Term paper Transpiration types and its significance,

Mechanism of opening and closing of stomata,

Guttation.

Submitted to:

Dr. Manoj Kumar.

Submitted by:

Irfan manzoor.

Roll no: RP7002B22.

Reg. no: 11000225.

Course: Bsc. Biotech 1.

Lovely professional university

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Contents

1. Transpiration 3

2. Types of transpiration 4-5

3. Significance of transpiration 6

4. Stomata 7

5. Opening and closing of stomata 8

6. Mechanism of Opening and

Closing of Stomata 9-10

7. Guttation 11

8. Bibliography 12

Transpiration

The process of transpiration in plants is the phenomenon of loss of water in the form of vapour from its living tissues of aerial parts of the plant body. Basically process of transpiration in plants is evaporation. There are various factors that

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influence the rate of transpiration like environmental factors like light, CO2 concentration, temperature, humidity of air, wind velocity and internal factors like root shoot ratio, leaf orientation, leaf area. The process usually occurs in the day. It is the loss of water vapor from parts of plants (similar to sweating), especially in leaves. Leaf surfaces are dotted with openings called, collectively, stomata, and in most plants they are more numerous on the undersides of the foliage. The stoma are bordered by guard cells that open and close the pore. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.

The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The amount of water lost by a plant depends on its size, along with surrounding light intensity, temperature, humidity, and wind speed (all of which influence evaporative demand). Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.

A fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced). Transpiration rate of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, and heat balance sap flow gauges.

Desert plants and conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis in which the stomata are closed during the day and open at night when transpiration will be lower.

Types of transpiration

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Plants show three types of transpiration as follows:

Cuticular transpiration : Transpiration which takes place through the cuticle is called cuticular transpiration. Cuticle is a waxy layer that covers the epidermis of leaves and young stems. It is meant to check transpiration. But in nature the cuticle shows many small cracks or cuts through which very little transpiration takes place. In many plants cuticle is thin and green and approximately 5% to 10% of the total water transpired through that.

In xerophytes the cuticle is comparatively thick and the extent of water transpired is significantly less.

ii. Lenticular transpiration : Lenticels are small slit like aerating opening in the bark of woody stems and fruits through which negligible amount of water vapour is lost (0.1 % of total loss).

Lenticular transpiration

 Stomatal transpiration : The loss of water jn the form of vapours through the stomata is called stomatal transpiration. The maximum amount (90 - 95%) of absorbed water is transpired through stomatal transpiration. Stomatal transpiration is commonly found in the leaves and stems of young plants.

The mesophyll is the ground tissue of the leaf enclosed on either side by the epidermal layer. The leaf in general has loosely arranged spongy parenchyma cells.

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The air spaces of parenchyma communicate with the outside atmosphere by means of minute pores called stomata present in the epidermis. The mesophyll cells of leaf get water from the xylem of the leaf vein by osmotic diffusion, and become turgid and saturated with water. When sunlight falls on the leaf the water evaporates from their moist walls into the intercellular spaces of mesophyll, which become saturated with water vapour. This water vapour then diffuses through the stomata into outer atmosphere which is unsaturated. More water then is taken into the intercellular spaces from the xylem through the mesophyll cells. Thus, the process of transpiration through stomata continues. If stomata are closed, the water evaporated from the mesophyll cells saturate the entire intercellular spaces with water vapour, which will diffuse out when stomata open.

Stomatal transpiration

Significance of transpiration

The process of transpiration in plants probably plays a vital role in absorption of water and ascent of sap in tall plants. The passive absorption of water

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through roots as well as ascent of sap is due to transpiration pull according to Cohesion-tension theory of Dixon and Jolly.

Water and mineral absorbed by plants from soil are transported upwards in the transpiration stream through xylem ducts.

The process of transpiration in plants prevents the heating of plant leaves during excessive day temperature.

The process of transpiration in plants is rightly known as a necessary evil. It is evil as huge amount of water (about 97%) absorbed by plants is lost through this process while it is necessary as it is helpful in many physiological processes.

Stomata

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A typical stomata is a microscopic and usually consists of a slit like opening which is surrounded by a pair of bean (kidney) shaped guard cells. The two guard cells and stomatal pore constitute stomatal apparatus. A stomatal pore is generally elliptical in surface view. The dimension of stomatal pore varies from species to species, but on an average, it measures about 20 jtm long and about 10-20 |j.m wide when fully open. Guard cells generally have thick and non-elastic walls towards the pore and thin and elastic walls on opposite side. The guard cells are living cells with cytoplasm and a central vacuole. The cytoplasm has a nucleus and many chloroplasts. The guard cells are surrounded by large epidermal cells called subsidiary cells. In turgid condition, stomata open and in flaccid condition they close.

The number and distribution of stomata varies in different groups of plants. In mesophytic dicotyledons, the lower surface of leaf has more number of stomata (hypostomatic). The monocots like grasses, maize etc have equal distribution of stomata on both the surfaces (amphistomatic). Among the aquatic, floating plants like Nymphaea and Lotus have stomata confined to the upper surface (epistomatic) decides the rate of transpiration. The number of stomata per square centimetre in plants may vary from 5,000 to 30,000. It is called stomata frequency.

Opening and closing of stomata

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The small pores on the epidermis of leaves are called stomata. Each stoma or stomata pore is surrounded by two guard cells. In dicot plants guard cells are kidney shaped or bean shaped. In monocot stomata, guard cells are dumb bell shaped .The inner wall of guard cells are thick and non-elastic. The outer wall is thin and elastic.The adjoining cell walls of two guard cells around the pore are free and not attached with each other and this help them to stretch laterally during stomatal opening

The epidermal cells surrounding the guard cells are called subsidiary cells. The stomata pore, guard cells and the subsidiary cells are together called stomata apparatus. Each guard cell contains a single nucleus and numerous chloroplasts.The guard cell chloroplast possess pigments of both photosystem I and II and thus perform photophosphorylation.They lack RUBISCO enzyme and NADP+ linked triose phosphate dehydrogenase enzymes.So carbon reduction cycle is absent Starch is synthesized in guard cell chloroplast by sugars transported from adjacent mesophyll cells and they are characterised by accumulation of starch during night(in dark) and their degradation during the day(in light). Mesophyll cells accumulate starch during the day and decrease during the night.This property helps in the opening and closing of stomata.

Mechanism of Opening and Closing of Stomata

There are two main theories about the mechanism of stomatal opening and closing

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1.Starch : Sugar hypothesis by J.D.Sayre and modified by Steward:-Photosynthesis occurs in light by absorbing carbondioxide which lowers the H+ ion of cell sap and PH of guard cell is increased.High PH favours the activity of enzyme  phosphorylase which converts starch into glucose and phosphate.It dissolves in the medium and increase the concentration of cell sap.This cause an increase in the osmotic pressure of guard cells and its  diffusion pressure deficit(DPD) is also increased which results in the movement of water in to the guard cells from surrounding cells.Guard cells become turgid and swell .Thus the stomata opens.

                  During dark ,the level of carbondioxide in substomatal cavity is increased which results in the decrease in the PH of guard cells.At low PH glucose is converted back to starch in the presence of enzyme phosphorylase .Synthesis of starch leads to the dilution of cell sap by consuming its dissolved glucose molecule. Thus osmotic pressure of cell sap is decreased and its DPD (diffusion pressure deficit)is decreased. The turgid cells lose water to surrounding cells and becomes flaccid and stomata closes

2.Theory of K+ ion transport by Levitt and elaboration by Raschke and Bowling:-In light starch is converted into phosphorylated hexoses and then to phosphoenol pyruvic acid which combines with carbon dioxide to produce malic acid. Malic acid dissociate into malate anion and H+ ion in the guard cell.H+ ions are  transported to epidermal cells   and K+ions are taken into the guard cells in exchange of H+ ions and is called ion exchange. Increased concentration oh H+ ions and malate ions in the vacuole of guard cells causes sufficient osmotic pressure to absorb water from surrounding cells. It results in the opening of stomata.

                   In the dark carbon dioxide concentration is increased in the substomatal cavity which prevents proton gradient across the protoplasmic membrane in guard cells.As a result active transport of K+ ions into guard cells ceases.As soon as the PH of guard cells decreases the abscissic acid inhibits K+ ion uptake by changing the diffusion and permeability of guard cells .Malate ion in the guard cell cytoplasm combine with H+ ion to  produce malic acid.These changes cause reversal of concentration movement  .So the K+ ion is transported out of guard cells into the surrounding epidermal cells.The osmotic pressure of guard cell is decreased which results in the movement of water from guard cells to surrounding cells and guard cells becomes flaccid and stomata closes .

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Guttation

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When leaves lose water as a liquid phase through special cells called hydathodes it is referred to as guttation. The main cause of guttation in plants is root pressure. During night when root pressure is high sometimes then due to this pressure watery drops ooze out with the assistance of hydathodes.

At night, transpiration usually does not occur because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant, creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes, forming drops. Root pressure provides the impetus for this flow, rather than transpirational pull . This is how guttation is carries out.

Guttation fluid may contain a variety of organic and inorganic compounds, mainly sugars, and mineral nutrients, and potassium. On drying, a white crust remains on the leaf surface.

Guttatation

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Guttation can only take place when there is more water in the soil than in the roots of the plant. The water potential in the roots has to be lower than the water potential in the soil (the amount of water in the roots is lower than the amount of water in the soil). This contributes to root pressure. Root pressure supplies the means by which guttation takes place. Root pressure occurs at night when stomata are closed and transpiration has stopped or during the day if transpiration is slow. If water potential is balanced between roots and soil and there is no transpiration, there will be no guttation.

Guttation in nature occurs much more frequently than commonly realized. In most cases it does not result in any visible injury to plants but it may be responsible for more or less continuous loss of nutrients under different climates. Various injuries from guttation may occur and an attempt has been made to classify them. According to cause: (1) those connected with loss and depletion of unusual amounts of vital nutrient substances; (2) others induced by accumulation and concentration of guttation products on localized areas of the plant, or (3) caused by entrance through hydathodes during active guttation periods of various foreign agents, such as pathogenic microorganisms, pesticides capable of interreacting with exuded substances, disinfecting and other gases, etc. According to symptoms and functional disturbances: (1) chlorosis (leaf-marginal, preceding necrosis, or as a deficiency symptom), (2) necrosis, caused by action of concentrated solutions on local areas near hydathodes or on the leaf blade, as in cantaloupes, squashes, onions, etc., or by depletion of potassium as in Cox's Orange Spot disease of apples, or by interaction of guttation salts with pesticides, (3) killing of entire leaves as a last stage of the above, and (4) defoliation and fruit-drop, seemingly due to excessive and drastic loss of nutrients in a short time.

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Bibliography

1. Goatley, James L.; Lewis, Ralph W. (March 1966). "Composition of Guttation Fluid from Rye, Wheat, and Barley Seedlings". Plant Physiology .

2. Study books from biotech.net.3. Wikipedia4. Google etc.

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Thank you

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