· Is water loss through transpiration harmful or beneficial for plants?
· Is there a relationship between transpiration rate and photosynthesis rate?
· If a bright and hot light is directed at a leaf and the stomata open, how could you determine if stomatal opening is due to the plant’s need for CO2 for photosynthesis or the plant’s need to cool itself?
· If you compare the top and bottom surfaces of a leaf, do you expect the top to have more stomata, fewer stomata, or about the same number of stomata?
· Stomata are found not only on leaves, but also on green stems. Would you expect to find them on woody stems, such as a large tree trunk? Why or why not?
Learning objectives for this week’s lab:
(1) Be able to explain transpiration to a fellow student.
(2) Be able to perform a leaf peel and identify stomata, accessory cells, guard cells, and epidermal cells.
(3) Be able to discuss why stomatal densities might differ within a leaf, among leaves from different parts of a large plant, and among leaves found on plant species specialized to different environments.
(4) Be able to generate and quantitatively test hypotheses about how light and leaf temperature influence transpiration rate in plants.
(5) Be able to interpret data from a potometer and explain what they tell us about transpiration rates.
Textbook Reading:
Cambell and Reece, 6th edition, pages 734, Fig. 35.19, 756-764, 825-826
LAB OVERVIEW
PLANT PHYSIOLOGY: TRANSPIRATION
THE BIG PICTURE
In recitation you learned that transpiration is central to the proper functioning and survival of plants. Today in laboratory we explore several important components of transpiration, including relationships between the organization of leaf tissue, stem tissue, and whole-plant water balance.
· reading today’s lab handout and the textbook readings;
· attending recitation lecture and reviewing your recitation notes;
DURING LAB / · Learn the basic structure of angiosperm stems and leaves and complete worksheet 4.
· Perform the Carnation transpiration experiment and record your observations.
· Perform the leaf epidermal imprint activity. Record your data on Worksheet 1 and contribute your data to the class pool.
· Quantify the effects of different variables on transpiration rates complete Worksheet 2.
ASSIGNMENT DUE AT THE BEGINNING OF LAB NEXT WEEK / Complete Worksheets 1 and 2 during lab, but do not turn them in. You will need them so that your group members can collaborate to answer the questions on Worksheet 3. As a group, complete and turn in Worksheet 3.
Individually, you will turn in worksheet 4. Remember to do your own work on this worksheet (and on others unless specifically instructed to work as a group. There have been some issues in the past with students turning in worksheets with the same answers word-for-word. Even if they are not word-for-word the same, this can still be a violation of the honor code. BE CAREFUL and if you have questions, please ask your instructor!!!!)
Angiosperm diversity: MONOCOTS AND DICOTS; work individually and complete worksheet 4
When one examines various flowering plants, they fall into two groups, each with distinctive features: the subclass Dicotyledonae (dicotyledons or dicots) and the subclass Monocotyledonae (monocotyledons or monocots). The following indicates some generalities. (As is usual in biology, there are exceptions.)
MONOCOTS VS DICOTS
Examine three of the plants available in the lab and determine whether they are monocots or dicots. Record your observations on your data sheet.
Angiosperm Stems: work individually and complete worksheet 4
In the lab this week, there will be prepared slides of cross-sections of stems, from both a dicot and a monocot. Examine the various cell and tissue types.
The outermost ring of cells is the epidermis. In the stem, epidermal cells are coated with a waxy cuticle. Inside the epidermis of the dicot stem is the cortex. This tissue consists of large, undifferentiated cells, called parenchyma cells. Some of them may contain chloroplasts, and thick-walled support cells.
Parenchyma cells make up most of the stem tissue and are stained light purple. In a dicot they also form the large, central pith. Now identify the vascular bundles (clusters of xylem and phloem). With toluidine blue, the phloem usually stains reddish-purple and lignin, a carbohydrate that stiffens cell walls and is often found in xylem, stains bluish-green.
Vascular bundles are arranged in different patterns in dicot and monocot stems. On your worksheet, draw an outline of your stem sections showing the epidermis and the arrangement of the vascular bundles for both types of stem. For the dicot stem, also label the cortex and pith. For the monocot you need only draw a representative sample of vascular bundles.
The xylem is most readily identified by the large diameter and thick walls of vessel members. The cells in the phloem are thinner walled, and consist of the sieve-tube members that transport organic compounds and their smaller, companion cells. The vascular bundles of corn are surrounded by a bundle sheath, made of fibers. Long strings of fibers associated with vascular bundles are the source of linen (made from flax, Linum) and natural rope (made from jute or hemp, Cannabis ).
A distinct difference between the dicot and monocot stems is the presence of brick-shaped cells of vascular cambium between the xylem and phloem. This secondary meristematic tissue gives rise to additional xylem and phloem in woody stems. Only dicots have vascular cambium and, therefore, only dicots can undergo lateral (woody) growth of the stem. Notice that the vascular cambium continues between the vascular bundles and encircles the stem.
Angiosperm Leaves: work individually and complete worksheet 4
The leaf is the principal lateral appendage of the stem and the principal organ of photosynthesis. Most leaves consist of a stalk, the petiole, and a broad, expanded part, the blade. Leaves are very diverse, and the structure of a leaf is to a great extent related to habitat and function. There are, however, basic differences between monocot and dicot leaves. Review these differences you learned during last week’s lab.
1. Syringa (lilac) leaf. Spend some time observing the prepared slide of a leaf under low power. It is important that you orient yourself. You will see variation in the size and orientation of the veins, or vascular bundles, which carry xylem and phloem in the leaf. The centrally located, largest vein is the midvein. Smaller veins are embedded in mesophyll, which is photosynthetic. Chloroplasts should be visible in cells in the mesophyll, as well as intercellular spaces. What is the function of these intercellular spaces?
If you examine a portion of the blade that is to one side of the midrib, you should be able to identify the upper epidermis. This has a relatively thin cuticle. Below it, there is a layer of palisade parenchyma. Running through the parenchyma are veins surrounded by rings of cells. Below the palisade parenchyma is spongy parenchyma. The palisade and spongy parenchyma together comprise the mesophyll. One the lower epidermis, you should see stomata, associated with substomatal chambers. What is the function of these chambers? More stomata occur on the lower than the upper surface. Why?
Water is a valuable resource for plants. The growth of plants is more likely to be limited by lack of water than by lack of any other factor — CO2, light, nitrogen or other minerals. Anyone who has neglected a houseplant knows this. Yet transpiration is the only way to lift water from a plant’s roots, and plants are constantly losing water through shoots, seeming to defeat the purpose of a plant’s extensive water-absorbing roots.
Why transpire, and, especially, why transpire so much? If a human being used as much water as a typical vascular plant, she would require consumption of about 40 liters of water every day. No one sweats that much. Think about why there is this difference. Plants need carbon dioxide to carry out photosynthesis in their chloroplasts and oxygen to carry out aerobic respiration in their mitochondria. People need oxygen to carry out aerobic respiration in their mitochondria. Why do plants move so much more water than we do?
This week’s lab focuses on this apparent paradox. Our focus is on individual leaves, but as you do the experiments you should think about transpiration as a normal function of individual plants, and of natural ecosystems, including the global ecosystem. Try to think about transpiration from two different perspectives.
First, explain how transpiration happens and how plants regulate the rate of transpiration.
Second, try to understand why transpiration happens and the manners in which plants (or leaves) acclimate and adapt to different environmental challenges such as high temperature or drought.
How transpiration happens
During the beginning part of the lab, you learned three things about the anatomy of vascular plants. First, leaves possess thick, impermeable cuticles to prevent the loss of water. Second, the inside of a leaf is occupied by intercellular air spaces that take up about 70% of the volume of the leaf. Third, the structures that connect these intercellular air spaces with the dry air outside the leaf are the microscopic openings in the leaf surface, the stomata.
Stomata can be opened and closed through the action of two guard cells, and many different factors regulate this opening or closing.
· A biological clock exists in the guard cells of most plants, with stomata opening in the morning and losing at night, even if the plant is kept in either 24 hours of light or 24 hours of dark.
· When a leaf is exposed to bright light, the blue wavelengths trigger guard cells to rapidly take up potassium ions (K+) through specialized channels in the cell membrane, an active transport process that requires ATP. Water then flows into the guard cell, increasing turgidity so that they stay open. These ions and the water will passively leave the guard cell when this light signal is removed.
· The hormone ABA plays a role in regulating the closure of stomata; this role was discovered by studying mutant plants that lack ABA and cannot close their stomata. In non-mutants, high temperatures or water deficiency in the soil can cause the synthesis of ABA in leaf tissue. ABA alters permeability of plant cell membranes as well as properties of the cell walls of the guard cells.
· Other environmental factors can trigger stomatal opening as well, including low concentrations of CO2 intracellularly, a consequence of high rates of photosynthesis that can rapidly deplete the leaf’s supply. In a complementary fashion, high CO2 concentrations intracellularly, or high temperatures, can promote the stomata to close.
In today’s lab, you will have the opportunity to investigate the role of light, temperature, and ABA in the regulation of transpiration.
Why transpiration happens: pros and cons
As we noted earlier, the downside of transpiration is that it causes plants to lose water. This is clearly a problem when water is scarce and the rate at which a plant transpires exceeds the rate at which roots can absorb water. Over time, this leads to wilting, and eventually plant death. Given this cost of transpiration, why do all plants do it? One answer is that transpiration is an inevitable consequence of the plant’s need to take up CO2 from the atmosphere for photosynthesis. The stomata must be open to permit gas exchange, and transpiration occurs whenever the stomata are open.
Another answer is that transpiration actually helps the plant maintain leaf temperature. As water evaporates, heat dissipates, thereby cooling the leaf. (The same principle is behind sweating in humans.) Imagine how hot leaves could become in the middle of a hot summer day, in the blazing sun, without a cooling system. Finally, transpiration is the only mechanism plants can use to lift water from roots deep in the soil, through the xylem, to branches many meters above the ground.
The take-home message is that there are pros and cons to transpiration. It should therefore not surprise you that plants have evolved a complex set of mechanisms for controlling transpiration, so that they can maximize its benefits and minimize its costs. You should also realize that the regulation of stomata is not the only strategy for regulating water loss. Species adapted to different environmental challenges may have evolved differences in the density of stomata. Other strategies for tolerating water loss include altering leaf position, hairiness on leaves, changes in leaf surface area and thickness, stomata located in chambers sunken inside the leaf surface, shedding of leaves during very dry seasons, or even alternative photosynthetic pathways.
Your textbook provides additional background on this topic:
ü pp. 756-759 (6th ed.) on ascent of water through the xylem
ü Fig. 36.13 (6th ed.) diagrams how guard cells open and close stomata
Objectives
· Demonstrate transport through a xylem in a stem.
· Determine how environmental conditions affect this phenomenon.
Equipment
2 (250 ml) narrow-mouth glass beakers
2 short-stemmed white Carnation flowers