Plant Transpiration and Guttation

Transpiration and Guttation

• Plants absorb large quantities of water from the soil
• Only a small amount of water absorbed is utilized
• Excess water is lost as water vapor from aerial parts of the plant
• This process of water loss is called Transpiration
• Transpiration can be demonstrated in a bell-jar experiment with a potted plant

Transpiration vs. Evaporation

• Transpiration is a vital physiological process in plants
• During Transpiration water is lost from the aerial parts of the plants as water vapor
• In transpiration living tissues are essential
• Evaporation, however, is a purely physical process
• During evaporation conversion of any liquid into vapors occur
• In evaporation living tissues are not essential

Kinds of Transpiration

• Stomatal transpiration: Most of it occurs through stomata, usually on the lower leaf sides
• In monocots and grasses, stomata are equally distributed on all sides of the leaf
• In aquatic plants with floating leaves, stomata are present on the upper surface of the leaf
• Cuticular transpiration: Some water loss happens through the cuticle despite it being impervious, which contributes to about 10% of total transpiration
• Lenticular transpiration: Water loss that happens through lenticels in woody stems, also known as foliar transpiration where transpiration occurs from the leaves

Mechanism of Stomatal Transpiration

• Takes place during the daytime
• 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 vapors from intercellular spaces to outer atmosphere through stomata

Opening and Closing of Stomata

• Stomata are easily recognized by their shape
• Epidermal cells that immediately surround the stomata cells that may be similar or different and specialized which would classify them as subsidiary cell
• Guard cells differ from other epidermal cells
  • The guard cells contain chloroplasts
  • Guard cells have peculiar thickenings on their adjacent surfaces (in closed stomata) or on surfaces adjacent to stomatal pore (in open stomata)
• There is radial micellation of cellulose microfibrils in the walls of the guard cells
• Cellulosic microfibrils radiate from the pore outward around their circumference
• Allows guard cells to elongate lengthwise and prevents their elongation crosswise when they become turgid
• Increase in osmotic pressure (O.P.) and diffusion pressure deficit (D.P.D.) of the guard cells is due to accumulation of osmotically active substances
• Osmotic diffusion of water from surrounding epidermal cells and mesophyll cells into the guard cells follows
• Increases the turgor pressure (T.P.) of the guard cells and they become turgid
• The guard cells swell, increase in length and their adjacent thickened surfaces stretch forming a pore meaning stomata open
• Decrease of O.P. and D.P.D. of guard cells is due to depletion of osmotically active substances relative to the surrounding epidermal and mesophyll cells
• Osmotic diffusion releases water back to the surrounding epidermal and mesophyll cells making guard cells flaccid which closes the stomata
• Osmotic diffusion of water into guard cells occurs when their osmotic potential and water potential decreases relative to surrounding cells with guard cells becoming flaccid when opposite reactions occur
• Movement of water takes place from a region of higher water potential to a region of lower water potential
• Multiple agents or mechanisms create osmotic potential and control stomatal movements such as hydrolysis of starch into sugars, synthesis of sugars of organic acids or active pumping of K+ ions in the guard cells

Starch-Sugar Interconversion Theory

• This theory is based on the effect of pH on Starch phosphorylase enzyme which catalyses the conversion of starch + inorganic phosphate into glucose-l-phosphate
• During the day, pH in guard cells is high
• Favors hydrolysis of starch into glucose-I-phosphate so osmotic potential becomes lower in the guard cells
• Water enters the guard cells by osmotic diffusion from the surrounding epidermal and mesophyll cells and become turgid so stomata open
• During dark reverse process occurs where glucose-l-phosphate is converted back into starch increasing the osmotic potential
• The guard cells release water, become flaccid and the stomata become closed

Synthesis of Sugars or Organic Acids in Guard Cells

• During daylight photosynthesis occurs in guard cells as they contain chloroplasts
• Soluble sugars formed contribute in decreasing water potential, resulting in stomatal opening
• Photosynthesis decreases CO2 concentration in guard cells, leading to increased pH
• Organic acids, chiefly malic acid, buildup during the period where (HCO)- combines with phosphoenol pyruvate (PEP) to form malic acid

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

• Growing evidence supports its operation for controlling stomatal movement
• The mechanism is more widely accepted than the classical starch hydrolysis theory
• States that there Potassium (K+) ions accumulate in the guard cells during daylight
• Protons (H+) are 'pumped out' from the guard cells into the adjacent cells and Potassium (K+) ions are pumped into them from adjacent cells
• H and K+ ions exchange are followed by entry of CI- anions into the guard cells which results in response to the electrical differential as a result of K+ ions accumulation
• H and K+ ions exchange is mediated through ATP making it an active process
• ATP is generated in non-cyclic photophosphorylation in photosynthesis in the guard cells
• ATP may also come through respiration
• Accumulation of Potassium (K+) is also accompanied by increased pH and organic acids build up
• Malic acid formation would produce protons that could operate in proton -Potassium (K+) exchange process

Factors Affecting Stomatal Movements

• Factors that greatly influence and control stomatal movements such as opening and closing of stomata
• Light
  • Has strong controlling influence on the stomatal movements
  • Stomata generally open in light and close in darkness
  • Amount of light required to achieve maximal stomatal openings varies with species with some requiring low-light and some requiring full sunlight
  • Plants showing CAM (Crassulacean Acid Metabolism) are exceptional and open at night and close during the day
  • Photosynthesis reduces the CO2 concentration in guard cells facilitating stomata opening
  • Osmotically active substances synthesized during photosynthesis decrease water potential of guard cells
  • High pH favors hydrolysis of starch decreasing the water potential of guard cells
• Carbon dioxide concentration:
  • Concentration of CO2 has a pronounced effect on stomatal movement
  • Reduced CO2 concentration favors opening while increased CO2 concentration promotes closing
  • It is CO2 inside the leaf (intercellular) that controls stomatal movement, not the outer atmosphere
• Temperature:
  • An increase in temperature usually results in increased stomatal opening provided water is not a limiting factor
  • Some plants do not open at very low temperatures, while others tend to close at high temperatures
• Water deficits and abscisic acid (ABA):
  • When the rate of transpiration exceeds the rate of water absorption, a water deficit is created
  • Plants begin to show signs of wilting and are known as water-stressed plants
  • Mesophytes close their stomata tightly to protect them from water shortage
• The cuticle present ensures stomata responds to the CO2 in the leaf rather than outer atmosphere

Factors Affecting Rate of Transpiration

External Factors

• Atmospheric Humidity
  • High humidity decreases the rate of transpiration
  • Dry atmosphere increases the rate of transpiration
• Temperature
  • An increase in temperature boosts rate of transpiration
  • Lowers the relative humidity
  • Opens the stomata wider
• Wind
  • Stagnant wind causes normal rates of transpiration
  • Gently blowing wind raises the rate of transpiration since it removes moisture
  • Violently blowing wind stops the rate of transpiration because it creates a hindrance in the outward diffusion of water and may close the stomata
• Atmospheric Pressure
  • Ultimate effect of atmospheric pressure is zero due to the positive effect being neutralized by low temperature
• Light
  • Increases the rate of transpiration
  • Opens stomata
  • Increases temperature
• Available soil water
  • Rate of transpiration will decrease if there is not enough water in the soil that the routes can easily absorb
• CO2
  • An increase in CO2 concentration leads towards stomatal closure and slows the rate of transpiration

Internal Factors

• Internal Water Condition
  • Water is essential for transpiration. Water deficiency will decrease rate of transpiration
• Structural Features
  • Affect rate of transpiration
  • Closing of stomata causes transpiration checks
  • Sunken Stomata help in reducing transpiration rate
  • Leaves in grooves or coated with hair decreases the rate of transpiration
  • Xerophytes leaves are reduced in size to check transpiration
  • Thick cuticle or wax reduces cuticular transpiration

Guttation

• Watery drops ooze out from the uninjured margins of the leaves where a main vein ends.
• Plants such as garden nasturtium, tomato, Colocasia exhibit this
• Occurs in the early morning when rate of water absorption and root pressure are higher
• Usually happens when the rate of transpiration is very low
• Watery drops consist of water that has dissolved organic and inorganic substances
• The salts and organic substances remain on the margins of the leaves in residue form after the drops dry
• Guttation needs special types of stomata that are called water stomata or hydathodes
• Hydathode has a water pore that remains permanently open
• Below it is a small cavity followed by epithem that joins association with the ends of the vascular elements of veins
• Under high root pressure water is given to the epithem by the xylem of the veins and epithem releases the extra water to the cavity

Transpiration vs. Guttation

• Transpiration: water is lost from aerial parts of plants in the form of invisible water vapours
• Guttation: watery solution oozes out from uninjured margins of aerial leaves only
• Transpiration: occurs in all vascular plants
• Guttation: occurs only in some plants
• Transpiration: occurs mostly through stomata, but also through cuticle and lenticels
• Guttation: occurs only through hydathodes
• Transpiration: takes place all day with maximum at noon
• Guttation: happens early in the morning when root pressure and rate of water absorption are higher

Ascent of Sap

• Water is distributed to all parts of the plant after being absorbed by the roots
• Upward movement of water through the stem is called Ascent of Sap
• Ascent can be studied regarding:
  • Path of Ascent of Sap
  • Mechanism of Ascent of Sap
• Ascent of sap takes place through xylem
• It can be shown in experiments like:
  • Placing leafy twig of Balsam plant in water with Eosin dye
  • Ringing Experiment a ring of bark(all the tissues outer to vascular Cambium) is removed

Mechanism of Ascent of Sap

• In small trees it is easily explained
• In big trees like Australian Eucalyptus, it rises up to 300-400'
• Theories:
  • Vital Theories
    • Ascent of sap is under the control of vital activities in the stem
    • Godlewski's theory of pumping activity says that ascent of sap takes place to the pumping activity of the cells of Xylem parenchyma which living
    • That cells of the medullary rays which are living, change O.P and draw water from the lower vessel and their becomes low
    • Water from the cells of xylem parenchyma is pumped into the above vessel, then repeated
    • Strasburger demonstrated ascent of sap continues in stems where living cells are killed by poison uptake, so disproved the theory
    • Bose's theory from 1923 said pulsatory activity of living cells of inner most cortical layer just outside the endodermis take part
    • But its experiment are not repeatable and found no correlation between pulsatory activity to ascent of sap
  • Root Pressure Theory
    • Root pressure which is developed in the xylem of roots, helps raises water
    • Magnitude of root pressure is very low so the results of this force are not so effective
    • Absent of sap continues if the roots are absent
    • Gymnosperms root pressure rarely has been observed

Physical Force Theories

• Various physical forces involved in ascent of sap:
  • Atmospheric Pressure: Not convincing because it cannot act on the water present in xylem in roots
  • Imbibition: Sachs supported that ascent of sap could take place by imbibition through the walls of xylem
  • Capillary Force: vessels are placed one above the other forming a sort of continuous channel similar to a capillary tube
  • Transpiration pull and cohesion of water theory
    • Forces that are involved
    • Very convincing
    • Based on Cohesive and Adhesive properties of water molecules
    • Water molecules remain joined to each other due to the presence of H-bonds between them helping develop stronger mutual force of attraction or Cohesive force _ Although H-bond is very weak when present in enormous numbers very strong for this reason it remain so in the xylem
    • Magnitude of this force is high therefore the continuous water column in the xylem cannot be easily broken
    • Adhesive properties are where there is attraction between the water molecules and the container walls (here the walls of xylem), this further ensure the continuity of water colum
    • Evaporation takes from upper parts of plant and water is going to be release from the mesophyll cells
    • A tension created in water
    • Downward tension pull upward the stem
    • Air bubbles presence and if any they do be unable to break column