Biological Science 4th Edition Freeman Instructors Manual
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Chapter 38 – Plant Nutrition
Learning Objectives: Students should be able to ¼
- Describe the array of essential nutrients that plants require to support growth.
- Explain how nutrients are absorbed into the plant.
- Explain the unique association among nitrogen, fungi, and roots.
- Describe a few specialized methods plants have for obtaining nutrients.
- Nutritional Requirements of Plants
- Where does a plant get its mass?
- Jean-Baptiste van Helmont researched this question through an experiment using willow trees. (Fig. 38.1)
- He planted a willow tree sapling in 200 pounds of soil. After 5 years, the plant had gained more than 164 pounds, but the soil had lost less than 1 pound.
- He erroneously concluded that the plant gained its mass from water.
- Further experiments demonstrated that a plant gains some mass from carbon dioxide and some from the soil.
- Which nutrients are essential?
- Essential nutrients are those absolutely required for normal growth and development. (Table 38.1)
- Three criteria are used to define an essential nutrient:
- It is required for growth and reproduction.
- No other element can substitute.
- It is necessary for a specific structure or metabolic function.
- Macronutrients are required in large quantities and are needed for the formation of cell macromolecules.
- Some are obtained from the atmosphere and water, and others are extracted from the soil.
- They include N, P, and K, the most common ingredients in fertilizers.
- Micronutrients are required in small quantities and are usually enzyme cofactors.
- What happens when key nutrients are in short supply?
- Deficiency in an essential nutrient results in abnormal growth. (Fig. 38.2)
- A study was conducted on the effect of a copper deficiency on tomato plants grown hydroponically. (Fig. 38.3)
- One set of tomato seedlings was grown in hydroponic culture with all essential nutrients. The other set was grown with all nutrients except copper.
- Tomatoes that lacked copper had stunted roots and shoots, curled leaves, no flowers, and dark foliage.
- Nutrient deficiency has severe effects on all plant tissues.
- A small amount of the nutrient will cure the deficiency.
- The effects of all essential nutrients have been studied.
- Soil: A Dynamic Mixture of Living and Nonliving Components
- The formation and texture of soil
- The breakdown (weathering) of rock yields particles of different size and composition. (Fig. 38.4)
- Decomposed organic matter is humus.
- Soil eventually becomes a mixture of inorganic and organic particles and living organisms. (Fig. 38.5)
- The texture and chemical composition of soil depend on the parent rock.
- Texture affects oxygen availability and root penetration.
- Chemical composition affects nutrient availability.
- The soil composition determines what plant species are capable of growth.
- The importance of soil conservation
- Soil erosion occurs when soil is carried away from a site by wind or water.
- Erosion can become a problem when an area has become devegetated.
- The Dust Bowl of the 1930s resulted from drought and poor farming practices. (Fig. 38.6a)
- Deforestation in the Dominican Republic and Haiti has resulted in mudslides and flooding. (Fig. 38.6b)
- Sustainable agriculture techniques can prevent or reduce unnecessary erosion:
- Planting rows of trees as windbreaks
- Minimizing tilling and plowing
- Adding organic matter such as manure, and planting over plowed-in crops
- What factors affect nutrient availability?
- Elements are found as ions in soil.
- Anions are negatively charged ions. (Fig. 38.7)
- Anions dissolve in soil water via hydrogen bonds, except phosphate ions.
- They are absorbed by plants but can leach out of soil with rainwater.
- Cations are positively charged ions. (Fig. 38.7)
- Cations bind to negatively charged organic matter and clay.
- They resist leaching but are difficult for plants to absorb because of the strong binding to clay.
- In the hot, wet weather conditions in tropical rain forests, cations can be leached, which leads to a decline in nutrient-containing cations.
- Soil pH affects the availability of nutrients.
- Acidic soil usually has high concentrations of carbonic acid, phosphoric acid, and nitric acid.
(1) The presence of acids in soils can increase the rate at which elemental ions are released from the soil.
(2) Cation exchange occurs when protons or other cations bind to negative charges on soil particles and cause bound cations such as magnesium or calcium to be released. (Fig. 38.8a)
(3) Cation exchange makes the cation nutrients available to nearby plant roots. (Fig. 38.8b)
(4) A higher concentration of protons in the soil speeds the rate of cation exchange and makes cations more available to plants. (Fig. 38.8c)
- Basic or alkaline soils are common where limestone is common.
- Summary: Negative ions stay in solution in soil water, and they are readily available to plants but may wash away easily. Positive ions tend to bind to soil particles but can be released by cation exchange. (Table 38.2)
III. Nutrient Uptake
- The root system is the site of nutrient uptake in most plants.
- The zone of maturation, just above the root tip, is the site of absorption.
- Epidermal cells in the zone of maturation have extensions called root hairs, which increase the surface area of roots for absorption. (Fig. 38.9)
- Root hairs contain many membrane proteins, which selectively facilitate ion passage into cells.
- Mechanisms of nutrient uptake
- Plants establish a proton gradient for nutrient uptake.
- Plants need to take up diffuse nutrients and concentrate them inside the cell.
- Uptake against a concentration gradient requires the expenditure of energy.
- Proton pumps (H+-ATPases) use the energy in ATP to accumulate protons outside of the cell. (Fig. 38.10a)
- This leads to a concentration gradient that favors proton movement into the cell.
- This also leads to a voltage or charge difference across the membrane; the exterior is more positive.
(1) For this reason, plant cells have a membrane potential of about –200 mV.
(2) Therefore, proton pumps create an electrical gradient as well as a concentration gradient, known as an electrochemical gradient.
- The electrical gradient produced by the proton pumps is enough to attract cations into the cell despite their concentration gradients.
- Evidence of active uptake occurs in experiments with radioactive K+.
- When the K+ concentration outside the cell is lower than the concentration inside, uptake into the cell continues. (Fig. 38.10b)
- Further research isolated the sequence for the gene that encodes for a protein that is responsible for the uptake of K+.
- Students should be able to draw a diagram showing how cations are imported into a cell via ion channels. They should also be able to add other diagrams showing what happens if H+-ATPases fail and if a molecule blocks a cation channel.
- Anions enter the cell via cotransporters. (Fig. 38.10c)
- Cotransporters are proteins that facilitate the diffusion of protons into the cell.
- These proteins use the energy released by the diffusion of protons to transport anions into the cell up their concentration gradients.
- Students should be able to draw a diagram showing how anions are imported into a cell via ion channels. They should also be able to add other diagrams showing what happens if H+-ATPases fail and if a molecule blocks an anion cotransporter.
- Mycorrhizae are fungi that live in association with plant roots.
- Ectomycorhizzal fungi (EMF) wrap around the exterior of the roots and radiate into the soil. (Fig. 38.11)
- Arbuscular mycorrhizal fungi (AMF) penetrate into root cells.
- The relationship between mycorrhizae and plant is mutually beneficial.
- Carbon dioxide fixed by plants is transferred to fungi as sugar.
- Nitrogen and phosphorus taken up by fungi are transferred to plant tissues.
- Fungi secrete digestive enzymes that break down dead plant material and make the nutrients available to both fungi and plants.
- Mechanisms of ion exclusion
- Passive exclusion
- The Casparian strip stops the movement of substances into the vascular tissue via the apoplastic pathway, forcing all substances to pass through endodermal cells to reach the vascular tissue.
(1) These cells have specific types of channels and transporters.
(2) This process limits the substances that are allowed into the vascular tissue of the roots. (Fig. 38.12)
- Root hairs also participate in passive exclusion.
(1) If root hairs lack the membrane protein required for a certain ion to enter the cell, the ion will not enter.
(2) Researchers hypothesize that the root hairs of salt-tolerant plants have fewer sodium channels than other plants do.
(3) This hypothesis has yet to be confirmed.
- Active exclusion by metallothioneins
- Many ions, such as copper, are toxic or dangerous to plants.
- One mechanism of neutralizing dangerous ions in plants involves metallothionein proteins that bind to and sequester metal ions.
(1) Natural populations of Arabidopsis thaliana vary in their ability to tolerate high concentrations of copper.
(2) Copper-tolerant A. thaliana individuals produce more metal-binding protein than copper-intolerant individuals do.
- Active exclusion by antiporters
- The tonoplast is the membrane that surrounds the vacuole.
- These transport proteins function as antiporters. (Fig. 38.13)
(1) Antiporters send protons out of the vacuole and bring sodium in.
(2) This may allow plants to survive in salty soil.
- Nitrogen Fixation
- Nitrogen fixation is the conversion of atmospheric nitrogen into compounds that plants can use.
- All living organisms require nitrogen in large quantities for the production of amino acids and nucleic acids.
- Molecular nitrogen, N2, is the most abundant component in the atmosphere; it is very stable and rarely participates in chemical reactions.
- With few exceptions, living organisms are unable to directly utilize N2 from the atmosphere.
- Only several species of bacteria can convert atmospheric N2 into ammonia (NH3), nitrites, or nitrates.
- The role of symbiotic bacteria
- Nitrogen-fixing bacteria in the genus Rhizobium form mutualistic relationships with pea-family plants.
- Bacteria provide plants with ammonia.
- Plants provide protection in root nodules and sugars to support bacterial growth. (Fig. 38.14)
- The relationship is symbiotic because the two different species are living in close association.
- How do nitrogen-fixing bacteria colonize plant roots?
- Colonization of roots by bacteria requires recognition. (Fig. 38.15)
- Roots cells produce flavenoids.
- When rhizobia contact flavenoids, they synthesize sugar-containing Nod factors.
- Nod factors bind to membrane proteins on the root hairs.
- Root hairs curl, and colonization of the root by bacterium proceeds.
- Nutritional Adaptations of Plants
- Epiphytic plants
- Epiphytic plants grow on the leaves or branches of trees and never make contact with soil. (Fig. 38.16a)
- They absorb nutrients from rainwater that collects on plants upon which they grow.
- Some have leaves that grow in rosettes that serve as tanks for holding water. (Fig. 38.16b)
- Parasitic plants
- Parasitic plants represent less than 1% of all plant species described thus far.
- Mistletoe taps into its host vascular tissue. (Fig. 38.17)
- Most parasitic plants are photosynthetic but obtain nutrients and water from the host-plant root system.
- Carnivorous plants
- Carnivorous species make their own carbohydrates via photosynthesis but use carnivory as a way to supplement the nitrogen available in the environment.
- In sundews, modified leaves develop hairs that exude a sticky substance and function like flypaper. (Fig. 38.18)
- Venus flytrap is a photosynthetic plant that grows in bogs with low levels of nitrogen.
- Through natural selection, Venus flytrap leaf has been modified to form a trap, showing phenotypic plasticity.
To emphasize the functional meanings of these terms, the list is organized by topic rather than by first occurrence in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.
zone of maturation
arbuscular mycorrhizal fungi/AMF mutualism
Horticulturist for a Day
Estimated duration of activity: 10–15 minutes
Students may benefit from this practical activity that requires them to link the symptoms of a plant nutritional deficiency and attempt to explain why that deficiency leads to that symptom.
Make index cards with the name of the nutrient on one side and the list of symptoms on the reverse side. Divide the class into small groups or pairs, and give each group a card. Ask students to use the information in Chapter 38 as well as the concepts they have learned about general plant (and eukaryotic cell) physiology to hypothesize why those symptoms occur when the plant is not receiving enough of that nutrient.
Give the students about 10 minutes to come up with their answers. The answers should be relatively basic. Ask each group to present its explanations to the class.
Nitrogen (N): Plant growth is stunted, particularly the growth of lateral shoots, so the plants become spindly. All leaves become light green to yellowish (chlorosis), and older leaves fall off early.
Sample answer: Nitrogen is a necessary component of amino acids and is therefore a prerequisite for new protein synthesis. Lack of nitrogen will mean that the plant cannot make new proteins, including enzymes. This will result in an overall slowing of plant growth. The leaves experience chlorosis because without enzymes chlorophyll cannot be made.
Phosphorus (P): Plant growth is stunted, particularly the growth of lateral shoots, so the plants become spindly. Leaves are dull dark green to bluish-green with purple petioles and veins.
Potassium (K): The leaves show chlorosis (loss of green color) and brown/black spots on their edges. These symptoms appear first on older leaves and then progress to younger leaves as the deficiency becomes more severe.
Calcium (Ca): New growth begins to die back. Leaves show slight chlorosis (loss of green color, or change in color), and new leaves may show brown or black scorch marks.
Magnesium (Mg): Leaves look marbled, since they are experiencing chlorosis (loss of green color) between the leaf veins. These yellow patches start in the center and move to the edges. When the deficiency is severe, black or brown scorch marks appear on the leaves.
Sulfur (S): Older leaves appear yellowish green, and stems become thin and hard. Some plants show orange and red tints rather than yellowing. The stems, though hard and woody, increase in length but not in diameter.
Note—All of the answers are similar to the sample answer provided for nitrogen. Students should realize that, no matter what specific role each nutrient plays, all ultimately allow for normal cellular function; therefore, without them, growth and photosynthesis ultimately suffer.
Debating the Benefits of Genetically Modified Foods
Estimated duration of activity: 15–20 minutes
Protein starvation is the most common type of malnutrition in the world. Agricultural research has brought about hybrid forms of rice, corn, and wheat that have increased amounts of protein. These crops require more nitrogen for growth, which is usually provided via fertilizers. Making these fertilizers is costly and requires large amounts of energy in the form of fossil fuels. One alternative to using fertilizers is enabling plants to fix nitrogen on their own. Researchers have identified the gene for nitrogenase and are working to genetically engineer plants that can make nitrogenase and fix nitrogen on their own. Have students debate the ethics of using such genetically engineered plants to feed starving populations in developing nations. Students will likely need a few days to research the issue on the Internet and decide what position to take. If students decide to argue against the use of genetically modified plants for this application, ask them to come to the debate ready to discuss alternatives.
In-class Challenge Questions
Discussion Ideas for a Class of Any Size
Here are three suggestions for in-class challenge questions.
- What is acid rain, and how does it influence the availability of soil nutrients?
Answer: Acid rain is atmospheric moisture that has a pH considerably lower than that of unpolluted rain. The burning of fossil fuels releases sulfur dioxide and nitrogen oxides, which react with water vapor to form sulfuric and nitric acids that are deposited in rain, snow, and fog. Acid rain can affect plants in several ways—direct damage to exposed plant parts (leaves, flowers, fruits, and so forth) and indirect damage due to the changing availability of soil nutrients. Direct damage to leaves, for example, can occur when acid solutions enter through stomata or thin cuticle and cause decreased photosynthesis and cell death. As the soil acidity (H+ concentration) increases due to acidic rainwater, the exchange of cations (calcium, magnesium, potassium) from soil particles occurs, and they are more readily leached from the soil. Removal of these important mineral nutrients results in plant deficiencies. Furthermore, increased acidity alters the solubility of other soil components (particularly heavy metals such as aluminum, lead, and mercury), increasing their concentration to potentially toxic levels. The effects of altered nutrients in soil are loss or damage to foliage, reduced root growth, death of roots, decrease in mycorrhizae, inhibition of seed germination, and so forth.
Note—This topic could lead to a much broader discussion on the buffering effects of soil ions, and how altering the pH leads to cation exchange and alteration of ion solubility. It could also lead to an even broader discussion about the outcome of acid rain on ecosystems downstream of forests—increased levels of leached cations can lead to eutrophication in lakes and streams, to subsequent effects on algal growth, to declines in fish populations in those bodies of water, and so on.
- The diversity in root systems enables plants living in the same environment to tap different nutrient sources. Plants with shallow, fibrous roots obtain nutrients on the upper surface of the soil; those with deeper taproots can obtain nutrients farther down in the soil layers. How would you determine whether plants draw on different nutrient sources in soil?
Answer: In the laboratory, germinate seeds of plants from the same habitat that have different types of root systems. Grow the seedlings in soil from their native habitat. Inject radioactive phosphorus into the lowest soil layer where roots are growing. Measure the concentration of accumulated isotopes in the different species. Repeat the experiment using different isotopes and injecting them into different soil layers and measuring uptake. This should tell you whether the plants draw on different sources in soil.
- Part 1. Design an experiment to determine the concentrations of nitrogen, magnesium, and calcium in a plant’s leaves, twigs, and wood. Would you expect the concentrations to vary in the different parts of the plant body? Why?
Answer: Obtain a plant to sample, and determine the total dry weight of the intact plant including both root and shoot systems. Take another sample; separate the leaves, twigs, and wood; and determine the percentage of the total dry weight of nitrogen, magnesium, and calcium in each of the three plant parts. This will tell you whether there is a difference in the concentration of these nutrients in different parts of the plant.
Yes, you would expect differences to occur in each of these nutrients because the plant parts have different functions. For example, because leaves carry out photosynthesis and Mg++ is needed as a component of the chlorophyll molecule, you would expect that concentrations of Mg++ would be higher in leaves than in twigs or wood.
Part 2. Using the experimental design you developed in part 1, calculate the concentrations of these nutrients in three different plant species that inhabit the same environment. Then calculate their percentages in the leaves, twigs, and wood. Assume that one is a deciduous canopy tree, one an understory evergreen tree species, and the third a deciduous shrub. Would you expect the nutrient concentrations to vary in the different plant species? Why?
Answer: You would anticipate that the concentrations will vary depending on the type of plant. The canopy tree has a larger mass and would be expected to have a higher concentration of nitrogen. However, the deciduous shrub may be expected to have a higher concentration of Mg++ because it has a high concentration of chlorophyll in its short-lived leaves.
Note—Discussion could lead into topics such as nutrient cycling in ecosystems, competition among organisms for limited resources, the role of fire in succession, and so forth.
- Which of the following nutrients is not obtained from the soil?
Bloom’s Taxonomy: Level 2 Comprehension
- Cation exchange is a process _____.
1a. in which anions are washed away by water moving through the soil
2b. in which protons replace positive ions bound to soil particles making them available for plants
3c. that is more efficient at high pH
4d. that is enhanced by sandy soil
5e. in which negatively charged ions are released from soil particles
Bloom’s Taxonomy: Level 2 Comprehension
- Roots use ATP during nutrient uptake _____.
1a. to pump sodium ions into cells
2b. to pump water across membranes
3c. to take up cations but not anions
4d. to create a high pH in the apoplast
5e. to pump protons out of root cells
Bloom’s Taxonomy: Level 2 Comprehension
- The enzyme nitrogenase _____.
1a. is inactivated by oxygen
2b. is encoded by the genomes of all plants
3c. uses NH3 gas to make N2
4d. is pink in color
5e. is found in bacteria associated with all plants
Bloom’s Taxonomy: Level 2 Comprehension
- Carnivorous plants trap and digest insects and other small animals _____.
1a. in order to acquire rare metal ions
2b. when growing in nutrient-rich soil
3c. in order to supplement their nitrogen nutrition
4d. when photosynthesis is not possible
5e. when they grow epiphytically and have no direct connection with soil
Bloom’s Taxonomy: Level 2 Comprehension