Melvy June Balasa
BS 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
accounts for most of the water loss by a plant, but some direct evaporation also takes place through the cuticle of the leaves and young stems. The amount of water given off depends somewhat upon how much water the roots of the plant have absorbed (Martin et al., 1976). Guttation, on the other hand, is the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface. 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 (Goatley et al., 1966). A potometer, or transpirometer, is a device used to measure the rate of transpiration, or the rate of loss of water, from the leaves of a plant. Potometer readings will typically vary according to factors in the environment, such as temperature, light, humidity, breeziness and the available supply of water for the plant (Slavík et al., 1974). 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. Xylem consists of numerous tiny channels which run vertically all the way up the plant. When water reaches the leaves, it evaporates through openings called stomata. As water molecules tend to stick together, this evaporation from the top of the plant exerts an upward pull on the vertical columns of water in the xylem. By setting up a potometer experiment, transpiration rates can be measured when various environmental factors are changed. 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). The potometer measures the rate of uptake of water. To measure transpiration rate directly, rather than the rate of water uptake, utilize a scientific instrument which quantifies water transfer at the leaves (Slavík et al., 1974). This experiment aims to familiarize with the physiological processes involved in water uptake. Also, it aims to learn the factors affecting the rate of transpiration and evaporation by comparison. It also aims to understand the principle involved in the methods used to measure the process.
II.
Materials and Methods
In this part of the experiment, the tissues concerned with water ascent in the stem were studied. Two leafy young shoots of kamantigue ( Impatiens balsamina) were secured. In one of the shoots, all leaves were detached. The bases of the shoots were cut under water and were immersed in separate test tubes which contained 2ml of 0.1% eosin. After 10 minutes, the shoots were removed from the tubes and were cut longitudinally. The length stained by the dye
in each shoot was then measured. The free hand cross-sections of the shoots midway between the bases and the highest point reached by the stain were prepared. This was then observed under the microscope and the tissues stained were identified and labeled. The lifting power of transpiration was measured in this experiment. 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 other leafy branch was left intact. With a sharp knife, a clean cut was made on the base of both shoots. This was done under water. The basal portion of each shoot was connected separately to a capillary tube-rubber tubing assembly. It was made sure that the capillary tube-rubber tubing assembly was completely filled with water prior to the placement of the 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. 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 mercury was returned in the test tube and the rate of ascent of mercury in the two set-ups was then computed. The next experiment dealt with the effect of light and wind on the rate of transpiration. For the first part, the Potometer method was used. An improvised photometer was set using a leafy shoot of the kamantigue (Impatiens balsamina) plant. The base of the shoot was ringed for about 4cm and was inserted in a single holed rubber stopper. It was made sure that there is a continuous column of water in the photometer. The bubbles in the capillary tube were removed by releasing water from the reservoir (thristle tube). To initiate measurements, an air bubble was introduced into the capillary tube by tapping the end of the bent tube with a finger. The operation was started by clipping the rubber tubing connected to the reservoir with a pinchcock. The distance it took the air bubble to move within 5 minutes was recorded. Three replicate measurements were made for low light intensity and still air, and high light intensity and moderate wind. The rate of transpiration (volume of water lost per minute) was computed.
III.
Results and Discussion A. Tissues concerned with water ascent in the stem As a mode of survival, plants evolved through time and developed vessels to suffice their water and nutrient requirements. The tracheary elements are the one responsible for water conduction. The distinguishing feature of vascular plants is the presence of vascular tissues, the xylem and phloem, which conduct water and nutrients between the various organs. Xylem tissue is responsible for the transport of water, dissolved minerals, and on occasion, small organic molecules upward throughout the plant from the root through the stem to aerial organs (Hopkins et al., 2009). Xylem consists of fibers, parenchyma cells and tracheary elements. The tracheary elements include both tracheids and vessel elements. Tracheary elements are the most highly specialized of the xylem cells and are the principal water-conducting cells. When mature and functioning, both tracheids and vessels form an interconnected network of nonliving cells, devoid of all protoplasm. The hollow, 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., 2009). a. Effect of defoliation on the ascent of water in the stem Species used: Impatiens balsamina
Defoliated Intact leaves
Length of stained portion (cm) 11.4cm 15.7cm
b. 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 free hand cross-sections of the shoots midway between the bases and the highest point reached by the stain were viewed under the microscope. As shown in the picture above, the areas stained were the xylem and the cortex. 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.
B. Lifting power of transpiration Species used: Impatiens balsamina Total time of observation: Maximum height (cm) 0.3cm 0.1cm
(1) Defoliated (2) Intact leaves
Rate of ascent (cm/min) 0.01cm/min 0.0033cm/min
The ascent of xylem sap is explained by combining transpiration with the cohesive forces of water. The three most prominent are root pressure, capillarity and the cohesion theory. The most widely accepted theory for movement of water through plants is known as the cohesion theory. 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. According to this theory, 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., 2009). The cohesion theory proved to be the working mechanism in this experiment. C. The effect of light and wind on the rate of transpiration Transpiration rate tends to be greatest under conditions of low humidity, bright sunlight and moderate winds because during these circumstances the stomates open up and carbon dioxide enters the plant and proceed to photosynthesis. Windier conditions increase transpiration because the le af’s boundary layer is smaller. When atmosphere is dry, then transpiration is more, because it receives water readily but when atmosphere becomes moist or saturated, then it can receive no more of water. The drier the atmosphere, the larger the driving force for water movement out of the plant, increasing rates of transpiration. Light levels as low as one thousandth of the sun can cause stomata to open. a. Potometer method A potometer sometimes known as a transpirometer — is a device used for measuring the rate of water uptake of a leafy shoot. The causes of water uptake are photosynthesis and transpiration. A bubble is introduced to the capillary; as water is taken up by the plant, the bubble moves. By marking regular gradations on the tube, it is possible to measure water uptake (Slavík et al., 1974). Species used: Impatiens balsamina Environmental condition (1) Low light & still air (2) High light & windy
1 15mm 23mm
Distance travelled 2 3 Mean 20mm 18.6mm 17.87mm 25mm 22.7mm 23.57mm
Transpiration rate (ml/h) 0.095 ml/h 0.126 ml/h
The transpiration rate for low light and still air is lower as compared to high light and windy air. The entire set-up to which the experiment was performed measured 4500mm and a 2-ml pipette was used. Wind speed has a marked effect on transpiration because it modifies the effective length of the diffusion path for exiting water molecules. This is due to the existence of the boundary layer introduced earlier. Before reaching the bulk air, water vapor molecules exiting the leaf must diffuse not only through the thickness of the epodermal layer, but also through the boundary layer.according to Fick’s law, this added length will decrease the rate of diff usion and, hence the rate of transpiration (Hopkins et al., 2009). The result presented above can be justified by the fact that the rate of transpiration is affected by light intensity. The rate of transpiration is directly proportional to light and wind. An increase in light and wind would also mean an increase in transpiration since during these conditions, the stomata tends to open.
Study Questions: 1. Why should the shoot be cut underwater (in procedures A & B)? -
In both procedures A&B, the shoot must be cut underwater to ensure that no air or air bubbles enter the stem.
2. Suppose you have water instead of mercury in the test tube in Procedure B, how would the ascent of water and solute be compared with ascent of mercury? What are the implications of the results of this experiment? -
3.
The ascent of water will be slower compared to the ascent of mercury.
Given the following water potential values in the different plant parts: (1) Leaf = -2.0 MPa (2) Soil = -0.3 MPa (3) Stem = -1.0 MPa (4) Root = -0.6 MPa In what order will the water move within the plant body? Give the reason for your answer. -
Soil, root, stem, then leaf.
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Water moves from area of high potential to low potential. The water will move from source(root) to sink(leaf).
4. What are the advantages and disadvantages of transpiration? ADVANTAGES
It helps in absorption and transport of mineral salts in plants.
It helps in sending out excessively absorbed water by plants.
It plays an indirect role in the translocation of organic solutes.
It regulates the leaf as well as other parts temperature.
The transpiration pull helps in ascent of sap.
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. Xerophytes show modifications of their leaves and other parts to minimize transpiration.
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. Due to this the physiological processes of plants are impaired.
IV.
Literature cited 1. Benjamin Cummins (2007), Biological Science (3rd ed). 2. Debbie Swarthout and C.Michael Hogan. (2010). Stomata . Encyclopedia of Earth. National Council for Science and the Environment, Washington DC. 3. Martin, J.; Leonard, W.; Stamp, D. (1976). Principles of Field Crop Production (Third Edition), New York: Macmillan Publishing Co., Inc. 4. Goatley, James L.; Lewis, Ralph W. (March 1966). "Composition of Guttation Fluid from Rye, Wheat, and Barley Seedlings". Plant Physiology 41 (3): 373 –375. 5. Slavík, BohdanZ; Jarvis, Margaret Susan (1974). Methods of Studying Plant Water Relations. Taylor & Francis. 6. Hopkins, William G.;Huner, Norman P. A. (2009 ). ―Introduction to Plant Physiology‖ (Fourth Edition), John Wiley & Sons, Inc.
