Transpiration Simulation
Climate and the environment often have drastic effects on the behavior and physiology of an organism. Increases in temperature, for example, can raise metabolic rates and affect the course of development. Humidity levels and the availability of water often disturb reproduction and/or the distribution of organisms. In a similar vein, the chemical composition water (pH, salinity, etc) affect development and various aspects of the animal's physiology (look at the effects of acid rain on wildlife, for example). Sometimes two or more environmental properties work together to challenge an animal (as wind speed increases, so do water losses in most terrestrial animals).
Each animal has an ecological minimum and maximum tolerance for temperature, humidity, salinity, and other environmental factors between which they can survive (albeit under stress and for only a short period of time). Thus, although you could tolerate being locked naked in a 0° walk-in freezer for several minutes, it's unlikely you'd survive for very long. Likewise, an animal may get by for short periods near their tolerance limits, but the stress can affect feeding, resistance to disease, and reproduction. Any condition that approaches the limits of an organism's tolerance range is said to be a limiting factor. Over a more narrow range animals have preferred or optimal ranges for temperature, humidity, and so forth. In this laboratory you will explore factors related to humidity and water balance in a variety of animals.
Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants absorb water in the soil through their roots. This soil can originate from deep in the soil (some desert plants have roots that extend 20 meters into the ground). Water is pumped through the plant by the evaporation of water through small pores called "stomata" (Figure 1, 2). A pair of guard cells surround the stoma on the lower epidermis of Tradescantia. The large dark bodies are nuclei. The lighter circular bodies within the guard cells are chloroplasts. In most plants guard cells are the only cells within a leaf's epidermis that contain chloroplasts. The production of sugar by photosynthesis in the daytime within guard cells and not in adjacent epidermal cells causes water to diffuse into the guard cells. This makes the guard cells swell, opening the stoma.
The stomata allow gas exchange between the surrounding air and the photosynthetic cells inside the leaf and are major avenues for the loss of water from the plant by evaporation. In most plants, stomata are more numerous on the bottom surface of the leaf than on the top (Figure 3,4). In figure 3, note the lack of stomata on the upper leaf surface while the lower surface shows numerous stomata (Figure 4). This adaptation minimizes water loss, which occurs more rapidly on the sunny upper side of a leaf.
Adaptations to dry environments include water storage in stems (no desert is completely without rain and cacti store the water when it rains; Figure 5). The leaves, a potential source of water loss, have been transformed into spines. The structure and toughness of the spines greatly cuts back on water loss compared to "normal" leaves and their great number provides some shade to the stem.
Cacti also have an important biochemical adaptation to desert life. Like all plants, cacti must undergo photosynthesis by using carbon dioxide and water, with sunlight as an energy source. Because photosynthesis requires light, it must be done during the day (don't confuse the dark reaction with this; the dark reaction doesn't require light, but it occurs during the day). Since carbon dioxide can only enter the plant through the stomata, opening the pores would result in a considerable amount of water loss during the day. In the desert, water loss can be life-threatening and during the hot daytime hours water loss would be at its worst. Cacti have a method for circumventing this problem. They open their stomata only at night, when air temperatures are much lower and water loss is at a minimum. The carbon dioxide taken in during the night is trapped by a special biochemical pathway and is stored in the form of organic acids. During the daytime cacti keep their stomata tightly closed, but they can still get the carbon dioxide they need from the organic acids produced during the night. The carbon dioxide is released from these organic acids during the day and photosynthesis is thus able to take place. This type of photosynthesis is called crassulacean acid metabolism (CAM). Although this method of photosynthesis is much slower than normal photosynthesis, the water savings more than offset the disadvantages, but cacti pay a price in that they are extremely slow growers.
Some desert plants avoid water loss through their leaves by dropping them during the driest parts of the year (Figure 6). These plants have tiny leaves to minimize water loss, but drop them when conditions get too dry. When it rains, they quickly produce new leaves and sometimes flowers. To further reduce water loss, desert plants have a thick cuticle, often with a waxy outer surface as well as a reduction in the number of stomata.
Physical and anatomical factors that affect the rate of transpiration can be explored with the transpiration model in this exercise (an executable version can be installed from here if your browser is acting up VISTA USERS READ THIS TO RUN THE PROGRAMS). Three physical factors affect water loss: temperature, humidity, and wind speed. Generally, as temperature and/or wind speed increase, so do water loss. Increases in relative humidity decrease water losses. The major anatomical features that affect water loss are the density of stomata on the leaf, as well as how wide the stomata are opened. Anatomical factors not modeled in this simulation include leaf shape, internal leaf structure near the stomata, and the distribution of the pores.
To run the simulation, first choose the parameter that will be varied, followed by values for each of the variables (temperature, humidity, etc., as appropriate; Figure 7). Then press the calculate button to produce a graph of transpiration rate In this example, the temperature was varied from 0 - 50° (along the X-axis) while relative humidity (10%), wind speed (10 m/s), stomata opening (10%) and stomata density (10/mm˛) remained constant. As you can see, transpiration losses increase greatly with increasing temperature.
In figure 8 the relative humidity was increased to 20% and the results were calculated. The red line is the original 10% relative humidity while the green line is for the new value. Up to 5 runs can be displayed for comparison. After five runs the oldest line is replaced with the newest. The graph can be cleared at any time by pressing the clear button.
To generate figure 9 I first ran the simulation with temperature as the varied parameter (red line), followed by relative humidify (green) and so forth to show the effects of each variable on transpiration rate (the color codes are next to the parameters). Note that the most important variable under the experimental conditions shown above is temperature. Note also that the response to changing humidity is linear.
Data can be copied from the built-in data sheet (yellow) to the clipboard by pressing the clipboard button. The data sheet will be automatically cleared. The data can then be pasted into Excel for further analysis. You can also clear the data sheet by pressing the "clear grid" button.
Transpiration Activity
Test the various parameters using extremes in temperature (0°, 25°, 50°), Relative humidity (0%, 50%, 100%), and the others.