Biological Sciences
Transpiration
Transpiration is the process by which plants lose water through the stomata in their leaves. This loss of water vapor helps to create a negative pressure in the plant, which in turn draws water and nutrients up from the roots. Transpiration also plays a role in cooling the plant and transporting minerals throughout the plant.
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9 Key excerpts on "Transpiration"
eBook - PDF- Paul J Kramer(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
im Transpiration Introduction 291 Importance of Transpiration 292 The Process of Transpiration 294 Evaporating Surfaces 294 The Water Vapor Pathway 295 Driving Forces and Resistances 295 Plant Factors Affecting Transpiration 305 Leaves 305 Stomata 317 Measurement of Stomatal Aperture or Resistance 328 Interaction of Factors Affecting Transpiration 330 Measurement of Transpiration 331 Gravimetric Methods 331 Volumetric Methods 333 Measurement of Water Vapor Loss 333 Velocity of Sap Flow 334 Bases for Calculating Transpiration Rates 335 Rates of Transpiration 336 Evaporation from Stands of Plants 337 Summary 339 Supplementary Reading 340 INTRODUCTION Transpiration can be defined as the loss of water from plants in the form of vapor. It is therefore basically an evaporation process, dependent on the supply of energy and the vapor pressure gradient between the evaporating surfaces and the ambient air. Viewed merely as an evaporation process, Transpiration is quite simple, but when viewed as a plant process, it is extremely complex. Unlike evaporation, Transpiration is modified by plant factors such as leaf structure and stomatal behavior, operating in addition to the physical factors that control 291 292 11. Transpiration evaporation. This chapter deals with the nature and importance of Transpiration from individual plants and plant communities and with factors affecting it. Importance of Transpiration Transpiration can be regarded as the dominant process in plant water relations. Evaporation of water produces the energy gradient that is the principal cause of water movement into and through plants, and it therefore controls the rate of absorption and the ascent of sap. In warm, sunny weather, Transpiration causes transient leaf water deficits almost daily, and when drying soil causes water absorption to lag behind loss by Transpiration, permanent water deficits develop that result in permanent wilting and finally in death by dehydration.
eBook - ePubWater at the Surface of Earth
An Introduction to Ecosystem Hydrodynamics
- David M. Miller(Author)
- 1982(Publication Date)
- Academic Press(Publisher)
In comparison with a water body, a vegetation-soil ecosystem has small heat storage, as good absorption of solar radiation, and a better linkage with the atmosphere by means of convective exchanges because it is rougher. These likenesses and differences suggest a little about the process of evaporation from ecosystems, the energy sources that power it and the external conditions that govern it.Transpiration of Water from Leaves
Transpiration plays a role in the operations of vegetation that is somewhat more than just a leakage of water that happens to occur when cells are opened to receive CO2 from the environment. It represents an “energy subsidy” (Odum, 1971, p. 43 ) to the ecosystem from outside. This subsidy powers the intake of nutrients from the soil and thus is a mechanism for nutrient conservation in the soil-vegetation system (Odum, 1971, p. 95 ). Also, by cooling the leaves it helps to “reduce the respiratory heat loss (i.e., the ‘disorder pump-out’) necessary to maintain the biological structure” (Odum, 1971, p. 43 ). While this book tends to deal more with the circulations of water outside plant tissue in ecosystems–delivery of rain, interception, infiltration, and so on–we cannot forget the important role of water within the tissues of the plants in the system.While Transpiration is a part of the functioning of life processes of plants, it is also at the same time a physical process. The usual number of kilojoules of energy are converted to latent heat, drawing on one or another source of energy, when a certain mass of water is vaporized; water has to be delivered to the site where vaporization can take place; and the vapor has to be carried away.Resistance to the Diffusion of Water Vapor
One characteristic of evaporation in an ecosystem not found in other evaporating systems is that the wet cell walls, the sites of vaporization, are not directly exposed to the free air, which is the ultimate sink for the vapor molecules. They open instead on intercellular spaces within the leaf, which interpose an additional resistance to molecular diffusion and tend to slow down the vaporization process.
eBook - ePub- Theodore T. Kozlowski, Stephen G. Pallardy(Authors)
- 1996(Publication Date)
- Academic Press(Publisher)
CHAPTER 12Transpiration and Plant Water Balance
TranspirationThe Process of TranspirationTranspiration as a Physical ProcessEnergy Use in TranspirationVapor Concentration Gradient from Leaf to AirResistances in the Water Vapor PathwayLeaf ResistancesExternal ResistancesFACTORS AFFECTING TranspirationLeaf AreaRoot/Shoot RatioLeaf Size and ShapeLeaf OrientationLeaf SurfacesStomataStomatal Control of TranspirationMeasurement of Stomatal ApertureINTERACTION OF FACTORS AFFECTING TranspirationMEASUREMENT OF TranspirationGravimetric MethodsCut-Shoot MethodVolumetric MethodsMeasurement of Water Vapor LossVelocity of Sap FlowTranspiration RATESWATER LOSS FROM PLANT STANDSMethods of MeasurementFactors Controlling EvapoTranspirationEffects of Changes in Plant CoverThinningRelative Losses by Evaporation and TranspirationChanges in Species CompositionMethods for Reducing TranspirationTranspiration Ratio and Water Use EfficiencyTHE WATER BALANCEDynamics of Plant Water StatusThe Absorption LagInternal Competition for WaterLong-Term Variations in Water ContentSeasonal Variations in Water ContentEFFECTS OF WATER STRESSADAPTATION TO DROUGHTDrought AvoidanceDrought ToleranceDrought HardeningSUMMARYGENERAL REFERENCESTranspiration
Transpiration is the loss of water vapor from plants. It is a dominant factor in plant water relations because evaporation of water produces the energy gradient that causes movement of water through plants. It therefore controls the rate of absorption and the ascent of sap and causes almost daily leaf water deficits. A single isolated tree may lose 200 to 400 liters of water per day, and a hardwood forest in the humid Appalachian Mountains of the southeastern United States loses 42 to 55 cm of water per year (Hoover, 1944
eBook - PDF- Paul Kramer(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
On the other hand, rapidly transpiring plants lose so much water on sunny days that the cells of young twigs and leaves lose turgor and wilt, stomata close reducing Transpiration 409 photosynthesis, and growth is reduced or stopped. The harmful effects of water stress will be discussed further in Chapters 13 and 16. We regard Transpiration as an unavoid-able evil, unavoidable because of the structure of leaves and evil because it often produces water deficits and causes injury and death by desiccation. The Process of Transpiration Because of the great importance of Transpiration in the overall water economy of plants, the nature of the process and the factors affecting it deserve careful attention. Transpiration is basically the physical process of evaporation which is controlled by physical factors. However, Transpiration is also a physiological process, and as such it is affected by plant factors such as leaf structure and exposure and the behavior of stomata. It usually occurs in two stages, evaporation of water from cell walls into intercellular spaces and the diffusion of water vapor into the outside air. Transpiration as a Physical Process The rate of evaporation of water from any surface depends on the energy supply available to vaporize water, the vapor pressure or vapor concentration gradient that constitutes the driving force, and the resistances in the diffusion pathway. Most water vapor escapes through the stomata, some passes out through the epidermis of leaves and its cuticular covering, and some escapes from the lenticels in the bark of branches and twigs of woody species.
eBook - PDFAtmosphere, Ocean and Climate Dynamics
An Introductory Text
- David H. Miller, John Marshall, R. Alan Plumb, J. Van Mieghem(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
* Such systems are not uncommon, being found where the spacing of rains is no more than a few days, in lands of high groundwater, and in artificially irrigated lands, including urban green areas. 274 Transpiration OF WATER FROM LEAVES 275 In comparison with a water body, a vegetation-soil ecosystem has small heat storage, as good absorption of solar radiation, and a better linkage with the atmosphere by means of convective exchanges because it is rougher. These likenesses and differences suggest a little about the process of evaporation from ecosystems, the energy sources that power it and the external conditions that govern it. Transpiration OF WATER FROM LEAVES Transpiration plays a role in the operations of vegetation that is somewhat more than just a leakage of water that happens to occur when cells are opened to receive C0 2 from the environment. It represents an energy subsidy (Odum, 1971, p. 43) to the ecosystem from outside. This subsidy powers the intake of nutrients from the soil and thus is a mechanism for nutrient conservation in the soil-vegetation system (Odum, 1971, p. 95). Also, by cooling the leaves it helps to reduce the respiratory heat loss (i.e., the 'disorder pump-out') necessary to maintain the biological structure (Odum, 1971, p. 43). While this book tends to deal more with the circulations of water outside plant tissue in ecosystems—delivery of rain, interception, infiltration, and so on—we cannot forget the important role of water within the tissues of the plants in the system. While Transpiration is a part of the functioning of life processes of plants, it is also at the same time a physical process. The usual number of kilojoules of energy are converted to latent heat, drawing on one or another source of energy, when a certain mass of water is vaporized; water has to be delivered to the site where vaporization can take place; and the vapor has to be carried away.
- Giuseppe Montanaro and Bartolomeo Dichio(Author)
- 2012(Publication Date)
- IntechOpen(Publisher)
This will be considered throughout this chapter. 2. Absorption and water flow through plants Independent of the species, plants require from the soil a water volume that overcomes its metabolic necessities. Through the Transpiration process plants transmit to the atmosphere the majority of the water absorbed from soil (generally around 90%). From this perspective, it is noted that the plant water requirements are defined primarily by the atmosphere evapotranspirative demand, which is a predominately passive process. Figuratively, and with some caveats, we can compare a plant water flow with the principles of oil flow in the wick of an old fashion lampion (Fig. 1). When it is fired, the oil that is burned on the upper extremity of the wick is quickly replaced by new one that is situated just below, and so on - following the physical forces of interaction between liquid and tissue - until reaching the level of the fuel reservoir, in the basal extremity of the wick. Applying this example to the plant, the burning of oil can be analogous to the Advances in Selected Plant Physiology Aspects 106 process of the loss of water vapour through their leaves, i.e. the Transpiration , which is caused by the pressure gradient of vapour between tissue saturated with water from the leaves and air, the “dry” atmosphere. The variations in this pressure gradient of the vapour will define the evaporative demand of the environment where the plant is. In the other extreme - where it represents the liquid reservoir of the lampion - we have water content present in the soil. In this scene, it is noted that the water flow through the plant is dependent on the energy formed by the gradient of the water content that is established between the soil and the atmosphere. However, we will see throughout this chapter that plants, unlike our lampion, can and must modulate this gradient in order to survive the wide variations of water availability between types of soils, weather and seasons.
eBook - PDFAgricultural Physics
The Commonwealth International Library: Physics Division
- C. W. Rose, W. Ashhurst, H. T. Flint(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
CHAPTER 8 A Physical Introduction to Plant-Water Relationships 8.1. Water Transport through the Soil-Plant-Atmosphere System The extremely important questions of the relation between the uptake of water and plant nutrients and the role of water in plant growth processes will be neglected here and attention focused on the Transpiration stream. Although water transport through plants can be under strong metabolic control in some circumstances (Kramer/ 1 ) Mees and Weatherley< 2 >) the process appears to be a predominantly passive one (Slatyer^ 3 *)· The concept of water moving in response to gradients in its total potential Ψ 9 discussed in Chapters 5 and 6 in the setting of water and soil, is equally relevant to the passive transport of water through the entire soil-plant-atmosphere system (Van den Honert< 4 >). Physically, the Transpiration stream may be regarded as a water flux from a source of finite capacity, the soil water, down a gradient in total water potential, to a sink of effectively infinite capacity, the atmosphere. With roots in moist soil this gradient may fall to almost zero at the end of a night. But with the dawning of another day, water evaporated from the plant cells into the air spaces of the leaf diffuses out through the now opening stomata to the surrounding air (Fig. 48). The local decrease in Ψ accom-panying this water loss causes upward flow through the xylem tissues of the stem, and subsequently through the roots. Thus absorption of water through the roots unavoidably lags behind the Transpirational loss, and the excess of total Transpiration over total absorption at any time gives the reduction in water stored in the plant. Decreasing water storage is reflected by lower turgor pressure within the plant cells and vice versa, and such turgor 200 PLANT-WATER RELATIONSHIPS 201 pressure fluctuations have a characteristic diurnal pattern under normal conditions.
eBook - PDF- Stanley W. Trimble(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
Under conditions of ele-vated CO 2 concentration, the CO 2 gradient between the atmosphere and the leaf is higher and CO 2 can pass through partially closed stomata at a rate similar to that under conditions of lower CO 2 and open stomata. The water vapor gradient remains the same at higher CO 2 , and the Transpiration is impeded. The net result is improved Transpiration efficiency by some plants. Under low n conditions, there is very little water vapor flux from leaf to air, and gas exchange can occur through open stomata with minimum loss of water. Under these conditions, the net result is also improved Transpiration efficiency. The advantage of intrinsic TE as a term is that it allows a direct comparison of intrinsic physiological considerations without confounding effects of differ-ences in temperature and humidity that may exist in a canopy situation. On the contrary, the disadvantage is that it only represents a ‘‘snap shot’’ of A / g s , and may not necessarily scale up to long-term considerations related to overall canopy productivity and growth. Transpiration efficiency at the entire plant level is defined as plant biomass (DM) accumulated per unit of water transpired (T) over a specified time interval, expressed as g kg 1 . At the entire plant level, TE can be more accurately determined by cumbersome and labor intensive gravimetric methods. [5–7] Encyclopedia of Water Science DOI: 10.1081/E-EWS-120010172 Copyright # 2008 by Taylor & Francis. All rights reserved. 1240 Tailwater–Virtual Variation in TE can occur due to both genetic and environmental factors. The following expression illus-trates the potential sources of variation in TE: TE ¼ A = g s ¼ ½ p a ð 1 p i = p a Þ = 1 : 6 ð e i e a Þ ð 2 Þ where e i and e a (mbar) are intercellular and atmo-spheric water vapor pressures. It can be seen from Eq. (2) that a reduction in p i / p a at a given e i e a , (i.e., the vapor pressure deficit (VPD) will increase TE.
- Arnold F. Moene, Jos C. van Dam(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
Plants cannot avert the onset of cav- itation. As soon as the water thread in a vessel is broken, the water flow will be diverted into neighbouring vessels by the bordered pits of a vessel below and above the obstructed vessel member (Figure 6.9). At the same time the expansion of the bubble from one vessel to the next is limited by the specific vessel structure. The holes at the perforations and the pits at the walls are good for liquid but not for gas transport. Because of the surface tension of water at the liquid–vapour interface, the holes will stop the air bubbles from escaping or steadily enlarging. During the night, when the Transpiration rate falls to zero and the tension eases, the air bubbles can be dissolved again. Thus the damage is repaired and the diversion will be closed (Ehlers and Goss, 2003). 6.4 Transpiration, Photosynthesis and Stomatal Control 6.4.1 Transpiration Plants take up liquid water, with nutrients dissolved in it, from the soil through their roots. The water is transported upward and most of it leaves the plant, as water vapour, whereas only a small fraction (about 1%) is used in the photosynthesis process. Los- ing this water is the unavoidable by-product of carbon exchange. The water vapour leaves the plants through the stomata. These are small openings that occur mainly on the plant leaves, but to a lesser extent on other plant organs as well. Stomata are the main path way for the exchange of both CO 2 and water vapour between the plant and the atmosphere because the cuticle is rather impermeable for gases (Figure 6.11). In herbaceous plants stomata occur at both the upper and the lower side of leaves, whereas trees have stomata only at the lower side (Willmer and Fricker, 1996). The density and size of stomata varies considerably between plant species, but there is a roughly inverse relationship between stomatal density and stomate size that leads to a rather constant total area of stomata (Hetherington and Woodward, 2003).
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