Learning Objectives
- Understand the phases and characteristics of plant growth
- Learn about plant growth regulators (phytohormones)
- Study photoperiodism and vernalisation
- Understand seed dormancy and its breaking
Key Concepts
Characteristics of Plant Growth
Plant growth is indeterminate (continuous due to meristems), measurable (increase in fresh weight, dry weight, length, area, volume, cell number). Growth is an irreversible permanent increase in size. Growth rate: Arithmetic growth (one daughter cell divides, other differentiates — linear, e.g., root elongation: Lt = L0 + rt) and Geometric growth (both daughter cells divide — exponential initially: Wt = W0ert). Sigmoid growth curve: lag phase → log/exponential phase → stationary/plateau phase.
Phases of Growth
Cell division phase: Cells in meristematic region divide rapidly (root/shoot apex). Cell elongation phase: Cells enlarge, vacuoles increase, new cell wall material deposited. Cell maturation/differentiation phase: Cells acquire specific shape and function. Dedifferentiation: mature cells regain capacity to divide (e.g., interfascicular cambium). Redifferentiation: Dedifferentiated cells again lose dividing capacity and mature.
Plant Growth Regulators (PGRs / Phytohormones)
Auxins: Discovered by F.W. Went from Avena coleoptile tips. IAA (Indole-3-Acetic Acid) is natural auxin. Functions: apical dominance, cell elongation, root initiation, prevents abscission, induces parthenocarpy, phototropism, geotropism. Synthetic auxins: 2,4-D (weedicide), NAA, IBA.
Gibberellins: Discovered from Gibberella fujikuroi fungus (foolish seedling disease of rice). GA3 is most common. Functions: stem elongation, bolting (rapid internode elongation before flowering in rosette plants), breaking seed dormancy, delays senescence, induces parthenocarpy, promotes maleness in cucumbers.
Cytokinins: Discovered by Skoog and Miller. Zeatin (natural, from maize). Kinetin (synthetic). Functions: promote cell division, delay senescence (Richmond-Lang effect), promote lateral bud growth (opposite to auxin's apical dominance).
Abscisic Acid (ABA): Called stress hormone and growth inhibitor. Functions: induces stomatal closure during water stress, promotes seed dormancy, inhibits growth, promotes abscission, promotes senescence.
Ethylene: Gaseous hormone. Functions: promotes fruit ripening, horizontal growth (nastic response), senescence, abscission, breaks seed and bud dormancy, promotes femaleness in cucumbers. Triple response in etiolated seedlings.
Photoperiodism
Response of plants to the duration of light/dark periods. Discovered by Garner and Allard. Long Day Plants (LDP): Require light period longer than critical day length to flower (e.g., wheat, radish). Short Day Plants (SDP): Require light period shorter than critical day length (actually long uninterrupted dark period) to flower (e.g., tobacco, chrysanthemum, rice). Day Neutral Plants: Flowering independent of photoperiod (e.g., tomato, cotton). Phytochrome is the photoreceptor pigment (Pr — red light absorbing, and Pfr — far-red light absorbing form).
Vernalisation
Low-temperature treatment to promote flowering. Required by biennials and winter varieties of cereals (wheat, barley). Qualitative (obligate) — must have cold treatment. Quantitative (facultative) — cold accelerates flowering.
Summary
Plant growth is indeterminate and follows a sigmoid curve. Growth involves cell division, elongation, and differentiation. Five major phytohormones regulate growth: auxins, gibberellins, cytokinins promote growth; ABA and ethylene generally inhibit growth or promote senescence. Photoperiodism and vernalisation regulate flowering in response to environmental cues.
Important Terms
- Apical dominance: Suppression of lateral buds by auxin from apical bud
- Parthenocarpy: Development of seedless fruits without fertilization (induced by auxin, GA)
- Bolting: Rapid stem elongation induced by gibberellins
- Senescence: Aging process in plants; delayed by cytokinins, promoted by ABA and ethylene
- Photoperiodism: Flowering response to relative lengths of day and night
- Vernalisation: Promotion of flowering by cold temperature treatment
- Phytochrome: Photoreceptor pigment involved in photoperiodism (Pr and Pfr forms)
- Abscission: Shedding of leaves, fruits, or flowers; promoted by ABA and ethylene
Quick Revision
- Auxin: apical dominance, phototropism, parthenocarpy, 2,4-D is synthetic auxin
- Gibberellin: stem elongation, bolting, break dormancy, from Gibberella fungus
- Cytokinin: cell division, delay senescence, promote lateral buds
- ABA: stress hormone, stomatal closure, promotes dormancy
- Ethylene: gaseous, fruit ripening, femaleness in cucumber
- SDP actually need long uninterrupted dark period
- Vernalisation: cold treatment for flowering (wheat, barley)
- Sigmoid growth curve: lag → log → stationary phase