06 Plant Water Relation - part 13 - Structure of stomatal apparatus

06 Plant Water Relation - part 13 - Structure of stomatal apparatus


Structure of stomatal apparatus : 

  • Typical stomatal apparatus consists of two guard cells - 
  1. Stoma 
  2. Accessory cells.
1. Stomata  :
  • Are minute, elliptical pores bounded by two kidney/ dumbbell shaped guard cells. 
  • Is a type of epidermal tissue which may be called as modified, epidermal parenchyma cell. 
  • Are kidney shaped in dicotyledons 
  • Dumbbell-shaped in grasses.
  • In Cyperus, both kidney- and dumb bellshaped guard cells are present. 
Guard cells

  • Are living, nucleated cells with unevenly thick walls. 
  • Inner wall : thick and inelastic;Lateral wall is thin and elastic. 
  • Contain few chloroplasts which are capable of poor photosynthesis. 
  • Guard cells have ability to change their size and form due to which stoma opens (widens) or closes (narrows). 
Stoma

  • Is an elliptical pore formed due to specific arrangement of guard cells. 
  • It is through the stoma, excess water is lost in the form of vapour. 
2. Accessory cells : 
  • These are specialized epidermal cellssurrounding the guard cells. 
  • Their number is variable and are the reservoirs of K+ ions. 
  • These are also called subsidiary cells. 
Opening and Closing of Stoma : 
  • Is controlled by turgor of guard cells. 
During day time
  • guard cells become turgid due to endosmosis. 
  • Thus turgor pressure is exerted on the thin walls of guard cells. 
  • Being elastic and thin, lateral walls are stretched out. 
  • Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma. 
During night time
  • Guard cells become flaccid due to exosmosis. 
  • Flaccidity closes the stoma almost completely. 
  • Endosmosis and exosmosis occur due to diurnal changes in osmotic potential of guard cells. 
  • Different theories are proposed to explain diurnal changes in osmotic potential. 
According to starch-sugar interconversion theory (Steward 1964) :

  • During day time, enzyme phosphorylaseconverts startch to sugar.
  • Thus increasing osmotic potential of guard cells cosing entry of water there by gaurd cells are stretched and stoma widens. 
  • The reverse reaction occures at night brining about the closure of stoma.
According to theory of proton transport (Levitt-1974) :
  • Stomatal movement occurs due to transport of protons H+ and K+ ions. 
  • During daytime, starch is converted into malic acid. 
  • Malic acid dissociates to form Malate and protons. 
  • Protons are transported to subsidiary cells and K+ ions are imported from them. 
  • Potassium malate is formed that increases osmolarity and causes endosmosis. 
  • Uptake of K+ ions is always accompanied with Cl¯ ions. 
  • At night, uptake of K+and Cl- ions is prevented by abscissic acid, changing the permeability of guard cells. 
  • Due to this guard cells become hypotonic and thereby become flaccid. 
Significance of Transpiration : 
Advantages: 
  • Removes excess of water.
  • Helps in the passive absorption of water and minerals from soil. 
  • Helps in the ascent of sap.
  • As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated. 
  • Maintains turgor of the cells.
  • Transpiration helps in reducing the temperature of leaf and in imparting cooling effect. 
Disadvantages: 
  • Excessive transpiration leads to wilting and injury in the plant. 
  • It may also lead to the death of the plant. 
Transpiration is `A necessary evil' : 
  • For stomatal transpiration to occur, stoma must remain open, during day time. 
  • When stomata are open then only the gaseous exhange needed for respiration and photosynthesis, will take place.
  • If stomatal transpiration stops, it will directly affect productivity of plant through the loss of photosynthetic and respiratory activity. 
  • Hence for productivity, stomata must remain open. 
  • Consequently transpiration can not be avoided. 
  • Hence, Curtis (1926) regarded transpiration as `a necessary evil'.

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