Plant Transpiration and Guttation

Chapter 4: Transpiration and Guttation

Transpiration

• Plants absorb large quantities of water from the soil but only a small amount is used
• Excess water is lost from aerial parts as water vapor through transpiration.
• Transpiration differs from evaporation as it's a vital physiological process in plants with water loss as vapor from aerial parts is essential for living tissues.
• Evaporation is a purely physical process turning any liquid into vapor, not necessarily reaching boiling point, and living tissues aren't essential.
• Demonstrated by keeping a potted plant under a bell-jar with the pot covered in a polythene bag to prevent evaporation from soil.
• Vaseline makes the apparatus airtight, and water droplets form on the inner bell-jar walls.

Kinds of Transpiration

• Stomatal transpiration accounts for most transpiration through stomata, mainly on the lower leaf sides, but equally distributed in monocots like grasses.
• Aquatic plants with floating leaves have stomata on the upper surface.
• Cuticular transpiration allows some water loss through the cuticle, contributing up to 10% of total transpiration.
• Lenticular transpiration involves water loss from woody stems through lenticels; transpiration from leaves is known as foliar transpiration.

Mechanism of Stomatal Transpiration

• Three steps involved during the daytime:
  • Osmotic diffusion moves water from the xylem in the leaf to intercellular spaces above stomata through mesophyll cells.
  • Opening and closing of stomata involves stomatal movement.
  • Simple diffusion moves water vapors from intercellular spaces to the outer atmosphere via stomata.
• Mesophyll cells draw water from xylem, become turgid, with decreasing diffusion pressure deficit (DPD) and osmotic pressure (OP), releasing water as vapor into intercellular spaces near stomata via osmotic diffusion.
• O.P. and D.P.D. of mesophyll cells then increase, drawing water from xylem via osmotic diffusion.

Opening and Closing of Stomata (Stomatal Movement)

• Stomata are recognized by the peculiar shape of surrounding epidermal cells, which may be similar or specialized into subsidiary cells.
• Guard cells differ from epidermal cells by containing chloroplasts, with unique thickenings on adjacent surfaces (closed stomata) or surfaces near the stomatal pore (open stomata).
• Radial micellation in guard cell walls, with cellulose microfibrils radiating outward, allows lengthwise elongation and prevents crosswise elongation when turgid.
• Increased osmotic pressure and diffusion pressure deficit in guard cells, due to accumulating osmotically active substances, causes osmotic diffusion of water from surrounding epidermal and mesophyll cells.
• This increases turgor pressure, causing guard cells to swell, lengthen, and stretch thickened surfaces, thus forming a pore and opening stomata.
• Decreasing O.P. and D.P.D. in guard cells (due to depletion of osmotically active substances) relative to surrounding cells releases water back into them, and guard cells become flaccid.
• Thickened surfaces of guard cells move closer, closing the stomatal pore.
• Osmotic diffusion of water into guard cells occurs when osmotic potential (ψs) and water potential (ψ) decrease (become more negative) relative to surrounding cells.
• Guard cells become flaccid when osmotic and water potential increase (become less negative) relative to surrounding cells and water moves from less to more negative potential regions.
• Various agents or mechanisms create osmotic potential in guard cells and control stomatal movements, like starch hydrolysis into sugars, synthesis of organic acids, and active K+ ion pumping into guard cells, balanced by Cl- or organic acid counter ions.

Factors Affecting Stomatal Movements

• Factors include:

Light

• Light strongly influences stomatal movements; stomata typically open in light and close in darkness.
• Light intensity needed for maximal stomatal opening varies - tobacco requires low intensities (2.5% of full daylight), while others need full sunlight.
• CAM plants are an exception, with stomata opening at night and closing during the day, absorbing CO2 and fixing it into organic acids nocturnally.
• Wavelength diversity affects stomatal opening; green light is ineffective.
• The action spectrum for the effect of light on stomata resembles photosynthesis, with a superimposed blue light action.
• Some plants lack a photosynthetic spectrum and are only sensitive to blue light, and a photosynthetic component may be due to photosynthesis in guard cells with chloroplasts.
• Light influence in stomatal opening:
  • Decreases CO2 concentrations in guard cells, which is a powerful stimulus.
  • Synthesizes osmotically active substances like soluble sugars during photosynthesis, decreasing water potential in guard cells.
  • Provides ATP via photophosphorylation to operate H+/K+ exchange pumps, decreasing water potential with pumped ions.
  • Illumination often increases pH in guard cells, which comes from reduced CO2.
  • High pH triggers starch hydrolysis into osmotically active glucose-1-phosphate by starch phosphorylase, decreasing water potential in the guard cells.

Carbon Dioxide Concentration

• CO2 concentration significantly affects stomatal movement; reduced CO2 promotes opening, and increased CO2 promotes closing.
• Stomata can be induced to open in darkness with significantly lowered CO2 below normal air levels.
• High CO2 above normal air causes stomata to close in light and dark.
• Stomata forced to close by high CO2 do not reopen rapidly by flushing with CO2-free air in the dark.
• Stomata reopen soon with subsequent light exposure as CO2 trapped inside the leaf is used during photosynthesis.
• Influential CO2 is from inside the leaf (intercellular), with the cuticle over guard cells and epidermis ensuring response to internal CO2 rather than outer atmosphere.

Temperature

• An increase in temperature usually increases stomatal opening if water isn't limiting.
• Camellia stomata, do not open at very low temperatures (below 0°C) even in strong light.
• Stomata may close at high temperatures (above 30°C), potentially due to increased CO2 caused by increased respiration and impaired photosynthesis.

Water deficits and abscisic acid (ABA)

• Water deficit created by transpiration exceeding water absorption can lead to wilting and water-stressed plants.
• Mesophytes close stomata tightly to protect from damage caused by extreme water shortage.
• Stomata usually reopen as water potential restores.
• Hydro passive control governs stomatal movement governed by water.

Significance of Transpiration:

• Plants expend much energy absorbing water, most of which is lost through transpiration
• Transpiration can regarded as advantageous or harmful to plants.

Factors Affecting Rate of Transpiration

• External factors:

Atmospheric Humidity

• Transpiration rate decreases in humid atmospheres due to moisture saturation hindering water vapor diffusion from intercellular spaces.
• Transpiration rate increases in dry atmospheres due to low relative humidity and unsaturated air.
• The amount of moisture in air is absolute humidity, expressed as a percentage for relative humidity.

Temperature

• Increased temperature increases transpiration by:
  • Decreasing relative humidity
  • Widening stomatal openings

Wind

• Stagnant wind maintains normal transpiration.
• Rate Increases with gentle wind because evaporation increases
• Gentle winds increase transpiration by removing moisture from the plant's vicinity, facilitating water vapor diffusion.
• Violent winds decreases transpiration rate because it blocks water vapour.

Atmospheric Pressure

• Atmospheric pressure has no ultimate effect on the transpiration rate.
• Positive effects at higher altitudes are counteracted by the temperature.

Light

• Light increases transpiration because:
  • Stomata open
  • Temperature increases
• Stomatal transpiration almost stops in the dark due to stomatal closure.

Available Soil Water

• Transpiration rate decreases if insufficient soil water is available for root absorption.

CO2

• Increased CO2 concentration, especially inside the leaf, leads to stomatal closure, hindering transpiration.

Internal Factors

• Internal water condition is vital for transpiration; deficient water reduces transpiration rate.
• Increased transpiration continuing for longer periods of time, leads to water deficits.

Structural Features

• Number, size, position, and movement of stomata affect transpiration rate.
• Stomata close in the dark, inhibiting stomatal transpiration.
• Sunken stomata reduce stomatal transpiration rate.
• Hairs and grooves reduce transpiration rate.
• Xerophytes reduce leaf size to check foliar transpiration.
• A thick cuticle layer or wax reduces cuticular transpiration.

Guttation

• Watery drops ooze from uninjured leaf margins, at the end of the main vein, in plants like garden nasturtium, tomato, and Colocasia.
• In guttation watery drops of inorganic and organic substances are released through special water stomata
• Guttation usually occurs early as water absorption and root pressure are high and transpiration is low.
• After drying, dissolved substances remain as residue on the leaf margins.
• A well-watered potted plant of garden nasturtium is kept under a bell-jar on a glass sheet, and watery drops appear on the margins of the leaves.
• Guttation is linked to unique stomata at leaf margins called water stomata, or hydathodes.
• Each hydathode has a permanently open water pore, a small cavity, and loose tissue termed epithem in close proximity to the vascular elements of veins.
• Higher root pressure delivers water to the epithem via the xylem, releasing it into the cavity, which then exudes as watery drops through the water pore.

Differences between Transpiration & Guttation

• Transpiration
  • Water lost from aerial plants, by invisible water vapor.
  • Happens in all vascular plants.
  • Happens mostly through Stomata, maybe cuticle and lenticels as well.
  • Rate Maximizes in noon.
• Guttation
  • Watery solution released uninjured by air.
  • Happens in some angiosperms such as tomatoes.
  • Happens through hydathodes only.
  • Happens when rate of water absorptions are high.

Chapter 5: Ascent of Sap

Ascent of Sap

• Water absorbed by roots is distributed to all plant parts; what's not used lost through transpiration.
• Water moves up the stem to reach topmost parts.
• The path and mechanisms for the ascent of sap include:

Path of Ascent of Sap

• Ascent of sap occurs through xylem, as proven by:
  • Cutting a leafy twig of Balsam (semi-transparent stem) underwater and placing it in water with Eosin dye shows colored lines moving upward in the stem.
  • Ringing Experiment, removing a bark ring (outer tissues to vascular cambium) while in water, maintaining green leaves.

Mechanism of Ascent of Sap

• Ascent of sap in tall trees with heights of 300-400' presents a problem with various theories to explain it.

Vital Theories

• Supporters suggests ascent of sap is due to the system.

Godlewski

• Ascend happens by the pumping activity of Xylem.
• High Water gets drown to low Water by the help of Xylem.
• Process repeats, water goes to the top.
• This was only a hypothetical experiment, that was countered by Strqsburger.

Bose

• Translocation takes place by cells of the inner cortical layers.
• Experiment of other scientists does not show relationship to ascent of Sap.

Root Pressure Theory

• Root Pressure will help to lift up the water but it might not be effective due to:
  • Magnitude.
  • Even if pressure is not there, sap continues.
  • Gymnosperms Pressure is rarely observed.

Physical Force Theories

Physical force might be involved in ascent water

• Atmospheric Pressure.
  • Effect cannot be achieved.
• Imbibition.
  • Insignificant in ascent sap due to walls.
• Capillary Force.
  • water goes to xlem
  • Objections:
  • Free surface is needed.
  • Force is low.
  • Narrow elements exist.
  • Gymnosperms never form continuous channels.

Transpiration Pull and Cohesion of Water Theory

• Dixon and Jolly (1894) proposed this theory, supported by many workers.
• Based on:
  • Cohesive and Adhesive properties of water molecules forming a continuous column in the xylem.
  • Transpiration pulls exerted on this water column.
  • Water molecules join each other owing to hydrogen bonds
  • Force cannot be broken easily