In this article we will discuss about:- 1. Definition of Stomata 2. Classification of Stomata 3. Mechanism.
Definition of Stomata:
Stomata are specialised epidermal cells which are distributed all over leaf surface but in case of terrestrial plants, mainly on lower surface of leaves. Therefore approximately 97% of transpiration takes place from the lower surface in such plants.Each stoma (open) has two kidney (or bean) shaped guard cells. Inner wall of guard cell is thick and outer wall is thin. Guard cells are surrounded by epidermal or subsidiary or accessory cells.
Classification of Stomata:
According to the distribution of stomata, plants are of five categories:
a) Apple and Mulberry Type:
In such plants, stomata are present on under surface only.
b) Potato Type:
More stomata on the lower (or under) surface than on upper surface.
c) Oat Type:
Stomata are equally distributed on both surfaces.
d) Water Lily Type:
Stomata only on upper surface.
e) Potamogeton Type:
In such plants stomata are either absent or functionless. Such plants are most of the submerged aquatic plants.
On the basis of daily movement of stomata, Loft field classified it into three main groups:
i) Alfalfa Type:
Such stomata are open throughout the day and night and are found mostly in thin leaved mesophytes e.g. pea, bean, radish, mustard, vitis etc.
ii) Potato Type:
Such stomata are open throughout day and night except for a few hours in the evening, e.g. onion, plantain, cabbage, pumpkin etc.
iii) Barley Type:
Such stomata are open only for a few hours during the day e.g. cereals.
Mechanism of Stomatal Opening and Closing:
Opening and closing of stomata are due to its turgidity and flaccidity respectively. It means stomatal movement is governed by turgor movement. When T.R of guard cells increases, stomata are opened and when decreases, stomata are closed.
1. Photosynthetic Production in the Guard Cells:
According to Von Mohl (1856):
Chloroplasts of guard cells synthesize osmotically active substances in the day which increases their osmotic pressure and thus endosmosis. This ultimately leads to stomatal opening and vice versa in the night.
In day:
i) Conc., of sugar in guard cells increases.
ii) DPD of guard cells increases.
iii) Water enters into guard cells by osmosis.
iv) TP of guard cells increases.
v) Stomata open.
In night, the process is reverse.
This proposition is not acceptable due to:
a) Increasing the CO2 conc., around leaf in bright light causes partial closure of stomata.
b) Chloroplasts of guard cells are either totally incapable of photosynthesis or can have only feable photosynthesis.
2. The Starch = Sugar Hypothesis:
Lloyd (1908) observed that the amount of starch in guard cells increases at night but decreases in day. It means turgidity of guard cells is governed by change in O.P caused by inter-conversion of starch and sugar. Scarth (1932) supported the Sayre’s hypothesis of starch = sugar inter-conversion. Sayre thought that the removal of CO2 by photosynthesis during the light period caused increase in the pH resulting in the conversion of starch into sugar. This inter-conversion is catalysed by phosphorylase.
But in 1964 steward said that O.P. of guard cells would not be affected appreciably unless glucose-1-PO4 was further converted to glucose and inorganic phosphate.
Drawbacks:
i) How does the change in CO2 raise the pH from 4.5 to 7.0.
ii) Either at the time of opening of stomata or disappearing of starch, sugar is never seen in guard cells. Starch is always changed into organic acid.
iii) In monocot, starch is not formed at all in guard cells.
3. Active K+ Transport Mechanism:
According to Immamura (1943), Yamashita (1952), Fischer and Hsiao (1968) there is a direct correlation between stomatal movements and K+ concentration of guard cells.
K. Raschke (1975) propounded that any of the following processes can initiate stomatal opening:
i) Disappearance of starch from the guard cells;
ii) Production of organic acids, particularly malic acid.
iii) Excretion of H+ from guard cells;
iv) Uptake of K+ into the vacuoles of the guard cells;
v) Uptake of CI– into the vacuoles.
According to Raschke, the excretion of H+ ion from the guard cells is of primary importance in the stomatal opening. The H+ ions are to be availed by dissociation of organic acids for exchanging K+ ions.
Abscisic acid (ABA) blocks the active excretion of H+ ions from guard cells and thus results in the stomatal closure.
4. Proton Transport Concept:
It deals with the matter under two heads viz., photoactive opening and scotoactive opening:
a) Photoactive Opening:
The proton transport concept of photoactive stomatal opening’ was proposed by Levitt (1974). Actually it is a synthetic theory in which good points of Scarth’s classical pH theory and active K+ transport theory.
b) Scotoactive Opening:
This theory was proposed by Pallas (1969) and Ehrler (1972). This theory deals with the opening mechanism of stomata during night especially in succulent plants. In succulent plants stomata open during night. In such plants initially stomata close when darkness sets in. It results in O2 deficiency and is more pronounced in thicker leaves where oxygen is utilised in respiration. This decreases the mitochondrial activity and gives the way to anaerobic respiration.
The mitochondrial induced proton transport to cytoplasm is stopped and the resultant acidification of cytoplasm is removed. This raises the pH of cytoplasm and larger amount of PEP (Phosphoenol Pyruvic Acid) is converted to R (COOH)2 (i.e., organic acid) by PEPC (Phosphoenol Pyruvate Carboxylase Enzyme). It is followed by K+ absorption. The remaining steps are same to those of photoactive opening.
i. Rate of transpiration is determined by Potometer i.e. Farmer’s Potometer and Ganong’s Potometer.
ii. Stomatal opening is measured by Knight’s poromete.
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