Comprehensive Study Material on Transpiration for UP TGT and PGT Agriculture Examinations

Transpiration is the biological process by which water is absorbed by the roots of plants, transported upward through the xylem tissue, and eventually lost in the form of water vapor primarily through the aerial parts of the plant, such as leaves and green stems. It is often described as a 'necessary evil' because while it causes substantial water loss, it is vital for mineral nutrient uptake, cooling of leaf surfaces, and maintaining internal plant turgor.

• Driving Force: The primary driving force for transpiration is the water potential gradient between the moist intercellular spaces of the leaf mesophyll cells and the drier outside atmosphere.
• Energy Requirement: It is a passive physical process driven by solar energy, wherein latent heat of vaporization is supplied by the sun.
• Agronomic Relevance: Understanding transpiration helps agronomists design efficient irrigation scheduling, calculate crop water requirement (CWR), and manage water use efficiency (WUE) in major field crops such as Triticum aestivum and Oryza sativa.

Based on the plant organs and structures involved, transpiration is classified into three distinct types: Stomatal, Cuticular, and Lenticular.

• Stomatal Transpiration: Accounts for 80% to 90% of the total water loss in plants. It occurs through microscopic pores called stomata located predominantly on the epidermis of leaves.
• Cuticular Transpiration: Occurs through the cuticle, a waxy layer covering the epidermal surfaces of leaves and stems. It accounts for 3% to 10% of total transpiration and is higher in young leaves or plants grown in shade.
• Lenticular Transpiration: Occurs through lenticels, which are raised porous tissues found on the periderm of woody stems and older roots. It accounts for less than 1% of total transpiration.
• Stomatal Mechanism: Guard cells regulate stomatal aperture through changes in turgor pressure. Potassium ion (K⁺) accumulation, malate synthesis, and starch-sugar interconversion play central roles in opening and closing stomata.

Transpiration rate is governed by a complex interplay of atmospheric and plant physiological factors.

• Light Intensity: Light triggers the opening of stomata during daytime (except in Crassulacean Acid Metabolism or CAM plants), thereby increasing transpiration.
• Relative Humidity (RH): There is an inverse relationship between RH and transpiration rate. High atmospheric humidity reduces the vapor pressure deficit (VPD), slowing down transpiration.
• Temperature: Higher temperatures increase the kinetic energy of water molecules and decrease relative humidity, thereby accelerating transpiration up to a threshold limit beyond which stomata close due to water stress.
• Wind Velocity: Moderate wind sweeps away the humid boundary layer surrounding the leaf, steepening the vapor pressure gradient and increasing transpiration. High winds, however, may cause mechanical closure of stomata.
• Soil Water Availability: Soil moisture deficit induces water stress, leading to the synthesis of abscisic acid (ABA), which triggers rapid stomatal closure to conserve water.

Accurate quantification of transpiration is essential for agro-meteorological studies, irrigation engineering, and crop modeling.

• Potometer: Laboratory devices used to measure the rate of water absorption by a cut shoot, which is assumed to be roughly equal to the rate of transpiration. Examples include Ganong's potometer and Darwin's potometer.
• Lysimeter: A field-scale weighing or drainage device used to measure actual evapotranspiration (ET) from a soil-crop system under natural conditions.
• Porometer: An instrument used to measure stomatal resistance and leaf conductance, providing direct insight into stomatal transpiration dynamics.
• Cobalt Chloride Paper Test: A qualitative method where blue cobalt chloride paper turns pink upon absorbing water vapor released from the leaf surface.

Agronomists manipulate transpiration to optimize crop yield per drop of water, especially in arid and semi-arid rainfed agriculture.

• Antitranspirants: Chemical compounds or physical agents applied to crops to reduce transpiration losses without significantly inhibiting photosynthesis.
• Types of Antitranspirants:
  • Stomatal Closing Type: Chemicals like Phenylmercuric Acetate (PMA), Abscisic Acid (ABA), and Atrazine that induce stomatal closure.
  • Film-Forming Type: Waxy or plastic emulsions like Silicon oils, Mobileaf, and Hexadecanol that form a physical barrier over stomata.
  • Reflective Type: Materials like Kaolin (White clay) that increase leaf albedo and reduce net radiation load.
  • Growth Retardants: CCC (Cycocoel) which reduces leaf area expansion and overall transpiration.
• Water Use Efficiency (WUE): Defined as the ratio of economic yield to the total water consumed by the crop through evapotranspiration (ET). C₄ plants like Zea mays and Sorghum bicolor exhibit higher WUE than C₃ plants like Triticum aestivum and Oryza sativa.