Plant Growth and Development

Plant growth is an irreversible permanent increase in size, volume, or dry weight. It primarily occurs at meristems (regions of active cell division) and proceeds through three phases:

• Meristematic phase: active cell division.
• Elongation phase: cells increase in size by vacuolation and wall expansion.
• Maturation phase: cells differentiate and acquire final form and function.

Plants show open growth because meristems remain active throughout the plant's life. Growth rate can be arithmetic (constant number of cells added per time) or geometric (each cell divides so the increase is exponential).

Development involves differentiation (irreversible change in structure and function), dedifferentiation (regaining division capacity, e.g., wound callus), and redifferentiation (cells produced by dedifferentiated tissue that differentiate again).

Plant growth regulators (PGRs) are small organic molecules that influence growth at low concentrations. The five major PGRs:

Plants respond to environmental stimuli by growth movements (tropisms) or turgor-pressure movements (nasties).

Tropisms — directional growth responses toward or away from a stimulus:

• Phototropism: shoot grows toward light (positive), root grows away (negative). Auxin redistributes to the shaded side, causing differential growth.
• Geotropism: root grows toward gravity (positive geotropism); shoot grows away (negative geotropism).
• Hydrotropism: roots grow toward water.
• Thigmotropism: tendrils coil around a support on contact (e.g., pea, gourd).
• Chemotropism: pollen tube grows toward the ovule guided by chemical signals.

Nastic movements — non-directional, independent of stimulus direction:

• Seismonasty: touch-sensitive folding of Mimosa pudica leaves due to rapid loss of turgor in pulvinus cells.
• Nyctinasty: sleep movements — leaves fold at night (legumes).
• Thermonasty: flower opening in response to temperature.

Photoperiodism is the response of a plant to the relative duration of light (photoperiod) and dark periods in a 24-hour cycle, primarily governing flowering.

• Short-day plants (SDP): flower when day length is shorter than a critical period — Chrysanthemum, Xanthium, rice, sugarcane, soybean.
• Long-day plants (LDP): flower when day length exceeds a critical period — wheat, spinach, henbane (Hyoscyamus niger).
• Day-neutral plants: flower regardless of photoperiod — tomato, maize, sunflower, cucumber.

The critical requirement in photoperiodism is actually the length of the dark period (night), not the light period. A brief light interruption during the night (night-break) can convert a short-day response to a long-day response.

Vernalization is the acceleration of flowering by prolonged exposure to low temperature (cold treatment). It is important in winter wheat, rye, and many biennials. It prevents premature flowering before winter.

Dormancy is a state in which a viable seed, bud, or organ refuses to grow even under apparently suitable conditions. ABA is the primary promoter of dormancy. Seed dormancy can be broken by stratification (cold treatment), scarification (breaking seed coat), or light.

Senescence is the final phase of growth leading to death. It involves chlorophyll degradation, protein breakdown, and nutrient remobilisation. Ethylene and ABA promote senescence; cytokinins delay it. Leaves turn yellow in autumn partly because nitrogen from degraded chlorophyll is transported out before the leaf dies.

Abscission is the active shedding of leaves, flowers, or fruits due to the formation of an abscission zone — a layer of weak cells at the organ base. Ethylene promotes abscission; auxin delays it.

Developmental plasticity: the same genotype can produce different phenotypes based on environment. Heterophylly (different leaf shapes on the same plant in different conditions) in Larkspur or cotton is a classic example.