Transpiration and Plant Growth Regulators
Deep FCI AG-III Technical Botany lesson on transpiration types, factors, significance, stomatal control, auxins, gibberellins, cytokinins, ABA, ethylene, agriculture links and conceptual clarifications.
Why This Lesson Matters
Transpiration and plant growth regulators, or PGRs, explain how plants balance water loss, nutrient movement, growth, flowering, fruiting, dormancy and stress response. For FCI AG-III Technical, these topics connect plant physiology with crop establishment, drought stress, grain filling, fruit ripening, post-harvest handling and storage quality.
Study this lesson in two blocks:
- Transpiration - water loss from aerial plant parts and its effect on water balance.
- Plant growth regulators - chemical messengers controlling growth and development.
Transpiration: Definition
Transpiration is the loss of water as vapour from aerial parts of plants, mainly through leaves.
Most absorbed water is lost through transpiration. Only a small fraction is used in photosynthesis, cell expansion and metabolism.
| Water movement stage | Meaning |
|---|---|
| Absorption | Roots absorb water from soil |
| Ascent of sap | Water moves upward through xylem |
| Evaporation | Water evaporates from mesophyll cell walls |
| Diffusion | Water vapour exits through stomata, cuticle or lenticels |
Types of Transpiration
| Type | Site | Contribution | Exam point |
|---|---|---|---|
| Stomatal transpiration | Stomata of leaves | Major share | Most important type |
| Cuticular transpiration | Cuticle of epidermis | Small share | More when cuticle is thin |
| Lenticular transpiration | Lenticels in woody stems | Very small share | Cannot be fully regulated |
Stomatal Transpiration
Stomata are pores surrounded by guard cells. When stomata open, CO2 enters for photosynthesis but water vapour escapes. Therefore, stomata create a trade-off between carbon gain and water loss.
Pro Content Locked
Upgrade to Pro to access this lesson and all other premium content.
₹99 charged monthly · Cancel anytime
- All Agriculture & Banking Courses
- AI Lesson Questions (100/day)
- AI Doubt Solver (50/day)
- Glows & Grows Feedback (30/day)
- AI Section Quiz (20/day)
- 22-Language Translation (100/day)
- Recall Questions (20/day)
- AI Quiz (15/day)
- AI Quiz Paper Analysis (100/day)
- AI Step-by-Step Explanations (100/day)
- Spaced Repetition Recall (FSRS)
- AI Tutor
- Immersive Text Questions
- Audio Lessons — Hindi & English
- Mock Tests & Previous Year Papers
- Summary & Mind Maps
- XP, Levels, Leaderboard & Badges
- Generate New Classrooms
- Voice AI Teacher (AgriDots Live)
- AI Revision Assistant
- Knowledge Gap Analysis
- Interactive Revision (LangGraph)
🔒 Secure via Razorpay · Cancel anytime · No hidden fees
Why This Lesson Matters
Transpiration and plant growth regulators, or PGRs, explain how plants balance water loss, nutrient movement, growth, flowering, fruiting, dormancy and stress response. For FCI AG-III Technical, these topics connect plant physiology with crop establishment, drought stress, grain filling, fruit ripening, post-harvest handling and storage quality.
Study this lesson in two blocks:
- Transpiration - water loss from aerial plant parts and its effect on water balance.
- Plant growth regulators - chemical messengers controlling growth and development.
Transpiration: Definition
Transpiration is the loss of water as vapour from aerial parts of plants, mainly through leaves.
Most absorbed water is lost through transpiration. Only a small fraction is used in photosynthesis, cell expansion and metabolism.
| Water movement stage | Meaning |
|---|---|
| Absorption | Roots absorb water from soil |
| Ascent of sap | Water moves upward through xylem |
| Evaporation | Water evaporates from mesophyll cell walls |
| Diffusion | Water vapour exits through stomata, cuticle or lenticels |
Types of Transpiration
| Type | Site | Contribution | Exam point |
|---|---|---|---|
| Stomatal transpiration | Stomata of leaves | Major share | Most important type |
| Cuticular transpiration | Cuticle of epidermis | Small share | More when cuticle is thin |
| Lenticular transpiration | Lenticels in woody stems | Very small share | Cannot be fully regulated |
Stomatal Transpiration
Stomata are pores surrounded by guard cells. When stomata open, CO2 enters for photosynthesis but water vapour escapes. Therefore, stomata create a trade-off between carbon gain and water loss.
Cuticular Transpiration
The cuticle is a waxy layer that reduces water loss. Xerophytes often have a thick cuticle to reduce cuticular transpiration.
Lenticular Transpiration
Lenticels are openings in woody stems used for gaseous exchange. Lenticular transpiration is minor but continues because lenticels do not open and close like stomata.
Stomatal Structure and Movement
Guard cells regulate stomatal aperture. In dicots, guard cells are usually kidney-shaped. In grasses, they are dumbbell-shaped.
| Guard cell condition | Stomatal state |
|---|---|
| Guard cells turgid | Stomata open |
| Guard cells flaccid | Stomata close |
Opening is linked with solute accumulation in guard cells, especially potassium ions and organic anions. Water enters by osmosis, guard cells become turgid, and the pore opens.
Closing occurs during darkness, water stress or high ABA levels.
conceptual confusion: Stomata regulate both transpiration and CO2 entry. Complete closure saves water but reduces photosynthesis.
Factors Affecting Transpiration
External Factors
| Factor | Effect |
|---|---|
| Light | Opens stomata and increases transpiration |
| Temperature | Increases evaporation and vapour pressure difference |
| Relative humidity | High humidity lowers transpiration; dry air increases it |
| Wind velocity | Moderate wind increases transpiration by removing humid boundary layer |
| Soil water | Low soil water causes stomatal closure |
| Atmospheric pressure | Lower pressure can increase diffusion |
Internal Factors
| Factor | Effect |
|---|---|
| Leaf area | Larger area usually increases water loss |
| Stomatal number | More stomata can increase transpiration |
| Stomatal position | Sunken stomata reduce water loss |
| Cuticle thickness | Thick cuticle reduces cuticular transpiration |
| Leaf rolling | Reduces exposed surface |
| Root-shoot ratio | Better roots support water absorption |
Xerophytic Adaptations
Plants adapted to dry conditions reduce water loss through:
- thick cuticle
- sunken stomata
- reduced leaf area
- leaf spines
- leaf rolling
- succulent tissues
- CAM photosynthesis
- deep or extensive roots
Significance of Transpiration
Transpiration has both benefits and risks.
Useful Roles
| Role | Explanation |
|---|---|
| Transpiration pull | Helps ascent of sap through xylem |
| Mineral transport | Carries dissolved minerals from roots to shoots |
| Leaf cooling | Evaporation removes heat |
| Maintains water movement | Keeps continuous flow from soil to atmosphere |
| Supports photosynthesis | Open stomata allow CO2 entry |
Harmful Effects
| Condition | Problem |
|---|---|
| Excess transpiration | Wilting |
| Hot dry wind | Rapid water loss |
| Low soil moisture | Stomatal closure and reduced photosynthesis |
| Drought during grain filling | Shrivelled grains and low test weight |
For FCI, transpiration links directly to crop yield and grain quality. Severe water stress during flowering and grain filling can reduce grain number, grain size and marketable quality.
Transpiration, Guttation and Bleeding
| Process | What happens | Main cause |
|---|---|---|
| Transpiration | Water vapour lost from aerial parts | Evaporation and diffusion |
| Guttation | Liquid water droplets appear at leaf margins | Root pressure through hydathodes |
| Bleeding | Sap exudes from cut or injured plant parts | Root pressure or internal pressure |
conceptual confusion: Guttation is liquid water loss through hydathodes; transpiration is vapour loss mainly through stomata.
Plant Growth Regulators: Overview
Plant growth regulators are organic substances produced in small amounts that regulate growth, development and responses to environment.
| PGR | Broad role |
|---|---|
| Auxins | Cell elongation, apical dominance, rooting |
| Gibberellins | Stem elongation, seed germination, bolting |
| Cytokinins | Cell division, shoot formation, delay of senescence |
| Abscisic acid | Stress hormone, stomatal closure, dormancy |
| Ethylene | Ripening, senescence, abscission |
Auxins, gibberellins and cytokinins are usually considered growth promoters. ABA and ethylene often act as growth inhibitors or stress and ripening regulators, though their effects depend on context.
Auxins
Auxins were among the first plant hormones discovered. The natural auxin is indole-3-acetic acid, or IAA.
| Feature | Auxins |
|---|---|
| Main natural form | IAA |
| Main site of synthesis | Shoot tips, young leaves, developing seeds |
| Transport | Polar transport, mainly from apex downward |
| Main effect | Cell elongation |
Functions of Auxins
| Function | Practical meaning |
|---|---|
| Cell elongation | Shoot growth |
| Apical dominance | Terminal bud suppresses lateral buds |
| Root initiation | Used in cuttings |
| Tropic responses | Phototropism and geotropism |
| Fruit development | Can induce parthenocarpy in some plants |
| Prevention of abscission | Delays leaf and fruit drop at certain concentrations |
| Weed control | Synthetic auxins can kill broadleaf weeds |
Synthetic auxins include IBA, NAA and 2,4-D. IBA and NAA are commonly linked with rooting. 2,4-D is known as a selective herbicide for broadleaf weeds in cereal crops.
conceptual confusion: Auxin promotes shoot elongation but high auxin concentration can inhibit root elongation. Roots are more sensitive to auxin than shoots.
Gibberellins
Gibberellins were discovered through the foolish seedling disease of rice caused by Gibberella fujikuroi. The common example is GA3, or gibberellic acid.
| Feature | Gibberellins |
|---|---|
| Common example | GA3 |
| Major effect | Stem elongation |
| Important in | Seed germination, bolting, flowering in some plants |
Functions of Gibberellins
| Function | Example / application |
|---|---|
| Stem elongation | Causes tall growth |
| Bolting | Promotes rapid internode elongation in rosette plants |
| Breaks dormancy | Helps seed and bud germination |
| Enzyme production | Induces alpha-amylase in cereal aleurone layer |
| Fruit growth | Increases size in some fruits |
| Delays senescence | Used in some horticultural contexts |
Cereal Germination Link
In germinating cereal grains, the embryo releases gibberellin. Gibberellin stimulates the aleurone layer to produce alpha-amylase. Alpha-amylase breaks starch into sugars for the growing embryo.
This is important for agriculture and storage because unwanted sprouting in moist grain changes starch quality and reduces market value.
Cytokinins
Cytokinins promote cell division. A natural cytokinin example is zeatin.
| Feature | Cytokinins |
|---|---|
| Main natural example | Zeatin |
| Major synthesis site | Root tips |
| Transport | Mainly upward through xylem |
| Main effect | Cell division |
Functions of Cytokinins
| Function | Practical meaning |
|---|---|
| Cell division | Promotes growth of tissues |
| Shoot initiation | Important in tissue culture |
| Breaks apical dominance | Promotes lateral bud growth |
| Delays senescence | Maintains chlorophyll and leaf freshness |
| Nutrient mobilization | Attracts nutrients to treated tissues |
Tissue Culture Ratio
| Auxin : cytokinin ratio | Result |
|---|---|
| High auxin, low cytokinin | Root formation |
| Low auxin, high cytokinin | Shoot formation |
| Balanced ratio | Callus formation |
conceptual confusion: Cytokinins are associated with cell division and delay of leaf senescence. Do not confuse them with gibberellins, which are strongly linked with stem elongation and seed germination.
Abscisic Acid
Abscisic acid, or ABA, is called the stress hormone. It helps plants survive drought and adverse conditions.
| Feature | ABA |
|---|---|
| Broad role | Growth inhibition and stress response |
| Stomatal effect | Causes closure |
| Seed effect | Maintains dormancy |
| Stress link | Drought, salinity, cold |
Functions of ABA
- closes stomata during water stress
- promotes seed dormancy
- inhibits precocious germination
- helps stress tolerance
- often antagonizes gibberellin in seed germination
FCI link: ABA-controlled dormancy matters for seed physiology and pre-harvest sprouting. In cereal grains, loss of dormancy under wet conditions can reduce grain quality.
Ethylene
Ethylene is a gaseous plant hormone.
| Feature | Ethylene |
|---|---|
| Physical nature | Gas |
| Main effect | Fruit ripening |
| Other roles | Senescence, abscission, stress responses |
Functions of Ethylene
| Function | Example |
|---|---|
| Fruit ripening | Banana, mango, tomato |
| Abscission | Leaf and fruit fall |
| Senescence | Ageing of tissues |
| Triple response | Short, thick and curved seedling growth |
| Flowering in pineapple | Ethylene-releasing compounds can induce flowering |
Ethylene is important in post-harvest handling. It can speed ripening and senescence. Ethylene-sensitive produce should be managed carefully during storage and transport.
conceptual confusion: Ethylene is a hormone even though it is a gas.
PGR Comparison Table
| Hormone | Main keyword | Promotes | Inhibits / controls | Agricultural clue |
|---|---|---|---|---|
| Auxin | Elongation | Rooting, apical dominance, tropism | Lateral buds under apical dominance | IBA/NAA for rooting, 2,4-D as herbicide |
| Gibberellin | Height | Stem elongation, germination, bolting | Dormancy | GA3, alpha-amylase in cereals |
| Cytokinin | Division | Cell division, shoots, delayed senescence | Apical dominance effects | Tissue culture shoots |
| ABA | Stress | Dormancy, stomatal closure | Germination, growth | Drought response |
| Ethylene | Ripening | Fruit ripening, abscission, senescence | Excess vegetative elongation in some contexts | Post-harvest ripening |
Transpiration and PGR Integration
Plant physiology topics are often connected in exam questions.
| Situation | Physiological explanation |
|---|---|
| Drought stress | ABA increases and closes stomata |
| Stomatal closure | Reduces transpiration but also reduces CO2 entry |
| Reduced CO2 entry | Lowers photosynthesis and yield |
| Moist grain sprouting | Gibberellin activates enzymes like alpha-amylase |
| Fruit ripening during storage | Ethylene accelerates ripening and senescence |
| Rooting of cuttings | Auxins promote adventitious roots |
| Tissue culture regeneration | Auxin-cytokinin ratio controls root or shoot formation |
FCI and Agriculture Relevance
| Topic | Why it matters for FCI/agriculture |
|---|---|
| Transpiration | Explains drought effect on crop yield and grain filling |
| Stomata | Balance water loss with photosynthesis |
| ABA | Drought survival and seed dormancy |
| Gibberellin | Cereal germination, sprouting and enzyme activity |
| Ethylene | Ripening and post-harvest quality |
| Auxin | Weed control and rooting |
| Cytokinin | Tissue culture and delayed senescence |
During grain filling, balanced water supply supports photosynthesis and assimilate transport to grains. Drought causes stomatal closure, reduced photosynthesis and shrivelled grains. During storage, unwanted germination or ripening-related changes reduce quality and shelf life.
Common Conceptual Confusions
- Transpiration is vapour loss; guttation is liquid water loss.
- Stomatal transpiration is the major type.
- Guard cells turgid means stomata open.
- High humidity reduces transpiration; dry air increases it.
- Wind increases transpiration only up to a point; very high wind can cause stomatal closure through stress.
- Auxin is linked with apical dominance and rooting.
- Gibberellin is linked with bolting, seed germination and alpha-amylase.
- Cytokinin is linked with cell division and delayed senescence.
- ABA closes stomata and maintains dormancy.
- Ethylene is a gaseous hormone and promotes fruit ripening.
- High auxin : cytokinin favours roots; high cytokinin : auxin favours shoots.
Summary
Transpiration is the loss of water vapour from aerial plant parts, mainly through stomata. It helps ascent of sap, mineral transport and cooling, but excessive transpiration causes wilting and yield loss. Its rate depends on light, temperature, humidity, wind, soil water, leaf area, stomata and cuticle. Plant growth regulators control growth and stress responses: auxins promote elongation and rooting, gibberellins promote stem elongation and germination, cytokinins promote cell division and delay senescence, ABA controls drought response and dormancy, and ethylene regulates ripening and abscission. For FCI AG-III, connect these ideas to drought stress, grain filling, sprouting, post-harvest quality and storage management.
Deep Revision Layer for Exam Mastery
Transpiration is driven by water potential gradient from soil to root to xylem to leaf air spaces to atmosphere. Stomata are the main regulators. When guard cells become turgid, stomata open; when guard cells lose turgor, stomata close. Light, low internal CO2 and adequate water usually favour opening. Water stress increases ABA, which promotes stomatal closure and reduces water loss.
The advantage of transpiration is mineral transport, cooling and maintenance of water flow. The disadvantage is water loss. This dual nature is why exam questions may ask "necessary evil." A crop under drought closes stomata to save water, but this also reduces CO2 entry and photosynthesis, lowering yield.
Plant Growth Regulator Master Table
| Hormone | Main memory cue | Agriculture link |
|---|---|---|
| Auxin | Apical dominance, rooting, elongation | Rooting cuttings, herbicide at high dose |
| Gibberellin | Stem elongation, bolting, germination | Break dormancy, cereal aleurone enzymes |
| Cytokinin | Cell division, delays senescence | Tissue culture shoots, leaf freshness |
| ABA | Stress hormone, dormancy, stomatal closure | Drought response, seed dormancy |
| Ethylene | Ripening, abscission, senescence | Fruit ripening, post-harvest handling |
Applied FCI Angle
Pre-harvest drought reduces grain filling through stomatal closure and lower photosynthesis. Excess humidity near harvest can encourage sprouting in some grains if dormancy is weak. Hormonal balance influences germination, dormancy and senescence, which connects plant physiology to seed quality and storage safety.
Exam-Safe Distinctions
Guttation is liquid water loss through hydathodes, usually at leaf margins. Transpiration is vapour loss, mainly through stomata. Bleeding is sap exudation from cut surfaces. ABA closes stomata; ethylene ripens fruit; gibberellin stimulates alpha-amylase in cereal germination.
Lesson Doubts
Ask questions, get expert answers