What is Transpiration in Plant? - Agrobotany

Transpiration and Related Aspects

Introduction

Transpiration is the continuous loss of water in the form of water vapor from the aerial parts of higher plants — chiefly leaves but also stems, flowers and fruits. Though it appears as a passive loss of water, transpiration is central to plant water relations because it generates the driving forces for uptake and long‑distance transport of water and mineral nutrients, contributes to leaf cooling and helps maintain cell turgor necessary for growth processes.

Types of Transpiration

1. Stomatal Transpiration

This is the predominant form of transpiration (roughly 70–90% in many mesophytic plants). Water vapor diffuses from the moist cell‑wall surfaces of mesophyll cells into the intercellular spaces and then escapes to the atmosphere through stomatal pores. Stomatal opening and closing are major regulators of this component.

2. Cuticular Transpiration

Water loss directly through the cuticle (a waxy layer on epidermis). Usually small in amount in most plants, but may be relatively more important in species with numerous or permeable cuticular cracks or in very dry environments where stomata are closed.

3. Lenticular Transpiration

Occurs through lenticels present on stems of woody plants. Lenticels are permanently open porous structures and contribute a small but continuous water loss from stems and twigs.

Steward’s Theory of the Mechanism of Transpiration

Steward proposed a clear physical explanation treating transpiration as an evaporation–diffusion process that follows water potential gradients. The sequence of events in his description may be summarized as follows:

1. Water movement to leaf tissues: Water absorbed by roots ascends via xylem to the leaf veins and diffuses from xylem into mesophyll cell walls.
2. Formation of a surface film: The walls of mesophyll cells remain covered by a thin film of water that is in dynamic equilibrium with the cell sap.
3. Evaporation into intercellular spaces: Heat energy (chiefly from solar radiation) causes evaporation of the surface film into the intercellular air spaces, producing water vapor.
4. Diffusion outwards: Water vapor moves by diffusion from intercellular spaces through stomatal apertures into the outer atmosphere following a concentration (vapour pressure) gradient — from a region of higher vapour pressure inside the leaf to a lower vapour pressure outside.

Steward emphasized that the physical gradient (vapour concentration and water potential differences) and diffusion through stomatal openings control the rate of transpiration; stomatal regulation and environmental parameters modify this basic physical process.

Significance of Transpiration

• Ascent of sap: Continuous transpiration establishes a negative pressure (transpiration pull) in the xylem that helps draw water and dissolved minerals from roots to shoots and leaves.
• Mineral transport: Transpiration stream carries essential ions from the soil to photosynthetic and growing tissues.
• Cooling of plant tissues: Evaporative water loss dissipates heat and prevents overheating of leaves under high irradiance.
• Maintenance of turgor and cell expansion: Adequate water movement maintains cell turgor, which is necessary for cell enlargement, stomatal function and growth.
• Influence on microclimate and gas exchange: Transpiration is coupled with CO₂ uptake for photosynthesis; stomatal behavior represents a trade‑off between water loss and carbon gain.
• Plant–environment interactions: Patterns of transpiration influence drought responses, water‑use efficiency and ecological distribution of species.

Factors Affecting Transpiration

External (Environmental) Factors

• Light: Promotes stomatal opening (thus increasing stomatal transpiration) and provides the energy for evaporation from cell surfaces.
• Temperature: Higher temperature raises saturation vapour pressure and molecular diffusivity; both favor increased transpiration rate.
• Humidity (Vapour Pressure Deficit): Low atmospheric humidity (high vapour pressure deficit between leaf interior and atmosphere) increases transpiration; high humidity reduces it.
• Wind/air movement: Removes the thin boundary layer of saturated air around leaves, increasing the vapour concentration gradient and hence transpiration.
• Soil moisture: Adequate soil water supports transpiration; soil drought limits water supply and often leads to stomatal closure, reducing transpiration.
• Atmospheric pressure: Lower barometric pressure (as at high altitudes) can increase evaporation rates.

Internal (Plant) Factors

• Stomatal number, size and behaviour: Density and aperture control the major pathway for water loss.
• Leaf anatomy: Thickness of cuticle, presence of trichomes, boundary layer characteristics and internal air space volume influence transpiration.
• Leaf area and orientation: Larger leaf area and leaf orientation relative to sun and wind change the effective transpiring surface and exposure.
• Hydraulic conductivity of xylem and root system: Limits to water supply from roots and through xylem influence leaf water status and stomatal behavior.
• Age and phenological stage: Young, rapidly expanding leaves may transpire differently from mature or senescent leaves.

Guttation

Guttation is the appearance of liquid water droplets on leaf margins or tips caused by root pressure forcing xylem sap out through specialized structures called hydathodes. Guttation typically occurs at night or in the early morning when soil moisture is high and transpiration is very low (stomata closed), so the pressure generated by active root absorption ejects excess water from the vascular system.

Mechanism

When transpiration is suppressed (e.g., at night) and soil moisture is abundant, osmotic absorption of water by roots generates a positive root pressure. This pressure drives water upward in the xylem; if water cannot escape to the atmosphere as vapour, it may be exuded as liquid through hydathodes at leaf margins. The exuded fluid is often enriched in dissolved minerals and organic solutes.

Antitranspirants

Antitranspirants are materials applied to leaves or plants to reduce transpiration. They are used in horticulture, forestry and agriculture to conserve water, reduce transplant shock and improve survival during drought spells. Antitranspirants may act by physiological modification of stomatal behaviour or by physical alteration of the leaf surface.

Main Categories

1. Stomatal closing types: Chemical substances (e.g., abscisic acid, some phenolic compounds) that induce partial or temporary stomatal closure and thus reduce water loss. These act by triggering guard cell responses (osmotic changes) or mimicking natural plant hormones.
2. Film‑forming types: Waxy or polymeric sprays (e.g., silicone oils, natural waxes, some commercial emollients) that form a thin semi‑permeable film over the leaf surface, reducing cuticular water loss and altering gas diffusion. Care is needed as they may also restrict CO₂ diffusion if used excessively.
3. Reflectant types: Materials (e.g., kaolin clay) that increase leaf reflectance, reduce leaf temperature and thereby indirectly reduce transpiration.
4. Growth retardants and size reducers: Compounds (e.g., CCC—chloromequat or similar growth regulators) that reduce leaf area and thus the total transpiring surface.

Advantages and Limitations

• Antitranspirants can help reduce water loss during transplanting and in water‑limited conditions, improving survival and establishment.
• Some may transiently reduce photosynthesis or gas exchange if stomatal closure is too severe; film‑forming agents may impede CO₂ diffusion at high doses.
• Selection of type, dose and timing is important to balance water conservation with carbon assimilation.

Difference Between Transpiration and Guttation

Feature	Transpiration	Guttation
Nature of water loss	Water lost in the form of water vapour	Exudation of liquid water (droplets)
Structures involved	Primarily stomata; also cuticle and lenticels	Hydathodes located at leaf margins/tips
Time of occurrence	Generally daytime when stomata are open	Usually at night or early morning when transpiration is low
Driving force	Vapour pressure gradient and transpiration pull (negative xylem pressure)	Positive root pressure
Physiological significance	Essential for ascent of sap, cooling, mineral transport and turgor maintenance	Minor; relieves excess root pressure and expels some solutes

Conclusion

Knowledge of transpiration, its controlling factors and related phenomena such as guttation is fundamental to plant physiology and agronomy. Understanding these processes helps in irrigation scheduling, breeding for water‑use efficiency, post‑transplant management and selecting appropriate antitranspirant strategies where water conservation is required.

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