Transpiration in Plants

Transpiration

• Plants absorb large quantities of water, but only utilize a small amount
• Transpiration is when excess water is lost from the aerial parts of plants as water vapor
• Transpiration differs from evaporation, as it is a vital physiological process in plants where water is lost from aerial parts as water vapors, and requires living tissues
• Evaporation is a purely physical process where liquid converts to vapor, and doesn't require living tissues

Kinds of Transpiration

• Stomatal transpiration: Most transpiration occurs through stomata, which are mainly on the lower side of leaves, equally distributed on all sides of monocots, and on the upper surfaces of aquatic plants with floating leaves
• Cuticular Transpiration: Some water is lost through the cuticle, contributing to a maximum of 10% of total transpiration
• Lenticular Transpiration: Water is lost through lenticels in woody stems, known as lenticular transpiration. Transpiration from leaves is called foliar transpiration

Mechanism of Stomatal Transpiration

• Stomatal transpiration happens during the daytime, and can be studied in 3 steps
• Osmotic diffusion of water from xylem to intercellular spaces through mesophyll cells
• Opening and closing of stomata movement
• Simple diffusion of water vapors from intercellular spaces to the outer atmosphere through stomata
• Inside the leaf, mesophyll cells interact with xylem and intercellular spaces above the stomata
• When mesophyll cells draw water from the xylem, they become turgid, decreasing diffusion pressure deficit (D.P.D.) and osmotic pressure (O.P.)
• Water is then released as vapor into intercellular spaces near stomata via osmotic diffusion
• The O.P. and D.P.D. of mesophyll cells increase, causing them to draw water from xylem by osmotic diffusion

Opening and Closing of Stomata

• Stomata are recognized by their shape, with surrounding epidermal cells that can be similar or specialized subsidiary cells
• Guard cells contain chloroplasts, unlike other epidermal cells, and have unique thickenings on adjacent surfaces
• Cellulose microfibrils in guard cell walls radiate from the pore outward, allowing lengthwise elongation when turgid but preventing crosswise expansion
• Osmotic pressure and diffusion pressure deficit (D.P.D.) increase in guard cells due to accumulation of osmotically active substances
• Water then osmotically diffuses from surrounding cells into the guard cells which increases turgor pressure (T.P.) of the guard cells, and they become turgid
• Guard cells swell, get longer, and their adjacent thickened surfaces stretch, forming a pore so stomata can open
• Osmotic potential (ψs) and water potential (ψw) decrease as water osmotically diffuses into guard cells which become more negative relative to surrounding cells
• When O.P. and D.P.D. decrease in guard cells, water is released back into surrounding cells by osmotic diffusion and the guard cells become flaccid
• Thickened surfaces of guard cells move closer, closing the stomatal pore
• Guard cells become flaccid when their osmotic potential and water potential increase, so become less negative relative to surrounding cells
• Water moves from higher water potential to lower water potential
• Osmotic potential is influenced by agents like starch hydrolysis into sugars, synthesis of sugars or organic acids, and active pumping of K+ ions with counter ions

Starch-Sugar Interconversion Theory

• This theory says pH impacts starch phosphorylase, an enzyme that reversibly catalyses starch conversion with inorganic phosphate into glucose-l-phosphate.
• A high pH during the day in guard cells favours starch hydrolysis into soluble glucose-I-phosphate, decreasing osmotic potential and allowing water to enter by osmosis
• The guard cells become turgid, opening the stomata
• The reverse process occurs in the dark; glucose-l-phosphate converts back to starch, increasing osmotic potential, causing guard cells to release water, become flaccid, and close the stomata
• Steward suggested that glucose-l-phosphate should convert into glucose and inorganic phosphate for stomata opening as inorganic phosphate and glucose-l-phosphate are equally osmotically active
• For stomata closing, metabolic energy in the form of ATP would be required which comes through respiration

Synthesis of Sugars or Organic Acids

• During daylight, photosynthesis in guard cells decreases their water potential and results in stomatal opening
• Photosynthesis decreases CO2 concentration in guard cells, increasing pH
• Organic acids like malic acid build up in guard cells (HCO combining with phosphoenol pyruvate (PEP) to form malic acid in the presence of PEP-Carboxylase)

ATP-Driven Proton - K+ Exchange Pump Mechanism

• It is believed a mechanism exists in plants that controls stomata movement, more accepted than the classical starch hydrolysis theory
• K+ ions accumulate in guard cells during the day
• Protons (H+) are pumped out of guard cells into adjacent cells, and K+ ions are pumped into guard cells
• Anions(Cl-) enter guard cells due to the electrical differential from K+ accumulation
• ATP mediates the exchange of H+ and K+ ions; ATP is generated by non-cyclic photophosphorylation in guard cells during photosynthesis
• ATP can also come through respiration. Adding ATP to epidermal strips in a KCl solution under light increased stomatal openings
• K+ accumulation is also accompanied by increased pH and the buildup of organic acids (malic acid), which produces protons for proton-K+ exchange

Final Steps of Stomatal Transpiration

• The final step of transpiration is the diffusion of water vapors diffusing from intercellular spaces to the outer atmosphere through open stomata
• Intercellular spaces have more moisture compared to the atmosphere near the stomata

Factors Affecting Stomatal Movements

• Factors affecting stomatal movements include:
• Light: It influences stomatal movements, with stomata opening in light and closing in darkness. The amount of light needed varies by species
• Plants with Crassulacean Acid Metabolism (CAM) open stomata at night and close during the day to fix CO2 into organic acids at night and release CO2 during the day
• Green light is ineffective in stomatal opening. Stomata's action spectrum has a resemblance to photosynthesis with blue light
• Light can stimulate stomatal opening by reducing CO2 in guard cells, synthesizing osmotically active substances like sugars, providing ATP via photophosphorylation for ion exchange pumps, and increasing pH within guard cells
• Carbon Dioxide Concentration: Reduced CO2 opens stomata, while increased CO2 closes them
• Stomata close because of high CO2 and do not reopen rapidly when flushing with CO2-free air, but reopen with subsequent light exposure which occurs as CO2 gets consumed with photosynthesis
• The cuticle is impermeable to CO2, so stomata respond to CO2 levels inside the leaf rather than outside
• Temperature: Increased temperature boosts stomatal opening with adequate water supply
• Camellia stomata do not open at low temperatures, while in some plants stomata close at high temperatures (over 30°C), which may be due to increased respiration and impaired photosynthesis
• Water Deficits and Abscisic Acid (ABA): When transpiration exceeds water absorption, a water deficit occurs, the plant wilts, and becomes water-stressed
• Mesophytes close stomata in such conditions to prevent damage and only reopen when water potential is restored. Hydro passive control controls stomatal movement

Significance of Transpiration

• Plants expand energy absorbing water that's ultimately lost through transpiration
• Opinions vary on whether transpiration is advantageous or harmful

Factors Affecting Rate of Transpiration

• External Factors:
• Atmospheric Humidity: Transpiration decreases in humid conditions, as the air becomes saturated, retarding water vapour diffusion from the leaf's intercellular spaces. The opposite happens in dry air
• Temperature: Transpiration increases with temperature by reducing relative humidity and opening stomata
• Wind: Stagnant wind results in normal transpiration rate
• During gentle wind conditions, transpiration is increased due to moisture is removed from the vicinity
• During violent wind conditions, transpiration rate hindered, and stomata may close
• Atmospheric Pressure: Atmospheric pressure's effect is considered as nil, due to the positive and negative effects are neutralized
• Light: Light increases transpiration because it opens stomata and it increases the temperature
• Available Soil Water: Transpiration declines with inadequate soil water
• CO2: Increased CO2 leads to stomatal closure, retarding transpiration
• Internal Factors:
• Internal Water Condition: Water deficiency reduces transpiration
• Structural Features: Stomata's number, size, position, and movement impact transpiration, so closed stomata halts transpiration in darkness
• Sunken stomata reduce transpiration
• Hairs or situated in grooves reduces transpiration further
• Xerophytes have reduced leaves, and thick cuticles or wax reduces transpiration

Guttation

• Guttation is when watery drops ooze out from uninjured margins, such as garden nasturtium, tomato, and colocasia
• Guttation happens early in the morning with high water absorption and root pressure, and low transpiration
• Watery drops are water with inorganic and organic substances
• After drying, salts and organic substances remain as residue on the leaf margins.
• Guttation is associated with water stomata or hydathodes, which are special stomata types located at the margins of leaves
• Hydathodes have a permanently open water pore, beneath is a small cavity followed by epithem that is closely associated with the vascular elements of veins
• Under high root pressure xylem transfers water to the epithem, which then releases it into the cavity and the excess water flows out through water pores

Differences Between Transpiration and Guttation

• Transpiration:
• Water is lost from aerial parts as water vapor
• Occurs in all vascular plants
• Occurs through stomata, cuticle, and lenticels
• Typically happens all day, peaking at noon
• Guttation:
• Watery solution oozes from uninjured leaf margins
• Only occurs in some plants like garden nasturtium, tomato etc
• Occurs only through hydathodes (water stomata)
• Occurs early in the morning when there is high root pressure and water absorption

Ascent of Sap

• Water absorbed by roots goes to all plant parts, with the excess lost through transpiration
• Water moves upward through the stem to reach plants topmost parts, known as ascent of sap
• The two areas to study ascent of sap include: Path of Ascent of Sap and Mechanism of Ascent of Sap

Path of Ascent of Sap

• Ascent of sap occurs through xylem, which can be proven by:
• Cutting leafy twig, then placing in water with eosin, xylem will start to show colored lines
• Ringing experiment on a twig which still remains fresh due to water supply through xylem
• In small trees and herbaceous plants, explaining ascent of sap can be simple, but in tall trees, like Australian eucalyptus, mighty sequoias ascent of sap becomes a problem.
• There are numerous theories

Vital Theories

• Vital theories says that ascent of sap is controlled by activities in the stem
• 2 uncommon theories include:
• Godlewski: Pumping activity of living xylem parenchyma cells causes O.P alteration, and water goes upwards through the xylem
• Bose: Pulsatory activity of living cells in the inner cortical layer causes translocation
• Strasburger rejected these claims and demonstrated that it continues even when living cells die by uptake of poisons

Root Pressure Theory

• Root pressure cannot rise very high, and therefore is not effective
• Ascent of sap happens even when root pressure is absent
• Gymnosperms do not display root pressure often

Physical Force Theories

• Physical forces that causes ascent of the sap include:
• Atmospheric Pressure: Ineffective to act on the xylem
• Imbibition: Ineffective because it occurs through xylem's lumen
• Capillary Force: Ineffective, since a free surface is necessary for rise, there is low magnitude, and vessels absent in gymnosperms

Transpiration Pull and Cohesion of Water Theory

• Formulated by Dixon and Jolly
• It is based on cohesive and adhesive, strong properties of water that forms a continuous column in xylem, and is pulled upwards due to transpiration
• Water molecules are joined by hydrogen bonds and exhibit attraction
• The pull creates some of the tension in xylem elements
• Water is pulled in a continuous flow to reach the surfaces