Summary: Plant Growth Development

Table of Contents
1. Introduction
2. Growth and its Characteristics
3. Phases of Growth
4. Growth Rates
5. Quantitative Comparison of Growth
6. Conditions for Growth
7. Differentiation, Dedifferentiation and Redifferentiation
8. Development and Plasticity
9. Factors Influencing Development
10. Plant Growth Regulators (PGRs) - Characteristics
11. Discovery (brief)
12. Major PGR Groups and Key Effects
13. Interactions and Roles
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Introduction

• Development = combination of growth and differentiation.
• Growth = permanent, irreversible increase in size; differentiation = formation of cell types with specific functions.
• Development begins with a fertilized egg, follows an ordered sequence to build organs (roots, leaves, branches, flowers, fruits, seeds).
• First visible step in a plant's life cycle is seed germination; when conditions are unsuitable, seeds enter a resting period and resume growth when favorable.

Growth and its Characteristics

• Growth is linked to metabolic activities (anabolic and catabolic) and is distinguishable from mere physical swelling caused by imbibition.
• Plants show largely indeterminate growth due to meristems - regions of dividing, self-perpetuating cells.
• Apical meristems at root and shoot tips drive primary growth (axis elongation); later-formed lateral meristems (including vascular cambium and cork cambium) drive secondary growth (increased girth).
• Growth is measurable by parameters proportional to protoplasm: fresh weight, dry weight, length, area, volume, cell number.

Phases of Growth

• Three main phases: meristematic, elongation, maturation.
• Meristematic phase: active cell division, rich protoplasm, thin primary walls.
• Elongation phase: cells enlarge, vacuolate and lay down new walls.
• Maturation phase: cells attain final size and specialized structures; most functional tissues form here.

Growth Rates

• Growth rate = growth per unit time. Two main patterns: arithmetic growth (linear increase; one daughter cell divides further) and geometrical growth (exponential increase; both daughters divide), the latter producing a typical sigmoid (S-) curve with lag, log and stationary phases.

Quantitative Comparison of Growth

• Absolute growth rate: total increase per unit time (no common basis).
• Relative growth rate: growth per unit initial value (enables comparison on a common basis).

Conditions for Growth

• Water: required for cell enlargement, turgidity and enzymatic medium for growth.
• Oxygen: needed for respiration to release metabolic energy.
• Nutrients: macro- and micronutrients for protoplasm synthesis and energy.
• Optimum temperature: each plant has a suitable temperature range for metabolic activity.
• Environmental signals (light, gravity): influence orientation and specific developmental phases.

Differentiation, Dedifferentiation and Redifferentiation

• Differentiation: maturation of meristem-derived cells into specialized types with structural and functional changes.
• Dedifferentiation: differentiated cells regain division ability and can form new meristems.
• Redifferentiation: cells produced by new meristems mature again into specialized forms.
• Callus in tissue culture is proliferating parenchyma induced to divide; tumors are localized uncontrolled divisions.

Development and Plasticity

• Development = entire sequence of changes from germination to old age, integrating growth and differentiation under intrinsic and extrinsic control.
• Plasticity: plants modify form or structure in response to environment or life stage; same genotype can produce different structures under different conditions.

Factors Influencing Development

• Intrinsic factors: genetic makeup and intercellular chemicals such as plant growth regulators (hormones).
• Extrinsic factors: light, temperature, water, oxygen and nutrition; many act via modulation of plant growth regulators.

Plant Growth Regulators (PGRs) - Characteristics

• Plant Growth Regulators (PGRs) are small molecules of diverse chemistry (indoles, adenine derivatives, carotenoid derivatives, terpenes, gases).
• PGRs are categorized by function: growth promoting (promote cell division, enlargement, patterning, flowering, fruiting) and growth inhibiting / stress response (dormancy, abscission, stress responses).

Discovery (brief)

• Historical discoveries identified five major groups of PGRs through physiological experiments and isolation of active substances.

Major PGR Groups and Key Effects

• Auxins: produced at growing tips; promote cell elongation, root initiation, apical dominance, parthenocarpy, xylem differentiation; synthetic auxins also act as herbicides.
• Gibberellins: acidic PGRs (GA family) that promote axis elongation, fruit improvement, delay senescence, accelerate certain developmental processes.
• Cytokinins: promote cell division (cytokinesis), leaf and chloroplast formation, lateral shoot growth, adventitious shoot formation, nutrient mobilization and delay senescence; natural forms produced in dividing regions.
• Ethylene: gaseous PGR produced in senescing and ripening tissues; influences seedling growth form, senescence, abscission, fruit ripening (increases respiration), dormancy break, internode elongation in flooded conditions; ethephon releases ethylene and is used agriculturally.
• Abscisic acid (ABA): generally a growth inhibitor and metabolic inhibitor; induces dormancy, inhibits germination, promotes stomatal closure and stress tolerance; often antagonistic to gibberellins.

Interactions and Roles

• PGRs interact (complementary, antagonistic or synergistic) to control events like dormancy, abscission, senescence and apical dominance.
• PGRs are one class of intrinsic control and act together with genomic programs and extrinsic signals to regulate plant growth and development.