Growth in plants is the permanent, irreversible increase in size, form, and number of cells. Unlike animals, plants show indeterminate (open) growth — they continue growing throughout their life due to the presence of permanent meristematic (dividing) tissues.
Characteristics of Plant Growth
- Growth in plants follows a sigmoid (S-shaped) curve when plotted against time:
- Lag phase: slow initial growth as cells are getting ready to divide
- Log phase (Exponential phase): rapid cell division and elongation; growth rate is maximum
- Stationary phase: growth rate slows as nutrients become limiting or cells mature
- Senescence phase: decline in growth rate
Arithmetic growth: One daughter cell continues to divide; other differentiates. Growth = constant increment per unit time. Graph: straight line. Example: root elongation.
Geometric growth: Both daughter cells continue to divide. Wt = W0 x ert where Wt = final size, W0 = initial size, r = growth rate, t = time. Graph: exponential.
Differentiation, Dedifferentiation, Redifferentiation
- Differentiation: process by which cells derived from meristems acquire specialised structures and functions (e.g., tracheids from cambium)
- Dedifferentiation: mature cells regain the capacity to divide under certain conditions (e.g., wound healing, callus formation)
- Redifferentiation: cells that have dedifferentiated can differentiate again into different types
Plant Growth Regulators (PGRs)
PGRs are small, simple molecules that influence physiological processes at very low concentrations. They can be promoters or inhibitors.
- 1. Auxins (IAA — Indole-3-acetic acid)
- Produced mainly in shoot apices and young leaves
- Promote cell elongation (by loosening cell wall)
- High concentration inhibits growth (especially in roots — more sensitive than shoots)
- Apical dominance: auxin from the shoot apex suppresses lateral bud growth
- Phototropism: unequal distribution of auxin causes shoots to bend toward light
- Gravitropism: redistribution causes roots to grow downward
- Synthetic auxins: 2,4-D (used as herbicide for broadleaf weeds), NAA, IBA (root initiation)
- 2. Gibberellins (GA3)
- Promote stem elongation (by stimulating cell division and elongation)
- Break seed dormancy and promote germination
- Stimulate flowering in long-day plants under non-inductive conditions
- Promote bolting (elongation of stem before flowering) in rosette plants
- Fruit development (parthenocarpy)
- In germinating cereal grains: GA promotes synthesis of alpha-amylase in aleurone layer → starch digestion for seedling
- 3. Cytokinins
- Promote cytokinesis (cell division)
- Produced in actively dividing cells (root apex, young fruits)
- Delay senescence (leaf yellowing) — Richmond-Lang effect
- Promote lateral bud growth (counteract apical dominance of auxin)
- Synergistic with auxin in tissue culture (ratio determines root vs shoot formation)
- 4. Abscisic Acid (ABA)
- The "stress hormone" — produced during drought, cold, salinity
- Promotes stomatal closure (reduces water loss)
- Promotes seed dormancy and inhibits germination
- Promotes abscission (leaf/fruit fall)
- Inhibits plant growth overall — acts as a growth inhibitor
- 5. Ethylene
- Only gaseous PGR; synthesised mainly in ripening fruits, aging tissues
- Promotes fruit ripening (climacteric fruits — banana, mango, tomato)
- Promotes abscission of flowers and fruits
- Promotes epinasty (downward curvature of leaves)
- Triple response in seedlings: horizontal growth, swollen hypocotyl, thickening; important test for ethylene
- Promotes root initiation; can break dormancy
Photoperiodism
- Photoperiodism is the response of plants to the relative duration of light and darkness (day length).
- Short-day plants (SDP): flower when day length is LESS than a critical photoperiod (darkness period is long). Example: Chrysanthemum, Xanthium
- Long-day plants (LDP): flower when day length EXCEEDS a critical photoperiod. Example: wheat, spinach, radish
- Day-neutral plants: unaffected by photoperiod. Example: tomato, maize
The critical factor is actually the length of the dark period, not the light period. Interrupting the dark period with a brief flash of light prevents flowering in SDPs.
The photoreceptor involved: Phytochrome (converts between Pr and Pfr forms depending on red/far-red light).
Vernalisation
Some plants (wheat, rye, carrot) require a prolonged cold period (winter) before they can flower — this is vernalisation. It prevents premature flowering and ensures flowering in spring.
Common mistakes
- Ethylene is a gas — the ONLY gaseous plant hormone.
- ABA promotes dormancy and inhibits growth (it is an inhibitor, not a promoter).
- In photoperiodism, it is the night length (dark period) that is critical, not day length.
- High auxin concentration INHIBITS root growth (roots are more sensitive than shoots).
Summary
Plant growth is characterised by meristematic activity and follows a sigmoid curve. Five major PGRs (auxin, gibberellin, cytokinin, ABA, ethylene) regulate diverse processes including cell elongation, dormancy, ripening, and stress responses. Photoperiodism and vernalisation are environmental cues plants use to time flowering.