Stomatal Regulation of Transpiration

Stomatal Regulation of Transpiration:-
General Introduction:-
> The loss of water in the vapour form from the exposed parts of a plant is called transpiration. 
> The loss of water due to transpiration is quite high:
i. 2 litres per day in Sunflower.
ii. 36 - 45 litres in Apple.
iii. Up to 1 tonne per day in Elm tree. 
> Rather 98-99% of the water absorbed by a plant is lost in transpiration. Hardly 0.2% is used in photosynthesis while the remaining is retained in the plant during growth.
Stomatal Transpiration:-
- It is the most important type of transpiration. 
- Stomatal transpiration constitutes about 50-97% of the total transpiration. 
- It occurs through the stomata. 
- The stomata are found mostly on the leaves. A few of them occur on the young stems, flowers and fruits.
- The stomata expose the wet interior of the plant to the dry atmosphere. Water vapours, therefore, pass outwardly through stomata by diffusion. 
- The stomatal transpiration continues till the stomata are kept open.
Structure of Stomata:- 
> Stomata are tiny pore complexes found in the epidermis of leaves and other soft aerial parts. 
> The size is 10- 14 µm (range 7-38 pm) in length and 3-12 µm in breadth. 
> The number of stomata per cm2 of leaf surface varies from 1000- 60,000 or 10-600/mm2.
> In mesophytic plants, stomata occur both on the upper (adaxial) and lower (abaxial) surfaces.
> Their number is roughly equal on the two surfaces in grasses and other monocot leaves. 
> In dicot leaves, the number of stomata on the upper surface is usually smaller, even absent in several
cases.
> Stomata are meant for the gaseous exchange but are also the main source of transpiration. 
> Each stomate or stoma is surrounded by two small but specialized green epidermal cells called guard cells. Because of their small size, they are rapidly influenced by turgor changes.
> The guard cells are connected with the adjacent epidermal cells through plasmodesmata. They contain a few small chloroplasts with peripheral reticulum characteristic of chloroplasts showing C4 photosynthesis. 
> The guard cells also possess small vacuoles and micro bodies. They store starch with the exception of a few. 
> The walls are differentially thickened and elastic. They have folds for expansion. Micro fibrils of these walls are oriented specifically to help in opening and closing of stomata.
> In most of the plants the guard cells are kidney shaped in outline. They are joined at their ends. The concavo-convex curvature of two guard cells is variable and causes stomatal pore to open and close. 
- The walls of these guard cells are thickened on inner side. They have one or two pairs of wall extensions or ledges to prevent entry of water drops into stomata.
- The walls are thinner and more elastic on the outer side. 
> When the stomata are to open, these guard cells swell up on the outer side by the development of a high turgor pressure. The inner concave sides also bend out slightly so as to create a pore in between two guard cells.
> During closure movement, reverse changes occur. 
> In cereals, members of cyperaceae and some plams the guard cells are dumb-bell shaped in outline. Their expanded ends are thin-walled while middle portions are highly thick-walled. In such cases opening and closing of the stomatal pore is caused by expansion and contraction of thin-walled ends of the guard cells.
Mechanism of Stomatal Movement:-
> Stomata function as turgor-operated valves because their opening and closing movement
is governed by turgor changes of the guard cells. 
> Whenever, the guard cells swell up due to increased turgor, a pore is created between them. With the loss of turgor the stomatal pores are closed.
> Stomata generally open during the day and close during the night with a few exceptions.
>The important factors which govern the stomatal opening are light, high pH or reduced CO2 and
availability of water. The opposite factors govern stomatal closure, viz., darkness, low pH or
high CO2 and dehydration.
> There are three main theories about the mechanism of stomatal movements:
a. Hypothesis of Guard Cell Photosynthesis:-
- Guard cells contain chloroplasts. 
- During day the chloroplasts perform photosynthesis and produce sugar. 
- Sugar increases osmotic concentration of guard cells. It causes absorption of water from nearby epidermal cells. 
- The turgid guard cells bend outwardly and create a pore in between. 
- However, photosynthetic activity of guard cell chloroplasts seems to be negligible.
b. Classical Starch Hydrolysis Theory:-
- The main features of the theory were spelled out by Sayre (1923). 
- It was modified by Steward (1964). 
- The guard cells contain starch. 
- At low carbon dioxide concentration (in the morning achieved through photosynthesis by mesophyll and guard cells), pH of guard cells rises. It stimulates enzyme phosphorylase. Phosphorylase converts starch into glucose 1- phosphate. The latter is changed to glucose 6-phosphate which undergoes hydrolysis to produce glucose and phosphoric acid. Glucose increases osmotic concentration of guard cells. On account of it, the guard cells absorb water from neighbouring cells, swell up and create a pore in between them.
- Evening closure of stomata is brought about by increased carbon dioxide content (due to stoppage of photosynthesis) of leaf. It decreases pH of guard cells and brings about phosphorylation of glucose. In the presence of phosphorylase, glucose 1-phosphate is changed into starch. As a result, osmotic concentration of guard cells falls. They lose water to adjacent epidermal cells. With the loss of turgidity, the guard cells shrink and close the pore in between them.
Objections:-
(i) Glucose is not found in guard cells at the time of stomatal opening.
(ii) Starch ↔ Sugar changes are chemically slow while opening and closing of stomata are quite
rapid.
(iii) Wide changes in pH of guard cells cannot be explained on the basis of carbon dioxide
concentration.
(iv) Onion and some of its relatives do not possess starch or related polysaccharide that can be
hydrolysed to the level of glucose.
(v) Blue light has been found to be more effective than other wavelengths for opening of
stomata. The same cannot be explained by starch hydrolysis theory.
(vi) Hydrolysis of starch theory cannot account for high rise in osmotic pressure found in guard
cells. 
c. Malate or K+ ion Pump Hypothesis (Modern Theory):- The main features of the theory were put forward by Levitt (1974). 
i. During Stomatal Opening:-
- According to this theory, pH of the guard cell can rise due to active H+ uptake by guard cell chloroplasts or mitochondria, CO2 assimilation by mesophyll and guard cells. 
- A rise in pH causes hydrolysis of starch to form organic acids, especially phosphoenol pyruvate. Starch → Hexose Phosphate → Phosphoenol Pyruvate.
- Phosphoenol pyruvate can also be formed by pyruvic acid of respiratory pathway. With
the help of PEP carboxylase (PEP case), it combines with available CO2 to produce oxalic acid
which gets changed into malic acid.
- Malic acid dissociates into H+ and malate. H+ ions pass out of the guard cells actively. In exchange, K+ ions pass inwardly. Same CI– ions may also enter guard cells along with K+ ions.
- Guard cells maintain their electroneutrality by balancing K+ with malate and Cl–. 
- In the combined state they pass into the small vacuoles and increase the osmotic concentration of the guard cells. As a result guard cells absorb water from the nearby epidermal cells through endosmosis, swell up and create a pore in between them.
ii. During Stomatal Closing:-
- The H+  ions diffuse out of the guard cell chloroplasts. It decreases pH of the guard cell cytoplasm. 
- Any malate present in the cytoplasm combines with H+  to form malic acid. 
- Excess of malic acid inhibits its own biosynthesis. High CO2 concentration also has a similar effect. 
- Un-dissociated malic acid promotes leakage of ions. As a result K+ ions dissociate from malate and pass out of the guard cells.
- Formation of abcisic acid (as during drought or midday) also promotes reversal of H+ = ↔ K+ pump and increases availability of H+ inside the guard cell cytoplasm. 
- Loss of K+ ions decreases osmotic concentration of guard cells as compared to adjacent epidermal cells. This causes exosmosis and hence turgidity of the guard cells decreases. It closes the pore
between the guard cells. Simultaneously the organic acids are metabolised to produce starch.