Transpiration in Plants: Definition and Types

Transpiration

• Plants absorb large quantities of water from the soil but only utilize a small amount and the excess water is lost from the aerial parts of plants as water vapor. This process is called transpiration
• Transpiration differs from evaporation, since it is a physiological process in plants, in which water is lost from their aerial parts in the form of water vapor and is essential for living tissues.
• Evaporation is a purely physical process in which any liquid converts into vapor without necessarily reaching the boiling point and living tissues are not essential for evaporation.
• Transpiration can be demonstrated by placing a potted plant under a bell jar, covering the pot with a polythene bag, and sealing the apparatus with Vaseline
• Water drops will be seen on the inner walls of the bell jar after some time

Kinds of Transpiration

• Stomatal Transpiration: Most of the transpiration happens through stomata, which are mostly on the lower sides of leaves, equally distributed on all sides in monocots like grasses, and present on the upper surface in aquatic plants with floating leaves.
• Cuticular Transpiration(peristomatal transpiration): Some water loss still occurs through the cuticle, even though it is impervious to water. This accounts for a maximum of 10% of the total transpiration.
• Lenticular Transpiration: Water is lost through lenticels in woody stems, and transpiration from leaves is called foliar transpiration

Mechanism of Stomatal Transpiration

• The mechanism of stomatal transpiration during the day occurs in 3 steps
• Osmotic diffusion of water from xylem to intercellular spaces through mesophyll cells
• Opening and closing of stomata (stomatal movement)
• Simple diffusion of water vapor from intercellular spaces to the atmosphere through stomata

Details of Stomatal Transpiration Steps

• Inside a leaf, mesophyll cells are in contact with xylem and intercellular spaces above stomata.
• Mesophyll cells become turgid after they draw water from the Xylem, decreasing their diffusion pressure deficit (D.P.D.) and osmotic pressure (O.P.), resulting in water release as vapor in intercellular spaces close to stomata via osmotic diffusion
• The O.P. and D.P.D. of mesophyll cells increase as they draw water from xylem by osmotic diffusion.

Opening and Closing of Stomata (Stomatal Movement)

• Stomata are identified from surrounding epidermal cells by their unique shape, where surrounding epidermal cells may be similar or differentiated into subsidiary cells
• The guard cells differ from other epidermal cells with chloroplasts and thickenings on adjacent surfaces
• Cellulose microfibrils radiate outward from the pore around their circumference. Guard cells elongate lengthwise but not crosswise when turgid.
• Osmotic pressure (O.P.) and diffusion pressure deficit (D.P.D.) increase in guard cells due to accumulation of osmotically active substances
• This action promotes osmotic diffusion of water from epidermal and mesophyll cells into guard cells, increasing turgor pressure (T.P.) and making them turgid.
• Guard cells swell, increase in length, and stretch thickened surfaces forming a pore, which opens the stomata

Stomatal Closing

• Osmotic pressure (O.P.) and diffusion pressure deficit (D.P.D.) of guard cells decrease relative to surrounding cells.
• Water is released back into epidermal and mesophyll cells by osmotic diffusion, causing guard cells to become flaccid
• The thickened surfaces then come close, closing the stomatal pore
• Osmotic diffusion of water into guard cells is aided when osmotic potential and water potential decrease relative to surrounding epidermal and mesophyll cells.
• Guard cells become flaccid when osmotic and water potential increase relative to surrounding cells

Osmotic Potential

• Osmotic potential is less negative when water moves from higher to lower water potential
• Osmotic potential in guard cells is created and controlled by several mechanisms
• Hydrolysis of starch into sugars
• Synthesis of sugars or organic acids,
• Active pumping of K+ ions (accompanied by CI- or organic acids)

Starch-Sugar Interconversion Theory

• This theory says pH affects Starch phosphorylase enzyme for starch and inorganic phosphate conversion into glucose-l-phosphate
• During the day, the pH in guard cells is high, which favors starch hydrolysis (insoluble) into glucose-I-phosphate (soluble).This process lowers osmotic potential.
• Water enters guard cells via osmosis from the surrounding cells. The guard cells become turgid, opening the stomata. The reverse happens in the dark.
• Glucose-l-phosphate transforms back to starch increasing osmotic potential and the guard cells release water, becoming flaccid closing the stomata

Steward's theory

• Steward (1964) suggests that glucose-l-phosphate converts into glucose and inorganic phosphate for stomata opening
• Metabolic energy as ATP is needed to close them, probably through respiration

Synthesis of Sugars or Organic Acids in Guard Cells

• During daylight photosynthesis occurs in guard cells because they contain chloroplasts, and the soluble sugars decrease guard cells water potential which opens stomata
• Photosynthesis decreases CO2 concentration and increases pH during daylight
• Organic acids, such as malic acid, may accumulate as (HCO)- combines with phosphoenol pyruvate (PEP) to form malic acid with PEP-Carboxylase.
• The creation of malic acid generates protons for an ATP-driven proton-K+ exchange pump
• The exchange pump then moves protons to the epidermal cells while K+ ions move into the guard cells, reducing water potential and leading to stomatal opening.

ATP-Driven Proton (H+) - K+ Exchange Pump Mechanism in Guard Cells

• Growing evidence says that this mechanism which controls stomata, is more widely accepted than starch hydrolysis theory
• K+ ions accumulate in guard cells during daylight
• Protons (H+) are pumped out of guard cells into adjacent epidermal cells, while also pumping K+ ions are into the guard cells
• Electrical differential due to K+ ion accumulation prompts Cl- anion entry-
• H+ and K+ ions are exchanged through ATP as an active process, and ATP is created in non-cyclic photophosphorylation
• The ATP needed in ion exchange along with respiration. The finding that more ATP added to epidermal strips floating in KCl solution in light resulted in greater openings shows that WIK+ exchange is an active process needing ATP
• K+ accumulation in guard cells increases pH and organic acids(malic acid) as formation of malic acid produces protons to operate in proton -K+ exchange

Stomata Movement

• K+ ions neutralize organic acid anions
• Accumulation of K+ ions and other ions reduce guard cell water potential during daylight
• Water enters from adjacent cells, which increases turgor pressure and opens the pore
• The stoma closes in the dark due to the reverse situation and no K+ accumulation
• MacaUum first noticed K+ accumulation in guard cells (1905), Lloyd (1925) confirmed it. Japanese workers said K+ movement and accumulation increase osmotic pressure to induce opening of stomata
• The last step in transpiration is water vapor diffusion to atmosphere through open stomata, since intercellular spaces are more saturated

Factors Affecting Stomatal Movements

• The factors that control stomatal movements such as opening and closing are:

Light

• Light strongly influences stomatal movements, and stomata generally open in light and close in darkness.
• The light needed for maximal opening varies by species
• Plants with CAM open stomata at night and close during the day
• The plants absorb CO2, fix into organic acids at night and during the day, CO2 releases from the organic acids for photosynthesis
• How long stomata remain open in daylight or closed at night differs, the same is true for different light wavelengths
• Opening of stomata is ineffective in green light
• The action spectrum of light resembles photosynthesis, with a superimposed blue light effect
• Light can influence stomatal opening by reducing CO2 in guard cells during photosynthesis
• Light synthesizes osmotically active substances to reduce water potential

Other Influences

• Photophosphorylation generates ATP for H+/K+ exchange
• H+ ions and K+ and Cl- ions decrease water potential
• Illumination raises pH in guard cells due to less CO2
• High pH favors starch hydrolysis into osmotically active sugars and decreases the water potential of guard cells

Carbon Dioxide Concentration

• CO2 affects stomatal movement, where decreases promote stomata to open, while an increase causes stomatal closure
• Lower CO2 concentration can open stomata even in the dark. A high CO2 concentration causes closure in light and darkness.
• Stomata forced closed by high CO2 do not quickly reopen if the leaf is flushed with CO2-free air in darkness
• CO2 within the leaf controls stomatal movement because the cuticle is impermeable and responds to internal CO2.

Temperature

• Increased temperature usually increases stomatal opening, provided water is not limited
• Some plants' stomata do not open at low temperatures, but tend to close at high temperatures
• Increased respiration and impaired photosynthesis increases internal CO2.

Water and Abscisic Acid

• Water Deficit: Transpiration exceeding water absorption causes plant wilting
• Most mesophytes close stomata under these conditions with stomata reopening when water potential is restored
• Hydro-passive control: Water controls stomatal movement
• Transpiration Significance: Plants expend energy absorbing water, much of which is lost

Transpiration Rate Factors

External

• Atmospheric Humidity: Transpiration decreases in humid air and increases in dry air due to water diffusion.
• Temperature: Transpiration increases with temperature via humidity decrease and stomata opening.
• Wind: Stagnant or normal wind maintains normal transpiration, gentle wind increases it. A violent wind can hinder outward water diffusion and close stomata.
• Atmospheric Pressure: Atmospheric pressure's effect on transpiration is negligible.
• Light: Light opens stomata and increases temperature, increasing transpiration with stomatal transpiration stopping in the dark.
• Available Soil Water: Decreased water will also decrease transpiration
• CO2: Higher atmosphere CO2 closes stomata and hinders transpiration.

Internal

• Internal Water Condition: Water deficiency will decrease transpiration
• Structural Features: Stomata number, size affect transpiration. Sunken stomata and hairs reduce transpiration.
• Reduced leaf size, thick cuticles, and wax reduce transpiration.

Guttation

• Watery drops ooze from uninjured leaf margins in some plants like garden nasturtium, tomato, Colocasia etc
• Guttation occurs early morning when water absorption and root pressure are high coupled with low transpiration
• Watery drops have dissolved inorganic and organic substances, leaving residue after drying

Demonstrating Guttation

• A well-watered, potted garden nasturtium is kept under a bell jar on a glass sheet
• The pot should be covered in a polythene bag, and the apparatus is sealed with vaseline and connected to an aspirator.
• Sucking air with the aspirator causes watery drops to appear on the leaf margins.
• Guttation involves water stomata (hydathodes) with permanently open water pores, a small cavity, and epithem tissue associated with vascular elements

Water Release

• Higher root pressure delivers water to epithem, releasing water into the cavity. Cavity saturation causes watery drops to ooze through the pore.

Transpiration

• Water is lost from aerial parts as invisible water vapor
• It happens in all vascular plants
• It happens mostly through stomata and also through the cuticle and lenticles
• It happens throughout the day, peaking at noon

Guttation

• Watery solution oozes from aerial leaves
• It only occurs in some Anzoisperms, like garden nasturtium and tomato etc
• It occurs only through hydathodes (water stomata)
• It only happens early in the morning with high root pressure and high rate of water absorption

Ascent of Sap

• Water is distributed to all plant parts after root absorption - extra water transpires.
• Water must move up the stem to reach the highest parts; this upward movement is referred to as Ascent of Sap.
• Ascent of sap has two studied components - path and mechanism.

Path of Ascent of Sap

• Ascent of sap happens through xylem, as demonstrated by two experiments
• A leafy twig of Balsam plant is cut underwater and placed in a beaker with water and Eosin dye
• Colored lines move upward in the xylem, and xylem elements fill with colored water when stem sections are cut
• Ringing: A leafy twig cut underwater with a ring of bark is placed in water
• Ring Removal: Bark (outer tissues of to vascular cambium) removed from stem
• Fresh leaves above the ring show that water is supplied through xylem

Mechanism of Ascent of Sap

• Ascent of sap is easy to explain in small trees and herbaceous plants, but it can be an issue in tall trees (Australian Eucalyptus, Sequoias) needing to rise 300–400'
• The mechanism is poorly understood but many theories exist to explain it

Vital Theories

• Supporters of vital theories think the ascent of sap is under stem control.
• Godlewski (1884) says ascent is by xylem parenchyma pumping
• Medullary ray cells change their O.P., drawing water from lower vessels
• Low O.P pumps water into the vessel, repeated for water rise in xylem
• This theory was discarded by Strasburger (1891, 1893), experiments continued with poison uptake
• Bose (1923) says upward movement is through pulsatory activity of inner cortical layer outside the endodermis
• This process could not be repeated and no correlation between pulsatory activity and ascent was ever found

Root Pressure Theory

• Root pressure in xylem can raise water to a height but appears ineffective in ascent of sap
• The magnitude of root pressure is low
• Ascent of sap continues without root pressure
• A leafy twig placed underwater remains fresh
• Root pressure can be rarely observed in gymnosperms

Physical Force Theories

• Physical forces may be involved in the ascent of sap:
• Atmospheric Pressure: Does not act on water in roots or cannot raise above 34'
• Imbibition: Ascent of sap occurs by imbibition through xylem walls. It is insignificant in ascent of sap since it happens through the lumen of xylem, not the walls
• Capillary Force: Xylem vessels create continuous channels like capillary tubes for water rise
• There are many objections
• Free surface is needed for capillarity
• The capillary force is low
• Spring wood has broader elements and autumn wood has narrow - against capillarity
• Vessels are absent in Gymnosperms

Transpiration Pull & Cohesion Theory

• Proposed by Dixon and Jolly, and elaborated by Dixon and it is widely supported
• Water cohesion and adhesion properties form a continuous water column in xylem
• H-bonds join water molecules. Cohesive force maintains continuous water column in the xylem
• The force’s magnitude stops from easily breaking
• The attraction between water molecules and xylem walls ensures water column continuity.
• As water evaporates from leaves water gets released into intercellular spaces and the mesophyll cells draw it from xylem and tension pulls water up through xylem elements of leaves, stem, and roots.
• The water remains unbroken and goes up to the top.
• Air bubbles are said to break water's continuity, but others dispute this claim