Stomata Discovered by Pfeffer Term stomata was given
Stomata • Discovered by Pfeffer • Term “stomata” was given by Malpigii
STOMATA (singular- stoma) §Microscopic pores embedded in the epidermis of the plant leaves, stems or other aerial organs. §Facilitate gaseous, water and vapor exchange. §Surrounded by a pair of guard cells (specialized parenchyma), meant to regulate the size of the stomatal aperture.
§Adjacent to each guard cell, one or more epidermal cells bordering the guard cells are the subsidiary cells or accessory cells. §Stomatal apparatus- the integrative complex of the pore, surrounding guard cell pair and the subsidiary cells collectively called as the stomatal apparatus. §In xerophytes and many gymnosperms, deeply embedded sunken stomata are found as an adaptation to check excessive transpiration.
Stomatal frequency No. of stomata present over a unit area of a leaf. Rate of water loss through transpiration can be assessed by determining the stomatal frequency and the total area covered by stomata.
Guard cells: ØDicots- Kidney-shaped; Exs. wheat, rice, asparagus, corn etc. Ø Monocots- Dumbell-shaped; Exs. Tomato, clovers, apples etc. A & B- Open & Closed Stomata in Dicot Leaf C & D- Open & Closed Stomata in Monocot Leaf
Arrangement of stomata In monocots stomata are arranged in parallel In dicots stomata are scatterd or irregularly arranged
TYPES On the basis of presence of stomata on leaf v. Polygonum type- present on upper epidermis only v. Mesophytic type-present on lower epidermis only v. Monocot type- stomata on both the surfaces
TYPES On the basis of ontogenic development (Florin 1931, 1933) • Haplocheilic type- A stoma mother cell directly functions as a guard mother cell which divides to produce a pair of guard cells only without cutting off any subsidiary cells • Syndetocheilic type- A stomatal initial functions as a guard mother cell after cutting off one or more subsidiary cells.
TYPES On the basis of the structure and position of the epidermal or subsidiary cells. (Metcalfe and Chalk 1950) 1) Anomocytic / Ranunculaceous/irregular-celled type 2) Anisocytic/ Cruciferous /unequal-celled type 3) Diacytic / Caryophyllaceous/cross-celled type 4) Paracytic /Rubiaceous/parallel-celled type
1) Anomocytic / Ranunculaceous/ irregular-celled type • The stoma is surrounded by a definite/limited number of cells • These cells are not different from the remainder of the epidermis. • so all the cells are normal epidermal cells without any modification • For example, in Digitalis, Lobelia
2) Anisocytic/ Cruciferous /unequal-celled type • The stoma is surrounded by three cells. Anisocytic • Out of those three cells, one is distinctly smaller than the other two. • For example, in Hyoscymus niger, Belladona
3) Diacytic/Caryophyllaceous/cross-celled type • The stoma is enclosed by a pair of subsidiary cells. • The common wall of subsidiary cells is at right angle to the guard cells. • Their Long axes is perpendicular to the pore. • For example, in Peppermint, Mentha piper
4) Paracytic /Rubiaceous/parallel-celled type • The stoma is accompanied on either side by one or more subsidiary cells. • The subsidiary cells are parallel to the long axis of the pore and guard cells. • For example, in Senna, Cocoa.
Stomatal movement
Stomatal movements The four important theories of stomatal movement are: (1) Theory of Photosynthesis in Guard Cells (2) Starch Sugar Inter-conversion Theory (3) Theory of Glycolate Metabolism (4) Active K+ Transport or Potassium Pump Theory
(1) Theory of Photosynthesis in Guard Cells Von Mohl (1856) observed that stomata open in light due to photosynthesis occur in guard cells in the presence of light resulting in the production of carbohydrate which increases their osmotic pressure. It is explained as. Light → Photosynthesis in guard cells → Formation of sugar, increase in osmotic pressure of cell sap → Endosmosis takes place from subsidiary cell to guard cell → Increase of TP in guard cells → Stomata open.
Demerits: 1. Increasing the CO 2 concentration around the leaves should lead to wide opening of stomata but here occurs their partial closure. 2. Chloroplast of guard cells are poorly developed and incapable of performing active photosynthesis.
2. Starch Sugar Inter-conversion Theory: i. According to Lloyd (1908), guard cells contain sugar during day time when they are open and starch during night when they are closed. ii. Sayre (1926) observed that stomatal movement is regulated by p. H due to inter-conversion of starch and sugar. Stomata open in neutral or alkaline p. H, during day time due to constant removal of CO 2 by photosynthesis and remain closed at night when there is no photosynthesis; accumulation of CO 2, causes the p. H to be acidic.
iii. Steward’s theory Steward (1964) proposed that conversion of starch to Glucose -1 phosphate is not sufficient, it should be converted to glucose in order to create sufficient osmotic pressure. For this, ATP is required which comes through respiration (in the presence of oxygen)
Based on Steward’s theory stomatal opening and closing can be summarized as:
Demerits of the starch-sugar interconversion theory: Ø In the presence of light when starch disappears from guard cells, malic acid appears and not the sugars. ØStarch has not been reported in the guard cells of many monocots such as Iris, Amatyllis, Allium. ØAccording to this theory O. P. of guard cells increases due to the formation of glucose-1 -phosphate in guard cells but it is found that the presence of phosphate ions causes the development of same O. P as does the presence of glucose-phosphate. ØThe theory could not explain the extra effectiveness of blue light at the time of stomatal opening.
3. Theory of Glycolate Metabolism: §According to Zelitch (1963) production of glycolic acid under low CO 2 concentration in the guard cells is an important factor in stomatal opening. §Glycolate gives rise to carbohydrate, thus raising the osmotic pressure and also that it could participate in the production of ATP, which might provide energy required for the opening of stomata.
Demerits(Theory of Glycolate Metabolism): 1. It fails to explain the opening of stomata in dark (e. g. , – in succulent plants). 2. In some plants stomata have been found to remain closed even during daytime. 3. It fails to explain the effect of blue light on stomatal opening.
4. Active K+ Transport or Potassium Pump Theory • The concept of K+ ion transport was given by Fujino and was supported and elaborated by Levitt & Rashke in 1975 • • It is an active mechanism and require ATP.
Factors affecting stomatal movements Following factors affects stomatal opening and closing in plant cells- 1) 2) 3) 4) Light Carbon Dioxide Concentration Temperature Water Deficits and Abscisic Acid.
1) Light • Stomata generally open in light and close in darkness. • The stomata CAM (Crassulacean Acid Metabolism) plants are exceptional as they open at night and close during the day. • Photosynthesis reduces the CO 2 conc. in guard cells which has powerful stimulus for opening the stomata. • Osmotically active substances (sugars) synthesised during photosynthesis contribute in decreasing the water potential of guard cells. • In many plants illumination of the guard cells results in an increase of p. H which favours hydrolysis of starch into osmotically active sugars (glucose-1 -phosphate)
The blue light component of action spectrum on the effect of light on stomatal opening is related to a different photoactive control through cryptochrome • Blue light is perceived by zeaxanthin in the chloroplast of guard cells. • The excitation of zeaxanthin by blue light then starts signal transduction process that includes, • (i) Isomerization of zeaxanthin, • (ii) Conformational changes of an apoprotein, • (iii) Transmission of blue light signal across the chloroplast membrane by a second messenger (most probably Ca++, phosphatases, calmodulin and IP 3) • (iv) Activation of H+–ATPase at the guard cell plasma membrane resulting in pumping of protons across the membrane and intake of K+ ions.
2. Carbon Dioxide Concentration: §Reduced CO 2 conc. favors opening of stomata while an increase in CO 2 conc. promotes stomatal closing. §Stomata can be induced to open even in dark if conc. of CO 2 is significantly lowered; while a marked increase in CO 2 conc. causes the stomata to close in dark as well as in light. §It is in-fact that CO 2 which is present inside the leaf (intercellular) rather than that of the outer atmosphere which has controlling influence on stomatal movement.
3. Temperature §An increase in temperature results in increased stomatal opening provided water does not become a limiting factor. §Stomata of some plants, e. g. , Camellia do not open at very low temperatures (below 0°C) even in strong light. §Stomata of some plants tend to close even at high temperatures (more than 30°C).
4. Water deficit and ABA • Under water deficit (transpiration rate exceeds rate of water absorption), mesophytes tends to close their stomata so as to avoid damage due to water stress. • The stomata re-open only when water potential of these plants is restored. This type of control of stomatal movement by water is called as hydro passive control. • Abscisic acid (ABA) accumulates in the guard cells of waterstressed plants , causes stomata of such plants to close. • When water potential is restored, the stomata reopen and ABA gradually disappears from the guard cells. This type of control of stomata by water (mediated through ABA) has been called as hydro active control.
ABA-mediated stomatal regulation under water deficit
ABA-mediated stomatal movement
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