Transpiration and Guttation in Plants

Transpiration and Guttation

• Plants absorb large quantities of water from the soil, but utilize only a small amount of it.
• Excess water is lost from the aerial parts of plants in the form of water vapor through transpiration.
• Transpiration can be demonstrated by keeping a potted plant under a bell-jar, covering the pot with a polythene bag, and applying Vaseline to make the apparatus air-tight, which will result in water drops forming on the inner walls of the bell-jar.
• Transpiration is a vital physiological process for plants where water is lost from aerial parts as water vapor, essential for living tissues.
• Evaporation is a purely physical process where liquid turns into vapor, not necessarily at boiling point, and doesn't need living tissues.

Kinds of Transpiration

• There are three kinds: stomatal, cuticular, and lenticular.
• Most transpiration occurs through stomata, which are usually more numerous on the lower sides of leaves.
• Monocots have stomata equally distributed, while aquatic plants with floating leaves have them on the upper surface.
• Cuticular transpiration involves water loss through the cuticle, contributing a maximum of about 10% of total transpiration.
• Lenticular transpiration is water loss through lenticels in woody stems.

Mechanism of Stomatal Transpiration

• Stomatal transpiration during the day happens in three steps: osmotic diffusion of water from xylem to intercellular spaces, opening and closing of stomata, and diffusion of water vapor to the atmosphere.
• Mesophyll cells are in contact with xylem and intercellular spaces above stomata.
• Mesophyll cells draw water from xylem, become turgid, decrease diffusion pressure deficit and osmotic pressure, releasing water as vapor into intercellular spaces.
• O.P. and D.P.D. of mesophyll cells increase, drawing water from xylem by osmotic diffusion.

Opening and Closing of Stomata (Stomatal Movement)

• Stomata can be recognized from the epidermal cells by shape.
• Epidermal cells surrounding the stomata may be similar or specialized, in which case they are called subsidiary cells.
• Guard cells differ from other epidermal cells by containing chloroplasts and thickenings on their adjacent surfaces.
• Cellulose microfibrils in guard cell walls radiate from the pore outward, allowing elongation lengthwise but preventing crosswise elongation when turgid.
• Increased osmotic pressure and diffusion pressure deficit in guard cells cause osmotic diffusion of water from surrounding epidermal and mesophyll cells into the guard cells.
• Increased turgor pressure causes guard cells to swell, stretch thickened surfaces to form a pore, and open the stomata.
• Decreased O.P. and D.P.D. in guard cells cause them to release water back into epidermal and mesophyll cells, becoming flaccid; thickened surfaces come together to close the stomatal pore.
• Osmotic diffusion of water into guard cells occurs when their osmotic and water potential decrease relative to surrounding cells.
• Guard cells become flaccid when osmotic and water potential increase relative to surrounding cells; water moves from higher to lower water potential areas.
• Agents or mechanisms create osmotic potential in guard cells and control stomatal movements, like starch hydrolysis into sugars, synthesis of organic acids, and active pumping of K+ ions accompanied by Cl- or organic acids.

Starch-Sugar Interconversion Theory

• It is based on the effect of pH on starch phosphorylase enzyme, which converts starch + inorganic phosphate into glucose-l-phosphate reversibly and high pH during the day.
• High pH favors starch hydrolysis into glucose-I-phosphate, lowering osmotic potential in guard cells.
• Water enters the guard cells by osmotic diffusion making them turgid.
• During the dark reverse process, glucose-l-phosphate converts back into starch increasing osmotic potential, the guard cells become flaccid and the stomata close.
• Steward suggested glucose-l-phosphate be further converted into glucose and it needs ATP for stomata to close probably through respiration.
• Starch-Sugar theory is not universally applicable because it may operate under certain circumstances only and does not find much support by recent workers.

Synthesis of Sugars or Organic Acids in Guard Cells

• During daylight, photosynthesis occurs in guard cells, decreasing their water potential and resulting in stomatal opening.
• Very small amounts of soluble sugars are extracted from guard cells but are insufficient to affect water potential.
• As a result of photosynthesis, CO2 concentration decreases and pH increases.
• Organic acids, like malic acid, build up by combining with phosphoenol pyruvate (PEP) to form malic acid (enzyme PEP-Carboxylase).

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

• Formation of malic acid produces protons that operate in an ATP-driven-proton-K+ exchange pump to move protons into the adjacent epidermal cells and K+ ions into the guard cells.
• This exchange contributes in decreasing water potential of the guard cells and leading to stomatal opening; reverse occurs in darkness.
• Accumulation of K+ ions in the guard cells during daylight involves 'pumping out' protons (H+) from guard cells into adjacent epidermal cells and pumping in K+ ions.
• Exchange of H+ and K+ ions is followed by entry of CI- anions into the guard cells, resulting from the electrical differential due to K+ ion accumulation and active process mediated through ATP.
• The ATP required in ion exchange may come through respiration; WIK+ exchange needs ATP, supported by ATP addition to epidermal strips in a KCl solution in light, greatly increasing stomatal openings.
• Accumulation of K+ in guard cells is accompanied by increased pH and organic acids, like malic acid, forming protons (proton -K+ exchange).
• Organic acid anions are neutralized by K+ ions, and accumulation of K+ ions with Cl- and organic acid ions significantly decrease the water potential of guard cells.
• Consequently, water enters producing turgor pressure and ultimately the stomatal pores open.

Factors Affecting Stomatal Movements

• Light has a strong controlling influence as stomata generally open in light and close in darkness.
• The amount of light required to achieve maximal stomatal openings can vary by species, for example, some plants like tobacco require low light intensities of 2.5% full daylight while others require full sunlight.
• Plants such as CAM (Crassulacean Acid Metabolism) open at night and close during the day, absorbing CO2 and fixing them into organic acids at night.
• During the day, CO2 is released from organic acids and is reduced photosynthetically.
• The duration of time stomata stay open in daylight and close at night varies among plant species; green light is ineffective, and the action spectrum of light on stomata is similar to photosynthesis with blue light.
• Some plants lack the photosynthetic spectrum and are only sensitive to blue light while the photosynthetic component is due to photosynthesis in guard cells which contain chloroplasts.
• Photosynthesis reduces CO2 in guard cells, and provides a powerful stimulus for opening stomata.
• Osmotically active substances, like soluble sugars, are synthesized during photosynthesis, contributing to decreasing water potential of guard cells.
• Photophosphorylation may provide ATP, which makes the transport of ions possible, and these ions, that enter the guard cells, decrease the water potential.
• pH increases from illumination in the guard cells; high pH favors hydrolysis of starch which reduces the water potential of the guard cells (hydrolysis declines when pH is low).

Carbon Dioxide Concentration

• Reduced CO2 favors opening of stomata, but increased CO2 promotes stomatal closing.
• Stomata can be induced to open in dark if CO2 is lowered below normal air or marked increase in CO2 can close stomata quickly in dark & light.
• CO2 present inside the leaf (intercellular) has a controlling influence, because the cuticle over the guard cells and epidermal cells is impermeable providing a response to CO2 in the leaf.

Temperature

• Increasing temperature results in increased stomatal opening only if water is not a limiting factor.
• Stomata of some plants do not open at very low temperatures even in strong light.
• Stomata may close even at hight temperature becuase of increased CO2 inside the leaves with heat-impaired photosynthesis .

Water Deficits and Abscisic Acid (ABA)

• A water deficit happens when the transpiration rate exceeds the water absorption rate, and the plants are called water-stressed plants.
• Most mesophytes close stomata to protect against extreme water shortage, and only reopen when water potential is restored
• The stomatal movement by water is called hydro passive control.

Significance of Transpiration

• Plants waste much energy absorbing large quantities of water, which is ultimately lost through transpiration.
• Transpiration is advantageous to plant even if it is unavoidable process that is rather harmful.

Factors Affecting Rate of Transpiration

• Atmospheric Humidity: High humidity decreases transpiration because the atmosphere is saturated and retards the passage of water to the air, where dry atmosphere increases transpiration.
• Temperature: Increased temperature brings lowering relative humidity and opening of the stomata increasing transpiration.
• Wind: Transpiration remains normal is air is stagnant, increases when blowimg gently that removes moisture and stops when wind is blowing violently and hinders the outward diffusion of the water vaPours.
• Atmospheric Pressure: the effect of this is nil because positive effect at hills is neutralized by the temperature.
• Light: Light increases the rate of transpiration for the stomata open.
• light also causes for the temperature .
• Available Soil Water: The rate of transpiration wll decrease if there is not enough of water in the soil.
• CO2: An increase of CO2 in the atmosphere can retard transpiration.
• Internal Water Condition: Deficiency will decrease transpirtion.
• Structural Features: These affect transpiration and includes the number, size, and position.

Guttation

• In some plants such as garden nasturtium, tomato, Colocasia etc., watery drops ooze out (watery) at the ends of the leaf.
• This is called, guttation and it happens in the early morning when the rate of water absorption and root pressure are high.
• The drops consist of a water that contains inorganic compounds.
• Guttation is with stomata that is called water stomata and hydaihodes.
• It consists of a water pore (remains open) with a small cavity that follow the water elements.
• The water then pass thrugh xylem to epithem that releases to the cavity.
• When the caviti is complete the watere oozes.

Differences between Transpiration & Guttation

• Transpiration: Water lost from aerial parts as water vapor vs. Guttation: Watery solution oozes out from uninjured margins.
• Transpiration: Occurs in all vascular plants vs. Guttation: Only in some plants.
• Transpiration: Mostly through stomata, cuticle, and lenticels vs. Guttation: Only through hydathodes (Water Stomata).
• Transpiration: All day, maximum at noon vs. Guttation: Early morning when water absorption is higher.

Ascent of Sap

• Water absorbed by roots is distributed to all plant parts through the stem, which is the upward movement of water called ascent of sap.
• Ascent of sap can be studied under Path of Ascent of Sap, and Mechanism of Ascent of Sap.

Path of Ascent of Sap

• It can be shown with experiemnts that this takes place through xylem.
• Balsam plant will demostrate it.
• Ringing experiment can also show this.

Mechanism of Ascent of Sap

• It can be explained easly, but the tall trees, such as the trees in Australia and Sequoias can be hard for the water to get up to the height of hundreds.
• Ascent Theories
• (A) Vital Theories that the system is alive in the stem.
• Godlewski believed that the activity in pumping comes from the system.
• Bose believed that translocation happens because of the inside cortecal layer.

Root Pressure Theory

• The theory can raise to a certain height but is not an effective
• This can be shown with a magnitude that is low (about 2 arms).
• Absence of experiments continue.
• Gymnosperms that is not observed.

Physical Force Theories

• May be involved in the ascent but is (1) is not Convincing because it does not act on water present in the tissue.
• (2) imbibition
• (3) Capillary Force
• (D) Transpiration Pull and Cohesion of Water Theory
• proposed by Dixon and Jolly (1894) and greatly supported and elaborated by Dixon (1914, 1924).
• (i)Cohesive and adhesive molecules form coninuous column in the molecule because the molecules contain H. Bond.