In Amphibious plants or emergent hydrophytes stomata remain always open throughout day and night, i.e. Equisetum type. How the rate of transpiration and absorption is maintained?
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Transpiration is of the following three types:
(i) Stomatal Transpiration: Transpiration takes place through the stomata which are present on the leaves of the plants and delicate organs, is called stomatal transpiration. The maximum amount of water is lost by this transpiration. About 80% to 90% transpiration occurs through the stomata.
(ii) Cuticular Transpiration: Loss of water through the cuticle which present on the herbaceous stem and leaves. Cuticle is a wax like thin layer present on epidermis. About 9% transpiration is cuticular.
(iii) Lenticular Transpiration: Minute pore like structure found on the stem of some woody plants and epidermis of some fruits is called lenticels. Some amount of water lost by lenticels is known as lenticular transpiration. However, it is approximately 0.1% to 1% of the total water lost.
Foliar transpiration: Total transpiration takes place through the leaves is called as foliar transpiration. Foliar transpiration = Stomatal + Cuticular, from the leaves.
Stomata are found on the aerial delicate organs and outer surface of the leaves in the form of minute pores. Stomatal pore is surrounded by two specialised epidermal cells called as guard cell. They are kidney shaped. The number of guard cells are two.
The structure of guard· cells in monocots (Gramineae) is dumbbell shaped.
Guard cells are epidermal cells. But due to presence of chloroplast they are different from that of epidermal cells.
The outer wall of the guard cells is thin and elastic while inner wall is thick and non-elastic.
Guard cells are surrounded by some specialized epidermal cells called subsidiary cells or accessory cells.
Stomata are found on both upper and lower surface. Stomata attached with air chambers and forms a cavity is called sub-stomatal-cavity.
In xerophytic plants position of stomata is deep in the surface of the leaf. Stomata are present in this position are called sunken stomata.
Types of stomata: On the basis of orientation of subsidiary cells around the guard cells, Metcalfe and Chalk classified stomata into following types:
Anomocytic: The guard cells are surrounded by a limited number of unspecialized subsidiary cells which appear similar to other epidermal cells. e.g., in Ranunculaceae family.
Anisocytic: The guard cells are surrounded by three subsidiary cells, two of which are large and one is very small. e.g., in Solanaceae and Cruciferae families.
Paracytic: The guard cells are surrounded by only two subsidiary cells lying parallel to the guard cells e.g., Magnoliaceae family.
Diacytic: The guard cells are surrounded by only two subsidiary cells lying at right angles to the longitudinal axis of the guard cells. e.g., Acanthaceae and Labiatae families.
Actinocytic: The guard cells are surrounded by four or more subsidiary cells and which are elongated radially to stomata.
Daily periodicity of stomatal movement: Loftfield (1921) classified the stomata into four types, depending upon the periods of opening and closing.
Alfalfa type (Leucerne type): The stomata remain open throughout the day but close during night, e.g., Pea, bean, mustard, cucumber, sunflower, radish, turnip, apple, grape.
Potato type: The stomata close only for a few hours in the evening, otherwise they remain open throughout the day and night e.g., Cucurbiia, Allium, Cabbage, Tulip, Banana etc.
Barley type: These stomata open only for a few hours in the day time, otherwise they remain closed throughout the day and night, e.g., Cereals.
Equisetum type: The stomata remain always open through out the day and night e.g., Amphibious plants or emergent hydrophytes.
1. Photosynthesis in guard cell hypothesis:
This theory was proposed by Schwendener & von mohl. According to this theory guard cell chloroplast perform photosynthesis during the day time. This produce sugars in guard cell which increases the O.P. of GC, compared to adjacent epidermal cells (subsidiary cells). Water enters in guard cells form subsidiary cells by endosmosis due to this guard cells become turgid & stomata will open.
(i) In CAM plants stomata open during dark/night.
(ii Chloroplast of monocot guard cells are nonfunctional (inactive) photosynthetically.
2. Starch Sugar interconversion theory:
This theory was proposed by Sayre (1926). First of all Lloyd stated that amount of sugar in GC increases during the day time & starch in night.
Detail study of this change was done by Sayre & give starch hydrolysis theory. According to Sayre, starch converts into sugars during day time when pH or guard cell is high. Sugar changes into starch during night at low pH in guard cells (Supported by Scarth). Sayre classified that CO2 reacts with water during night. Due to accumulation of H2CO3 pH in guard cell decreases. Hanes: Stated that this change takes place by phosphorylase enzyme.
Yin & Tung reported the presence of phosphorylase enzyme in guard cells.
Stewards Modification: According to Steward (1964) appreciable change in O.P. of GC is possible after the conversion of glucose-1 P into Glucose & ip (inorganic phosphate)
Steward give stomatal mechanism as follows.
(i) Starch is absent in GC of some monoccts like onion.
(ii) Formation of organic acids is observed during stomatal opening.
3. Active K+ H+ exchange theory or active proton transport mechanism.
Given by Levitt (1973-74). This is modern & most accepted theory for stomatal opening & closing. First of all Fujino observed that influx of K+ ions in guard cells during stomatal opening. (Supported by Fisher & Hsiao). Detail study of this· phenomenon was done by Levitt, who proposed this theory. According to him stomata opens by following mechanism.
(i) Carbohydrates Closing of stomata: Plant hormone ABA-acts on guard cells, which interfere the exchange of K+ H+ ions in guard cells, results in reverse of rxn. of opening of stomata, hence stomata closed. pH of guard cells decreases during night, which favours stomatal closing.
High concentration of K+ ions in guard cells is electrically balanced by uptake of Cl– and malate ions in guard cells. (4) Ca-ABA second messenger model:
Given by Desliva & Cowman (1985). This is modern explanation of stomatal closing only.
Ramdas & Raghvandra suggested that A TPs for stomatal movement comes from cyclic ETS.
Bowlings: Malate switch hypothesis.
Raschke: K+ ions in guard cells comes from subsidiary cells.
Stomata opens during night in succulent plants and closes during the day. This nature of stomata in Opuniia is called scotoactive stomata.
In CAM plants organic acid is formed during night which is broken down during day & CO2 is liberated is used in photosynthesis.
Stomatal opening in succulent plants (Scotoactive stomata): The stomata in succulent plant or CAM plants (like Opuniia, Bryophyllum etc.) open during night (darkness) and remain closed during the day time and is found in lower surface. This type of stomatal opening is called 'Scotoactive type' and the stomata which open during day are called as photoactive. Stomata is closed and opened due to the activity of water. This types of stomata is known as hydroactive stomata. The opening and closing mechanism of scotoactive stomata was explained by Nishida (1963). In succulent plants, during night, there is incomplete oxidation of carbohydrates and accumulation of organic acids (e.g., malic acid) without release of CO2, During day time the accumulated organic acids breakdown rapidly releasing excess amount of CO2 for photosynthesis as well as to keep the stomata closed. During night: C6H12O6 + 3O2 → 3C4H6O5 + 3H2O During day: C4H6O5 + 3O2 → 4CO2 + 3H2O
Light: In most of the plants stomata open during the day except succulent xerophytic plants and close during the dark. Opening of stomata completes in the presence of blue and red light. Blue light is most effective and causes stomatal opening.
Temperature: Loft Field show temperature quotient of opening stomata is [Q10] = 2
Stomata are sensitive towards the internal CO2 conc. in the leaves.
It is internal leaf CO2 cone rather than the atmospheric CO2 conc. that ditactes stomatal opening.
Stomata opens at low concentration of CO2 while closed at high concentration of CO2.
Cytokinin hormone induce opening of stomata. It increase the influx of K+ ions and stimulates the stomata for opening.
While ABA stimulate the stomata for closing. This hormone oppose the induction effect of cytokinins,
ABA effects the permeability of the guard cells. It prevent the out flux of H+ ions and increase the out flux of K+ ions. Because of this, pH of the guard cells decreased.
Cl- ions also plays important role in stomatal movement.
Above mentioned effects also found in high amount of CO2.
ABA is formed due to high water stress in chloroplast of leaves.
Atmospheric humidity: Stomata opens for long duration and more widen in the presence of humid atmosphere, while stomata remains closed in dry atmosphere or partial opening at higher atm humidity. Transpiration is stops but stomata remain completely open.
Factors effecting the rate of transpiration are divided into two types:
(A) External factors (Environmental factor)
(B) Internal Lactors
(A) External factors:
Atmospheric humidity: Tr - 1/Atmospheric Humidity
This is the most important factor. The rate of transpiration is higher in low atmospheric humidity while at higher atmospheric humidity the atmosphere is moistened, decreasing the rate of transpiration.
Therefore, the rate of transpiration is high during the summer and low in rainy season.
Tr µ Temperature
The value of Q10 for transpiration is 2. It means by increasing 10°C temperature, the rate of transpiration is approximately double. (By Loftfield)
Water vapour holding capacity of air increases at high temperature, resulting in increase in rate of transpiration.
On contrary vapour holding capacity of air decreases at low temperature so that the rate of transpiration decreases.
Light: Light stimulates transpiration by heating effect on leaf.
Action spectrum of transpiration is blue and red. Rate of transpiration is faster in blue-light than that of red light, because stomata are completely opened at their full capacity in the blue light. Wind velocity: Tr µ Wind velocityTranspiration is less in constant air but if wind velocity is high the rate of transpiration is also high, because wind removes humid air (saturated air) around the stomata.
Transpiration increases in the beginning at high wind velocity [30 - 35 km./hour] But latter on it cause closure of stomata due to mechanical effect and therefore transpiration decreases.
Atmospheric Pressure:The speed of the air increases at low atmospheric pressure, As a result rate of the diffusion increases which in turn increases the rate of transpiration.
The rate of transpiration is found maximum in the high intensity of light at high range of hills.
Transpiration ratio (TR): Moles of H2O transpired/moles of CO2 assimilated.
Ratio of the loss of water to the photosynthetic CO2 fixation is called TR.
TR is low for C4 plants (200-350) while high for C3 plants (500-1000). It means C4 conserve water with efficient photosynthesis.
CAM plants passes minimum TR (50-100)
Anti transpirants: Chemical substances which reduce the rate of transpiration are known as antitranspirants. Anti transpirants are as follows:
Phenyl mercuric acetate (PMA], Aspirin (Salicyclic acid), Abscisic acid [ABA], Oxi-ethylene, Silicon oil, CO2 and low viscous wax.
PMA closed the stomata for more than two weeks partially.
Antitranspirants are used in dry farming.
(B) Internal factors: These factors are concerned with structure of plants these are of following types:
Transpiring area: Pruning increase the rate of transpiration per leaf but overall reduce the transpiration.
Anatomical characteristics of leaf and leaf orientation:
The following characteristics are found in leaf to reduce the transpiration.
(i) Leaves modified into spines.
(ii) Leaves transformed into needle e.g. Pinus.
(iii) Folding and unfolding of leaves by Bulliform cells. e.g. Amophilla, Poa etc.
(iv) Small size of the leaves.
(v) Presence of thick waxy layer on the leaves. e.g. Banyan tree.
Stomatal characteristics: Transpiration is effected by the structure of stomata, position of stomata, distance between the stomata, number of stomata per unit area and activity of the stomata.
By Salisbury – Stomatal Index (SI) =I= S/E+S
SI = Stomatal index,
S = No. of stomata/unit area
E = No. of epidermal cells in same unit area. (4) Leaf-orientation
(5) Water status of leaves
(6) Root: Shoot Ratio:
The rate of transpiration-decreases with decrease in root - shoot ratio.The rate of transpiration increases with increase root - shoot ratio.
In regulation of temperature: Cooling effect on the surface of leave is produced by the process of transpiration due to which the temperature remain constant of the plants.
In mineral absorption
In ascent of sap
In water absorption
Distribution of absorbed salts
Control of hydrological cycle