Introductory Crop Physiology (1+1)

Mechanism of stomatal Transpiration

The mechanism of stomatal transpiration can be studied in 3 steps.


1. Osmotic diffusion of water in the leaf, from xylem to intercellular space above the
stomata through the mesophyll cells.
2. Opening and closing of stomata (stomatal movement).
3. Simple diffusion of water vapour from intercellular spaces to outer atmosphere



Osmotic diffusion


• Inside the leaf, the mesophyll cells are in contact with xylem, and with intercellular spaces above the stomata.
• When mesophyll cells draw water from the xylem they become turgid and their diffusion pressure deficit (DPD) and osmotic pressure (OP) decreases.
• Then, water is released in the form of vapour into intercellular spaces close to stomata by osmotic diffusion.
• Now, the OP and DPD of mesophyll cells become higher and hence, they draw water form xylem by osmotic diffusion.


Opening and closing of stomata (Stomatal movement)

Courtesy:http://academic.kellogg.edu/herbrandsonc/bio111/animations/0021





The Reactions in opening and closing








• The stomata will be opened by the action of light.
• When the light falls on the leaf the CO2 content of guard cells will be converted into bicarbonates which results in a decrease of CO2 concentration.
• Bicarbonate formed will be combined with phosphoenol pyruvic acid to form malic acid.
• The malic acid will be dissociated into H+ and malate ions.
• H+ ions will be exchanged with the K+ ions from other epidermal cells as a result there will be an influx of K+ ions into the guard cells.
• This K+ ion combines with malate ions to form potassium malate which will be transported into the vacuoles.
• Thus the solute potential will be decreased which leads to a decrease in the water potential also. Because of this decrease in water potential a gradient in water potential will be developed between guard cells and other epidermal cells. With the result water will be taken into the guard cells.
• Thus the turgidity of guard cells increases which leads to the opening of stomata.

TOP


• The stomata are easily recognized from the surrounding epidermal cells by their peculiar shape.
• The epidermal cells that immediately surround the stomata may be similar to other epidermal cells or may be different and specialized.
• In the latter case, they are called as subsidiary cells. The guard cells differ from other epidermal cells also in containing chloroplasts and peculiar thickening on their adjacent surface (in closed stomata) or on surfaces.
• Consequent to an increase in the osmotic pressure (OP) and diffusion pressure deficit (DPD) of the guard cells (which is due to accumulation of osmotically active substances), osmotic diffusion of water from surrounding epidermal cells and mesophyll cells into guard cells follows.
• This increase the turgor pressure (TP) of the guard cells and they become turgid. The guard cells swell, increase in length and their adjacent thickened surfaces stretch forming a pore and thus the stomata open.
• On the other hand, when OP and DPD of guard cells decrease (due to depletion of osmotically active substances) relative to surrounding epidermal and mesophyll cells, water is released back into the latter by osmotic diffusion and the guard cells become flaccid. The thickened surfaces of the guard cells come close to each other, thereby closing the stomatal pore and the stomata.
• Osmotic diffusion of water into guard cells occur when their osmotic pressure increases and water potential decreases (i.e. become more negative) related to those of surrounding epidermal and mesophyll cells.
• The guard cells become flaccid when their osmotic pressure decreases relative to the surrounding cells (Movement of water takes place from a region of higher water potential to a region of lower water potential).


The different mechanisms that create osmotic potential in the guard cells and control stomatal movements are,


a. Hydrolysis of starch into sugars in guard cells
b. Synthesis of sugars or organic acids in guard cells
c. The active pumping of K+ ions and, Cl- ions or organic acid counter ions into the guard cells


a. Hydrolysis of starch into sugars in guard cells
Starch – Sugar Interconversion theory:


• This classical theory is based on the effect of pH on starch phosphorylase enzyme which reversibly catalyses the conversion of starch + inorganic phosphate into glucose -1 phosphate.
• During the day, pH in guard cells wil be high which favours hydrolysis of starch (insoluble) into glucose -1- phosphate (soluble) and osmotic pressure is increased in guard cells. Consequently water enters into the guard cells by osmotic diffusion from the surrounding epidermal and mesophyll cells.
• Guard cells become turgid and the stomata open.
• During dark, the reverse process occurs. Glucose 1- phosphate is converted back into starch in the guard cells thereby decreasing osmotic pressure. The guard cell release water, become flaccid and stomata become closed




• According to Steward (1964), the conversion of starch and inorganic phosphate into glucose-1-phosphate does not cause any appreciable change in the osmotic pressure because the inorganic phosphate and glucose-1-phosphate are equally active osmotically.
• In this scheme it is suggested that, Glucose-1-phosphate should be further converted into glucose and inorganic phosphate for the opening of stomata. Metabolic energy in the form of ATP would be required for the closing of stomata which probably comes through respiration.

Steward’s scheme of reactions involved in the opening and closing of stomata



However, starch – sugar inter-conversion theory is not universally applicable as this scheme operates under certain circumstances.



Difference between epidermal cells and guard cells:


• Guard cells have chloroplast, epidermal cells do not.
• Guard cells are much smaller than the epidermal cells
• They have bean or Kidney shape in dicots and dumbell shape in monocots especially grasses
• The cell walls of guard cells are not uniform . Inner walls are thicker than the outer walls . Epidermal cells are uniformly thin .

TOP