.
• Day-neutral plants (DNP) flower irrespective of night length. E.g. tomato, cucumber, maize, some varieties of rice.

Solved Examples

Example: A root tip has cells that are small, dense, thin-walled and actively dividing. In which phase of growth are these cells? Name the zone in which they are located.

Solution: These cells are in the meristematic phase of growth. They are found in the zone of cell division (apical meristem) at the root tip. These cells have dense cytoplasm, large nuclei and no large central vacuole, which are hallmarks of actively dividing meristematic cells.

Example: Explain why a short-day plant like soybean fails to flower when grown under long-day conditions in summer, but flowers when days become shorter in autumn.

Solution: Soybean is a short-day plant that requires a continuous dark period exceeding a critical length to flower. In summer, nights are short (less than the critical duration), so the plant remains vegetative. As autumn approaches, nights lengthen beyond the critical threshold, Pfr levels decline sufficiently during the long dark period, and the floral stimulus is generated at the shoot apex, inducing flowering.