Melvy June BalasaBS Biology III Exercise 3 Uptake and water movement of water in Plants I. Introduction Water is very essential for plant growth, metabolism, transport, transpiration and guttation. As plants evolved, they have developed functional organs that would effectively suffice with their daily requirements, especially water and minerals. The roots mainly served this purpose, not only does it aid in anchoring the plant, but it also help in nutrient and water uptake. And from here on, when the roots have grown in search for water and nutrients, all this essential plant requirements must be equally distributed throughout the plant. Plants are composed of conducting vessels that efficiently transport water and minerals. For water conduction, plants have xylems. From the root hairs, water is absorbed by neighboring cells of the cortex to the endodermis then to the xylem tracheids or vessels. Absorbed water will then be transported into all parts of the plant body through the system of conductive tissues and distributed to leaves, flowers, fruits and growth apex. Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants (similar to sweating), especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells that open and close the pore (Cummins, 2007). 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. Mass flow of liquid water from the roots to the leaves is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem (Swarthout et al., 2010). Despite the fact that transpiration accounts for water loss in plants, it is not simply a hazard of plant life. It is the engine that pulls water up from the roots to supply photosynthesis, bring minerals from the roots for biosynthesis within the leaf and also serves in cooling plants. There are several factors affecting the rate of transpiration. These are light, temperature, humidity, wind and soil water. The rate of transpiration is directly related to the evaporation of water molecules from plant surface, especially from the surface openings, or stoma, on leaves. Stomatic transpiration such as grasses. it aims to learn the factors affecting the rate of transpiration and evaporation by comparison. The length stained by the dye . By setting up a potometer experiment.. Xylem consists of numerous tiny channels which run vertically all the way up the plant. humidity. Plant roots take up water and minerals from the soil and transport them up the stem to the leaves through specialized tissue known as xylem. rather than transpirational pull (Goatley et al. the tissues concerned with water ascent in the stem were studied.. on the other hand. the shoots were removed from the tubes and were cut longitudinally. As water molecules tend to stick together. transpiration rates can be measured when various environmental factors are changed. The amount of water given off depends somewhat upon how much water the roots of the plant have absorbed (Martin et al.. from the leaves of a plant. which condenses from the atmosphere onto the plant surface. Potometer readings will typically vary according to factors in the environment. such as temperature. A potometer.. When there is a high soil moisture level. hydathodes. utilize a scientific instrument which quantifies water transfer at the leaves (Slavík et al. because the water potential of the roots is lower than in the soil solution. water will enter plant roots. but some direct evaporation also takes place through the cuticle of the leaves and young stems. light. Two leafy young shoots of kamantigue (Impatiens balsamina) were secured. 1974). Guttation is not to be confused with dew. The Potometer does not measure the rate of transpiration accurately because not all of the water that is taken by the plant is used for transpiration (water taken might be used for photosynthesis or by the cells to maintain turgidity). or the rate of loss of water. it evaporates through openings called stomata. 1966). The bases of the shoots were cut under water and were immersed in separate test tubes which contained 2ml of 0. In one of the shoots. After 10 minutes.1% eosin. Also. The water will accumulate in the plant. The root pressure forces some water to exude through special leaf tip or edge structures. The potometer measures the rate of uptake of water.accounts for most of the water loss by a plant. all leaves were detached. When water reaches the leaves. Materials and Methods In this part of the experiment. Root pressure provides the impetus for this flow. or transpirometer. is a device used to measure the rate of transpiration. is the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants. To measure transpiration rate directly. this evaporation from the top of the plant exerts an upward pull on the vertical columns of water in the xylem. At night. forming drops. rather than the rate of water uptake. creating a slight root pressure. transpiration usually does not occur because most plants have their stomata closed. Guttation. 1974). II. This experiment aims to familiarize with the physiological processes involved in water uptake. 1976). breeziness and the available supply of water for the plant (Slavík et al. It also aims to understand the principle involved in the methods used to measure the process. The next experiment dealt with the effect of light and wind on the rate of transpiration.in each shoot was then measured. and high light intensity and moderate wind. The distance it took the air bubble to move within 5 minutes was recorded. The mercury was returned in the test tube and the rate of ascent of mercury in the two set-ups was then computed. an air bubble was introduced into the capillary tube by tapping the end of the bent tube with a finger. The initial height of the mercury was noted and the maximum height of mercury within 30 minutes time was also recorded in both defoliated and intact shoot. The free end of each capillary tube was immersed into their respective test tube with mercury and the set-up was secured in a place against a support. This was then observed under the microscope and the tissues stained were identified and labeled. It was made sure that the capillary tube-rubber tubing assembly was completely filled with water prior to the placement of the shoot. The lifting power of transpiration was measured in this experiment. It was made sure that there is a continuous column of water in the photometer. For the first part. This was done under water. The operation was started by clipping the rubber tubing connected to the reservoir with a pinchcock. Two leafy shoots of kamantigue (Impatiens balsamina) was secured. All the expanded leaves were removed from one of the shoot and a small amount of lanolin paste was applied on the exposed cuts. The basal portion of each shoot was connected separately to a capillary tube-rubber tubing assembly. To initiate measurements. An improvised photometer was set using a leafy shoot of the kamantigue (Impatiens balsamina) plant. The rate of transpiration (volume of water lost per minute) was computed. the Potometer method was used. . With a sharp knife. The other leafy branch was left intact. The bubbles in the capillary tube were removed by releasing water from the reservoir (thristle tube). Three replicate measurements were made for low light intensity and still air. a clean cut was made on the base of both shoots. The base of the shoot was ringed for about 4cm and was inserted in a single holed rubber stopper. The free hand cross-sections of the shoots midway between the bases and the highest point reached by the stain were prepared. the xylem and phloem.7cm Defoliated Intact leaves b. dissolved minerals. The tracheary elements include both tracheids and vessel elements. 2009). The hollow.. which conduct water and nutrients between the various organs.4cm 15. devoid of all protoplasm. When mature and functioning. plants evolved through time and developed vessels to suffice their water and nutrient requirements. Xylem consists of fibers. the areas stained were the xylem and the cortex. both tracheids and vessels form an interconnected network of nonliving cells. tubular nature of these cells together with their extensive interconnections facilitates the rapid and efficient transport of large volumes of water throughout the plant (Hopkins et al. Tracheary elements are the most highly specialized of the xylem cells and are the principal water-conducting cells. small organic molecules upward throughout the plant from the root through the stem to aerial organs (Hopkins et al. The location to which the stain was seen is reasonable since conduction of water and nutrients from the roots to the stem going up is one of the xylem and phloem’s actions. 2009). a. Xylem tissue is responsible for the transport of water. Tissues concerned with water ascent in the stem As a mode of survival. Illustrate and label a cross-section of the stained portion of the stem and explain the above results Cross-section of Impatiens balsamina: Cortex xylem & phloem In the experiment. The tracheary elements are the one responsible for water conduction. As shown in the picture above. the free hand cross-sections of the shoots midway between the bases and the highest point reached by the stain were viewed under the microscope. and on occasion. Results and Discussion A. . parenchyma cells and tracheary elements. Effect of defoliation on the ascent of water in the stem Species used: Impatiens balsamina Length of stained portion (cm) 11. The distinguishing feature of vascular plants is the presence of vascular tissues..III. the larger the driving force for water movement out of the plant. The effect of light and wind on the rate of transpiration Transpiration rate tends to be greatest under conditions of low humidity. Species used: Impatiens balsamina Environmental condition 1 (1) Low light & still air 15mm (2) High light & windy 23mm Distance travelled 2 3 Mean 20mm 18. the driving force for water movement in the xylem is provided by the evaporation of water from the leaf and the tension or negative pressure that results (Hopkins et al. as water is taken up by the plant. a.0033cm/min (1) Defoliated (2) Intact leaves The ascent of xylem sap is explained by combining transpiration with the cohesive forces of water.3cm 0. According to this theory. The drier the atmosphere. then it can receive no more of water. it is possible to measure water uptake (Slavík et al. A bubble is introduced to the capillary..095 ml/h 0. increasing rates of transpiration.57mm Transpiration rate (ml/h) 0.01cm/min 0. By marking regular gradations on the tube. the bubble moves. This theory depends on there being a continuous column of water from the tips of the roots through the stem and into the mesophyll cells of the leaf. The causes of water uptake are photosynthesis and transpiration.7mm 23. The three most prominent are root pressure. Windier conditions increase transpiration because the leaf’s boundary layer is smaller.126 ml/h . The most widely accepted theory for movement of water through plants is known as the cohesion theory.. 2009). The cohesion theory proved to be the working mechanism in this experiment.B. Potometer method A potometer sometimes known as a transpirometer— is a device used for measuring the rate of water uptake of a leafy shoot. then transpiration is more. capillarity and the cohesion theory. When atmosphere is dry. Lifting power of transpiration Species used: Impatiens balsamina Total time of observation: Maximum height (cm) 0.6mm 17. Light levels as low as one thousandth of the sun can cause stomata to open. because it receives water readily but when atmosphere becomes moist or saturated. bright sunlight and moderate winds because during these circumstances the stomates open up and carbon dioxide enters the plant and proceed to photosynthesis. C.87mm 25mm 22. 1974).1cm Rate of ascent (cm/min) 0. this added length will decrease the rate of diffusion and.0 MPa (2) Soil = -0.The transpiration rate for low light and still air is lower as compared to high light and windy air. This is due to the existence of the boundary layer introduced earlier. how would the ascent of water and solute be compared with ascent of mercury? What are the implications of the results of this experiment? The ascent of water will be slower compared to the ascent of mercury. 2009). then leaf. hence the rate of transpiration (Hopkins et al. The water will move from source(root) to sink(leaf). The result presented above can be justified by the fact that the rate of transpiration is affected by light intensity. 3. stem. An increase in light and wind would also mean an increase in transpiration since during these conditions. water vapor molecules exiting the leaf must diffuse not only through the thickness of the epodermal layer. Water moves from area of high potential to low potential.. Given the following water potential values in the different plant parts: (1) Leaf = -2. Study Questions: 1. Before reaching the bulk air. Suppose you have water instead of mercury in the test tube in Procedure B. Soil. the shoot must be cut underwater to ensure that no air or air bubbles enter the stem. but also through the boundary layer. the stomata tends to open. . The rate of transpiration is directly proportional to light and wind.3 MPa (3) Stem = -1. root. 2.0 MPa (4) Root = -0. Why should the shoot be cut underwater (in procedures A & B)? In both procedures A&B.according to Fick’s law. Wind speed has a marked effect on transpiration because it modifies the effective length of the diffusion path for exiting water molecules. The entire set-up to which the experiment was performed measured 4500mm and a 2-ml pipette was used.6 MPa In what order will the water move within the plant body? Give the reason for your answer. It regulates the leaf as well as other parts temperature. . It helps in absorption and distribution of water in plants. DISADVANTAGES Deciduous trees shed their leaves during dry spell to avoid transpiration to protect the plant. It plays an indirect role in the translocation of organic solutes.4. The transpiration pull helps in ascent of sap. Due to this the physiological processes of plants are impaired. Most of the absorbed water is lost without being utilized. If transpiration rates exceed water absorption rates the leaf cells loose turgor and show wilting. Xerophytes show modifications of their leaves and other parts to minimize transpiration. It helps in sending out excessively absorbed water by plants. What are the advantages and disadvantages of transpiration? ADVANTAGES It helps in absorption and transport of mineral salts in plants. .Michael Hogan. William G. Plant Physiology 41 (3): 373–375. 4. New York: Macmillan Publishing Co. Methods of Studying Plant Water Relations. ―Introduction to Plant Physiology‖ (Fourth Edition). Hopkins. J.Huner. (2010). Stomata. 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