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Chap37 Chap37 Document Transcript

  • 37 Plant Nutrition Food is an essential commodity that separates prosperous nations from struggling ones. For instance, North Korean agriculture met that en- tire country’s food needs until about a decade ago. The country’s farm- ers were highly efficient and productive. Its food crisis began with the collapse of the Soviet Union, which had provided North Korea with chemicals and petroleum. This loss of support was followed by three years of drought, hailstorms, and floods. Today, North Korea is a starving country with a failed farming system. Why should a desperate shortage of chemicals and petroleum affect a nation’s agriculture? Crop production depends on several factors, but the one that is most commonly limiting is a supply of nitrogen in a form usable by plants. All plants re- quire the element nitrogen, which is an abundant component of proteins and nucleic acids as well as chlorophyll and many other important biochemical compounds. If a plant cannot get enough nitrogen, it cannot synthesize these compounds at a rate ad- equate to keep itself healthy. To meet their crops’ need for nitrogen and other miner- als, farmers in all parts of the world apply fertilizers of one kind or another. The in- Nitrogen is Essential for Plant Growth dustrial production of fertilizers is an energy-intensive process, and the energy In this experimental wheat field in needed is most commonly obtained from petroleum. Without petroleum, North Bangladesh, nitrogen was withheld from Korea cannot begin to provide the fertilizer needed to restore its crop production. the plot on the left. The resulting plants were stunted and unhealthy. In addition to nitrogen, plants need other materials from their environment. In this chapter, we will explore the differences be- tween the basic strategies of plants and of an- imals for obtaining nutrition. Then we will look at what nutrients plants require and how they acquire them. Because most nutrients come from the soil, we will discuss the for- mation of soils and the effects of plants on soils. As any farmer can tell you, nitrogen is the nutrient that most often limits plant growth, so we will devote a section specifi- cally to nitrogen metabolism in plants. The chapter concludes with a look at carnivorous and parasitic plants, which supplement their nutrition in special ways. The Acquisition of Nutrients Every living thing must obtain raw materials from its environment. These nutrients include
  • PLANT NUTRITION 717 the major ingredients of macromolecules: carbon, hydrogen, sources that are somehow brought to it. Most sessile animals oxygen, and nitrogen. Carbon and oxygen enter the living depend primarily on the movement of water to bring them world in the form of atmospheric carbon dioxide through the raw materials and energy in the form of food, but a plant’s carbon-fixing reactions of photosynthesis. Hydrogen enters supply of energy arrives at the speed of light from the sun. living systems through the light reactions of photosynthesis, However, with the exception of carbon and oxygen in CO2, which split water. For carbon, oxygen, and hydrogen, pho- a plant’s supply of nutrients is strictly local, and the plant tosynthesis is the gateway to the living world, and these may use up the water and mineral nutrients in its local envi- elements are in plentiful supply. ronment as it develops. How does a plant cope with the In the remainder of this chapter, we shall focus our atten- problem of scarce nutrient supplies? tion on nitrogen, which is in relatively short supply for One way is to extend itself by growing in search of new plants. The movement of nitrogen into organisms begins resources. Growth is a plant’s version of movement. Among with processing by some highly specialized bacteria living in plant organs, the roots obtain most of the mineral nutrients the soil. Some of these bacteria act on nitrogen gas, convert- needed for growth. By growing through the soil, they mine ing it into a form usable by plants. The plants, in turn, pro- the soil for new sources of mineral nutrients and water. The vide organic nitrogen (and carbon) to animals, fungi, and growth of leaves helps a plant secure light and carbon diox- many microorganisms. ide. A plant may compete with other plants for light by out- In addition to nitrogen, other mineral nutrients are es- growing and shading them. sential to living organisms. The proteins of organisms con- As it grows, a plant—or even a single root—must deal with tain sulfur (S), and their nucleic acids contain phosphorus (P). a variable environment. Animal droppings create high local There is magnesium (Mg) in chlorophyll, and iron (Fe) in concentrations of nitrogen. A particle of calcium carbonate in many important compounds, such as the cytochromes. the soil may make a tiny area alkaline, while dead organic mat- Within the soil, these and other minerals dissolve in water, ter may make a nearby area acidic. Such microenvironments forming a solution—called the soil solution—that contacts encourage or discourage the proliferation of a root system. the roots of plants. Plants take up most of these mineral nu- trients from the soil solution in ionic form. Mineral Nutrients Essential to Plants As roots grow through the soil, what important mineral nu- Autotrophs make their own organic compounds trients do plants take up from their environment, and what Plants, some protists, and some bacteria are autotrophs; that are the roles of those nutrients? Table 37.1 lists the mineral is, they make their own organic (carbon-containing) com- nutrients that have been determined to be essential for plants. pounds from simple inorganic nutrients—carbon dioxide, Except for nitrogen, they all derive from rock. All of them are water, nitrogen-containing ions, and a few other soluble min- usually taken up from the soil solution. eral nutrients. The plants provide carbon, oxygen, hydrogen, There are three criteria for calling something an essential nitrogen, and sulfur to most of the rest of the living world. element: Heterotrophs are organisms that require preformed organic The element must be necessary for normal growth and compounds as food. All heterotrophs depend directly or in- reproduction. directly on autotrophs as their source of nutrition. The element cannot be replaceable by another element. Most autotrophs are photosynthesizers—that is, they use The requirement must be direct—that is, not the result of light as their source of energy for synthesizing organic com- an indirect effect, such as the need to relieve toxicity pounds from inorganic raw materials. Some autotrophs, caused by another substance. however, are chemosynthesizers, deriving their energy not from light, but from reduced inorganic substances, such as In this section, we’ll consider the symptoms of particular hydrogen sulfide (H2S), in their environment. All chemosyn- mineral deficiencies, the roles of some of the mineral nutri- thesizers are bacteria. As we’ll see below, some chemosyn- ents, and the technique by which the essential elements for thetic bacteria in the soil contribute to the nutrition of plants plants were identified. by increasing the availability of nitrogen and sulfur. There are two categories of essential elements: macronu- trients and micronutrients (see Table 37.1). How does a stationary organism find nutrients? Plants need macronutrients in concentrations of at least Many heterotrophs can move from place to place to find the 1 gram per kilogram of their dry matter.* nutrients they need. An organism that cannot move, termed *Dry matter, or dry weight, is what remains after all the water has been a sessile organism, must obtain nutrients and energy from removed from a plant tissue sample.
  • 718 CHAPTER THIRT Y-SEVEN 37.1 Mineral Elements Required by Plants ELEMENT ABSORBED FORM MAJOR FUNCTIONS Macronutrients Nitrogen (N) NO3– and NH4+ In proteins, nucleic acids, etc. Phosphorus (P) H2PO4– and HPO4 2– In nucleic acids, ATP, phospholipids, etc. + Potassium (K) K Enzyme activation; water balance; ion balance; stomatal opening 2– Sulfur (S) SO 4 In proteins and coenzymes 2+ Calcium (Ca) Ca Affects the cytoskeleton, membranes, and many enzymes; second messenger Magnesium (Mg) Mg2+ In chlorophyll; required by many enzymes; stabilizes ribosomes Micronutrients Iron (Fe) Fe2+ In active site of many redox enzymes and electron carriers; chlorophyll synthesis Chlorine (Cl) Cl– Photosynthesis; ion balance Manganese (Mn) Mn2+ Activation of many enzymes Boron (B) B(OH)3 Possibly carbohydrate transport (poorly understood) Zinc (Zn) Zn2+ Enzyme activation; auxin synthesis Copper (Cu) Cu2+ In active site of many redox enzymes and electron carriers Nickel (Ni) Ni2+ Activation of one enzyme 2– Molybdenum (Mo) MoO4 Nitrate reduction Nitrogen deficiency is not the only cause of chlorosis. In- Plants need micronutrients in concentrations of less adequate iron in the soil can also cause chlorosis because than 100 milligrams per kilogram of their dry matter. iron, although it is not contained in the chlorophyll molecule, These two categories differ only with regard to the amounts is required for chlorophyll synthesis. However, iron defi- required by plants. Both the macronutrients and the mi- ciency commonly causes chlorosis of the youngest leaves, cronutrients are essential for the plant to complete its life cy- with their veins sometimes remaining green. The reason for cle from seed to seed. How do we know if a plant is getting this difference is that nitrogen is readily translocated in the enough of a particular nutrient? plant and can be redistributed from older tissues to younger tissues to favor their growth. Iron, on the other hand, cannot Deficiency symptoms reveal inadequate nutrition Before a plant that is deficient in an essential element dies, it usually displays characteristic deficiency symptoms, such as 37.2 Some Mineral Deficiencies in Plants discoloration or deformation of its leaves. Table 37.2 describes DEFICIENCY SYMPTOMS the symptoms of some common mineral deficiencies. Such symptoms help horticulturists diagnose mineral nutrient de- Calcium Growing points die back; young leaves are yellow and crinkly ficiencies in plants. With proper diagnosis, appropriate treat- Iron Young leaves are white or yellow with green ment can be applied in the form of a fertilizer (an added veins source of mineral nutrients). Magnesium Older leaves have yellow in stripes between Nitrogen deficiency is the most common mineral defi- veins ciency in both natural and agricultural environments. Plants Manganese Younger leaves are pale with stripes of dead in natural environments are almost always deficient in nitro- patches gen, but they seldom display deficiency symptoms. Instead, Nitrogen Oldest leaves turn yellow and die pre- their growth slows to match the available supply of nitrogen. maturely; plant is stunted Crop plants, on the other hand, show deficiency symptoms if Phosphorus Plant is dark green with purple veins a formerly abundant supply of nitrogen runs out. The visible and is stunted symptoms of nitrogen deficiency include uniform yellowing, Potassium Older leaves have dead edges or chlorosis, of older leaves. Chlorophyll, which is responsible Sulfur Young leaves are yellow to white with yellow veins for the green color of leaves, contains nitrogen. Without ni- trogen there is no chlorophyll, and without chlorophyll, the Zinc Young leaves are abnormally small; older leaves have many dead spots yellow carotenoid pigments in the leaves become visible.
  • EXPERIMENT Question: Is a particular ingredient of a growth medium an essential plant nutrient? be readily redistributed. Younger tissues that are actively growing and synthesizing compounds needed for their METHOD Grow seedlings in a medium that lacks the element in question (in this case, nitrogen) growth show iron deficiency before older leaves, which have already completed their growth. Several essential elements fulfill multiple roles Essential elements may play several different roles in plant Seedling grown Seedling grown cells—some structural, others catalytic. Magnesium, as we in a complete in a medium growth medium. lacking nitrogen. have mentioned, is a constituent of the chlorophyll molecule and hence is essential to photosynthesis. It is also required as a cofactor by numerous enzymes involved in cellular respi- ration and other metabolic pathways. RESULTS Phosphorus, usually in phosphate groups, is found in many organic compounds, particularly in nucleic acids and in the intermediates of the energy-harvesting pathways of photosynthesis and glycolysis. As we saw in Chapter 7, the transfer of phosphate groups occurs in many energy-storing and energy-releasing reactions, notably those that use or pro- duce ATP. The addition or removal of phosphate groups is Growth is normal. Growth is abnormal. also used to activate or inactivate enzymes. Calcium plays many roles in plants. Its function in the pro- Conclusion: Nitrogen is an essential plant nutrient. cessing of hormonal and environmental cues is a subject of great biological interest, as we’ll see in the next chapter. Cal- 37.1 Identifying Essential Elements for Plants This dia- cium also affects membranes and cytoskeletal activity, partic- gram shows the procedure for identifying nutrients essential to plants, using nitrogen as an example. ipates in spindle formation for mitosis and meiosis, and is a constituent of the middle lamella of cell walls. Other elements, such as iron and potassium, also play multiple roles in plants. All of these elements are essential to the life of all plants. that they provided micronutrients that the investigators How did biologists discover which elements are essential? thought they had excluded. Furthermore, because some mi- cronutrients are required in such tiny amounts, there may be enough in a seed to supply the embryo and the resultant sec- Experiments were designed to identify ond-generation plant throughout its lifetime and leave essential elements enough in the next seed to get the third generation well An element is considered essential to plants if a plant fails to started. Indeed, simply touching a plant may give it a signif- complete its life cycle, or grows abnormally, when that element icant supply of chlorine in the form of chloride ions from is not available, or is not available in sufficient quantities. The sweat. Such difficulties make it necessary to perform nutri- essential elements for plants were identified by growing plants tion experiments in tightly controlled laboratories with spe- hydroponically—that is, with their roots suspended in nutrient cial air filters (to exclude microscopic salt particles in the air) solutions without soil (Figure 37.1). In the first successful ex- and to use only chemicals that had been purified to the high- periments of this type, performed a century and a half ago, est degree attainable by modern chemistry. Only rarely are plants grew seemingly normally in solutions containing only new essential elements reported now. Either the list is nearly calcium nitrate, magnesium sulfate, and potassium phosphate. complete, or perhaps, we will need more sophisticated tech- Omission of any of these compounds made the solution inca- niques to add to it. pable of supporting normal growth. Tests with other com- Where does the plant find its essential mineral nutrients? pounds including these elements soon established the six How does it absorb them? macronutrients— calcium, nitrogen, magnesium, sulfur, potas- sium, and phosphorus—as essential elements. Identifying essential elements by this experimental ap- Soils and Plants proach proved to be a more difficult task in the case of the Most terrestrial plants live their lives anchored to the soil. Of micronutrients. In the nineteenth-century experiments on course, soils offer mechanical support for growing plants, but plant nutrition, some of the chemicals used were so impure there are many other plant-soil interactions, some of which
  • 720 CHAPTER THIRT Y-SEVEN are much more complex. Plants obtain their mineral nutri- ents from the soil solution. Water for terrestrial plants also A horizon comes from the soil, as does the supply of oxygen for the Topsoil roots. Soil harbors bacteria, some of which are beneficial to plant life. Soils may also contain organisms harmful to plants. In this section, we will examine the composition, structure, B horizon and formation of soils. We will consider their role in plant nu- Subsoil trition, their care and supplementation in agriculture, and their modification by the plants that grow in them. Soils are complex in structure Soils are complex systems made up of living and nonliving C horizon Weathering components. The living components include plant roots as parent rock well as populations of bacteria, fungi, protists, and animals (bedrock) such as earthworms and insects (Figure 37.2). The nonliving portion of the soil includes rock fragments ranging in size from large rocks through sand and silt and finally to tiny par- ticles called clay that are 2 µm or less in diameter. Soil also contains water and dissolved mineral nutrients, air spaces, 37.3 A Soil Profile The A, B, and C horizons can sometimes be seen and dead organic matter. The air spaces are crucial sources in road cuts such as this one in Australia. The dark upper layer (the A of oxygen (in the form of O2) for plant roots. The character- horizon) is home to most of the living organisms in the soil. istics of soils are not static. Soils change constantly through natural phenomena such as rain, temperature extremes, and the activities of plants and animals, as well as human activi- horizons, lying on top of one another. Mineral nutrients tend ties—agriculture in particular. to be leached from the upper horizons—dissolved in rain or The structure of many soils changes with depth, revealing irrigation water and carried to deeper horizons, where they a soil profile. Although soils differ greatly, almost all soils con- are unavailable to plant roots. sist of two or more recognizable horizontal layers, called Soil scientists recognize three major horizons (A, B, and C) in the profile of a typical soil (Figure 37.3). Topsoil is the A horizon, from which mineral nutrients may be depleted by leaching. Most of the dead and decaying organic matter in the soil is in the A horizon, as are most plant roots, earth- Organic matter (from plants, worms, insects, nematodes, and microorganisms. Successful animals, and fungi) agriculture depends on the presence of a suitable A horizon. Topsoils are composed of different proportions of sand, silt, and clay. In pure sand there are abundant air spaces be- tween the relatively large particles, but sand is low in water and mineral nutrients. Clay contains many mineral nutrients and more water than sand does, but the tiny clay particles Air pack tightly together, leaving little space to trap air. A little Bacteria bit of clay goes a long way in affecting soil properties. A loam is a soil that has significant amounts of sand, silt, and clay, and thus has sufficient levels of air, water, and nutrients for Quartz Air plants. Loams also contain organic matter. Most of the best Air H2O topsoils for agriculture are loams. Air and water Below the A horizon is the B horizon, or subsoil, which is the zone of infiltration and accumulation of materials leached Aggregates of from above. Farther down, the C horizon is the parent rock clay particles 25 µm that is breaking down to form soil. Some deep-growing roots 37.2 The Complexity of Soil Even a tiny crumb of soil has both extend into the B horizon to obtain water and nutrients, but organic and inorganic components. roots rarely enter the C horizon.
  • PLANT NUTRITION 721 A clay particle, which is negatively Soils form through the weathering of rock charged, binds cations. The type of soil in a given area depends on the type of par- Root hair ent rock from which it formed, the climate, the landscape features, the organisms living there, and the length of time H+ that soil-forming processes have been acting (sometimes mil- lions of years). Rocks are broken down into soil in part by K+ mechanical weathering, which is the physical breakdown— without any accompanying chemical changes—of materials H+ – – – – – Clay – by wetting, drying, and freezing. The most important parts –– – – – H+ of soil formation, however, include chemical weathering, the chemical alteration of at least some of the materials in the rocks. CO2 + H2O H2CO3 HCO3– + H+ Both the physical and chemical properties of soils depend on the amounts and kinds of clay particles they contain. These tiny particles, which bind mineral nutrients and ag- The cations are exchanged for hydrogen ions obtained from carbonic acid (H2CO3 ) or from the plant itself. gregate into larger particles, are extremely important to plant growth. Clay is not produced merely by the mechanical 37.4 Ion Exchange Plants obtain mineral nutrients from the soil grinding up of rocks. In addition to mechanical weathering, primarily in the form of positive ions; potassium is the example shown here. several types of chemical weathering are required: Oxidation by atmospheric oxygen makes some essential elements more available to plants. Reaction with water (hydrolysis) releases some mineral plant growth, called soil fertility, is determined in part by its nutrients from the rock. ability to provide nutrients in this manner. Acids, carbonic acid in particular, free some essential ele- Clay particles effectively hold and exchange cations, and ments from their parent salts. cations tend to be retained in the A horizon. However, there These reactions leave the surface of clay particles with an is no comparable mechanism for exchanging anions, the abundance of negatively charged chemical groups, to which negatively charged ions. As a result, important anions such certain mineral nutrients bind. Let’s see how roots take up as nitrate (NO3–) and sulfate (SO42–)—the primary and direct these mineral nutrients from clay particles. sources of nitrogen and sulfur, respectively—leach rapidly from the A horizon. As a consequence of this leaching, the primary soil reservoir of nitrogen is not in the form of nitrate Soils are the source of plant nutrition ions. Most of the nitrogen in the A horizon is found in the The availability of mineral nutrients to plant roots depends organic matter in the soil, which slowly decomposes to re- on the presence of clay particles in the soil. The negatively lease nitrogen in a form that can be absorbed and used by charged clay particles bind the cations of many minerals plants. that are important for plant nutrition, such as potassium (K+), magnesium (Mg2+), and calcium (Ca2+). To become available to plants, these cations must be detached from the Fertilizers and lime are used in agriculture clay particles. Agricultural soils often require fertilizers because irrigation This task is accomplished by reactions with protons (hy- and rainwater leach mineral nutrients from the soil and be- drogen ions, H+). These protons are released into the soil by cause the harvesting of crops removes the nutrients that the roots, which also release CO2 through cellular respiration. plants took up from the soil during their growth. Crop yields The CO2 dissolves in the soil water and reacts with it to form decrease if any essential element is depleted. Mineral nutri- carbonic acid, which then ionizes to form bicarbonate and ents may be replaced by organic fertilizers, such as rotted free protons (CO2 + H2O ~ H2CO3 ~ H+ + HCO3–). These manure, or by inorganic fertilizers of various types. protons bind more strongly to the clay particles than do the mineral cations, so they trade places with the cations in a ORGANIC AND INORGANIC FERTILIZERS. The three elements process called ion exchange (Figure 37.4). Ion exchange puts most commonly added to agricultural soils are nitrogen important cations back into the soil solution, from which they (N), phosphorus (P), and potassium (K). Commercial fertil- are taken up by the roots. The capacity of a soil to support izers are characterized by their “N-P-K” percentages. A
  • 722 CHAPTER THIRT Y-SEVEN 5-10-10 fertilizer, for example, contains 5 percent nitrogen, Plants affect soil fertility and pH 10 percent phosphate (P2O5), and 10 percent potash (K2O) The soil that forms in a particular place depends on the types by weight.* Sulfur, in the form of a sulfate, is also occasion- of plants growing there. Plant litter, such as dead fallen ally added to soils. leaves, is the major source of the carbon-rich materials that Either organic or inorganic fertilizers can provide the nec- break down to form humus—dark-colored organic material, essary mineral nutrients for plants. Organic fertilizers release each particle of which is too small to be recognizable with the nutrients slowly, which results in less leaching than occurs naked eye. Soil bacteria and fungi produce humus by break- with a one-time application of an inorganic fertilizer. How- ing down plant litter, animal feces, dead organisms, and ever, the nutrients from organic fertilizers are not immedi- other organic material. Humus is rich in mineral nutrients, ately available to plants. Organic fertilizers also contain especially nitrogen that was excreted by animals. In combi- residues of plant or animal materials that improve the struc- nation with clay, humus favors plant growth by trapping ture of the soil, providing spaces for air movement, root supplies of water and oxygen for absorption by roots. Look- growth, and drainage. Inorganic fertilizers, on the other ing at the big picture, we see that successful plant growth can hand, provide a supply of soil nutrients that is almost im- create conditions that support further plant growth. mediately available for absorption. Furthermore, inorganic Plants also affect the pH of the soil in which they grow. fertilizers can be formulated to meet the requirements of a Roots maintain a balance of electric charges. If they absorb particular soil and a particular crop. more cations than anions, they excrete H+ ions, thus lower- ing the soil pH. If they absorb more anions than cations, they pH EFFECTS ON NUTRIENTS. The availability of nutrient ions, excrete OH– ions or HCO3– ions, raising the soil pH. whether they are naturally present in the soil or added as The mineral nutrient most commonly in short supply, in fertilizer, is altered by changes in soil pH. The optimal soil both natural and agricultural situations, is nitrogen, despite pH for most crops is about 6.5, but so-called acid-loving the fact that elemental nitrogen makes up almost four-fifths crops such as blueberries prefer a pH closer to 4. Rainfall of Earth’s atmosphere. What is the reason for this scarcity? and the decomposition of organic substances lower the pH Let’s consider how nitrogen is made available to plants. of the soil. Such acidification can be reversed by liming— the application of compounds commonly known as lime, such as calcium carbonate, calcium hydroxide, or magne- Nitrogen Fixation sium carbonate. The addition of these compounds leads to The Earth’s atmosphere is a vast reservoir of nitrogen in the the removal of H+ ions from the soil. Liming also increases form of nitrogen gas (N2). However, plants cannot use N2 di- the availability of calcium to plants. rectly as a nutrient. It is a highly unreactive substance—the Sometimes, on the other hand, a soil is not acidic enough. triple bond linking the two nitrogen atoms is extremely sta- In this case, sulfur can be added in the form of elemental sul- ble, and a great deal of energy is required to break it. How, fur, which soil bacteria convert to sulfuric acid. Iron and some then, is nitrogen made available for the synthesis of proteins other elements are more available to plants at a slightly acidic and nucleic acids? pH. Soil pH testing is useful for home gardens and lawns as well as for agriculture. The test results indicate what amend- ments should be made to the soil. SPRAY APPLICATION OF NUTRIENTS. Spraying leaves with a nutrient solution is another effective way to deliver some essential elements to growing plants. Plants take up more copper, iron, and manganese when these elements are applied as foliar (leaf) sprays than when they are added to the soil. Such foliar application of mineral nutrients is increasingly used in wheat production, but fertilizers are still delivered most commonly by way of the soil. The relationship between plants and soils is not a one-way affair—soils affect plants, but plants also affect soils. *The analysis is by weight of the nutrient-containing compound and not as weights of the elements N, P, and K. A 5-10-10 fertilizer actually does contain 5 percent nitrogen, but only 4.3 percent phosphorus and 8.3 per- 37.5 Root Nodules Large, round nodules are visible in the root cent potassium on an elemental basis. system of a pea plant. These nodules house nitrogen-fixing bacteria.
  • PLANT NUTRITION 723 A few species of bacteria have an enzyme that enables Bacteria of the genus Rhizobium fix nitrogen only in close, them to convert N2 into a more reactive and biologically use- mutualistic association with the roots of plants in the legume ful form by a process called nitrogen fixation. These prokary- family. The legumes include peas, soybeans, clover, alfalfa, otic organisms—nitrogen fixers—convert N2 to ammonia and many tropical shrubs and trees. The bacteria infect the (NH3). There are relatively few species of nitrogen fixers, and plant’s roots, and the roots develop nodules in response to their biomass is small relative to the mass of other organisms their presence. The various species of Rhizobium show a high that depend on them for survival on Earth. This talented specificity for the species of legume they infect. Farmers and group of prokaryotes is just as essential to the biosphere as gardeners coat legume seeds with Rhizobium to make sure the are the photosynthetic autotrophs. bacteria are present. Some farmers alternate their crops, planting clover or alfalfa occasionally to increase the avail- able nitrogen content of the soil. Nitrogen fixers make all other life possible The legume–Rhizobium association is not the only bacterial By far the greatest share of total world nitrogen fixation is association that fixes nitrogen. Some cyanobacteria fix nitro- performed biologically by nitrogen-fixing prokaryotes, which gen in association with fungi in lichens or with ferns, cycads, fix approximately 170 million Mg (megagrams or metric or nontracheophytes. Rice farmers can increase crop yields tons) of nitrogen per year. About 80 million Mg is fixed in- by growing the water fern Azolla, with its symbiotic nitrogen- dustrially by humans. A smaller amount of nitrogen is fixed fixing cyanobacterium, in the flooded fields where rice is in the atmosphere by nonbiological means such as lightning, grown. Another group of bacteria, the filamentous actino- volcanic eruptions, and forest fires. Rain brings these atmos- mycetes, fix nitrogen in association with root nodules on pherically formed products to the ground. woody species such as alder and mountain lilacs. Several groups of bacteria fix nitrogen. In the oceans, vari- How does biological nitrogen fixation work? In the four ous photosynthetic bacteria, including cyanobacteria, fix nitro- sections that follow, we’ll consider the role of the enzyme ni- gen. In fresh water, cyanobacteria are the principal nitrogen fix- trogenase, the mutualistic collaboration of plant and bacter- ers. On land, free-living soil bacteria make some contribution ial cells in root nodules, the need to supplement biological to nitrogen fixation, but they fix only what they need for their nitrogen fixation in agriculture, and the contributions of own use and release the fixed nitrogen only when they die. plants and bacteria to the global nitrogen cycle. Other nitrogen-fixing bacteria live in close association with plant roots (Figure 37.5). They release up to 90 percent of the nitrogen they fix to the plant and excrete some amino acids Nitrogenase catalyzes nitrogen fixation into the soil, making nitrogen immediately available to other Nitrogen fixation is the reduction of nitrogen gas. It proceeds organisms. The plant obtains fixed nitrogen from the bac- by the stepwise addition of three pairs of hydrogen atoms to terium, and the bacterium obtains the products of photosyn- N2 (Figure 37.6). In addition to N2, these reactions require thesis from the plant. Such associations are excellent exam- three things: ples of mutualism, an interaction between two species in which both species benefit. They are also examples of sym- biosis, in which two different species live in physical contact 37.6 Nitrogenase Fixes Nitrogen Throughout the chemical reac- tions of nitrogen fixation, the reactants are bound to the enzyme for a significant portion of their life cycles. nitrogenase. A reducing agent transfers hydrogen atoms to nitrogen, and eventually the final product—ammonia—is released. 2 A reducing agent transfers 3 The final products—two molecules three successive pairs of of ammonia—are released, freeing 1 The enzyme nitrogenase binds the nitrogenase to bind another N2 hydrogen atoms to N2. a molecule of nitrogen gas. molecule. Substrate: + 2H + 2H + 2H H H H H Nitrogen gas (N2) N H N H N N H H H H H H H H Product: N N N N H N N H H N N H Ammonia (NH3) Reduction Reduction Reduction Nitrogenase Nitrogenase Binding of substrate
  • Root hairs Cortical cells Root hair Rhizobia 1 Root hairs release chemical signals that attract Rhizobium. Infection thread 2 Rhizobium proliferates and causes an infection thread to form. Root tip 3 The infection thread grows into the cortex of the root. 4 The infection thread releases bacterial cells, which become bacteroids in the root cells. Nod factors from bacteria cause cortical cells to divide. Nodule Bacteroids in Uninfected cell infected cell Nodule 37.7 A Nodule Forms Rhizobium develops the ability to fix nitrogen only after entering a legume root. The diagrams show the sequence of events in nodule formation. The photograph shows bacteroids of Rhizobium japonicum in vesicles within a soybean root cell. A portion of an uninfected root cell is seen on the right. Bacteroids 5 The nodule forms from rapidly dividing, infected cortical cells. a strong reducing agent to transfer hydrogen atoms to protein leghemoglobin in the cytoplasm of the nodule cells. N2 and to the intermediate products of the reaction Leghemoglobin is a close relative of hemoglobin, the oxy- a great deal of energy, which is supplied by ATP gen-carrying pigment of animals. Some plant nodules con- the enzyme nitrogenase, which catalyzes the reaction tain enough of it to be bright pink when viewed in cross sec- (Depending on the species of nitrogen fixer, either respiration tion. Leghemoglobin, with its iron-containing heme groups, or photosynthesis may provide both the necessary reducing transports enough oxygen to the bacteroids to support their agent and ATP.) respiration. Nitrogenase is so strongly inhibited by oxygen that its presence in biochemical extracts was obscured and its dis- covery delayed because investigators had not thought to seek Some plants and bacteria work together to fix nitrogen it under anaerobic conditions. It is therefore not surprising Neither free-living Rhizobium species nor uninfected legumes that many nitrogen fixers are anaerobes and live in environ- can fix nitrogen. Only when the two are closely associated in ments with little or no O2. Because this crucial enzyme is so root nodules does the reaction take place. The establishment inhibited by O2, it was at first surprising that legumes respire of this symbiosis between Rhizobium and a legume requires aerobically, as do Rhizobium. Investigation of the root nodules a complex series of steps, with active contributions by both where nitrogenase is found revealed how the enzyme could the bacteria and the plant root (Figure 37.7). First the root re- operate there. leases flavonoids and other chemical signals that attract soil- Within a root nodule, O2 is maintained at a low level suf- living Rhizobium to the vicinity of the root. Flavonoids trig- ficient to support respiration, but not so high as to inactivate ger the transcription of bacterial nod genes, which encode nitrogenase. The plant makes this possible by producing the Nod (nodulation) factors. These factors, secreted by the bac-
  • PLANT NUTRITION 725 teria, cause cells in the root cortex to divide, leading to the Plants and bacteria participate in the global formation of a primary nodule meristem. The meristem gives nitrogen cycle rise to the plant tissue that constitutes the nodule. The nitrogen released into the soil by nitrogen fixers is pri- Among the products of the meristem is a layer of cells that marily in the form of ammonia (NH3) and ammonium ions excludes O2 from the interior of the nodule. The function of (NH4+). Although ammonia is toxic to plants, ammonium leghemoglobin is to carry O2 across this barrier. Within a nod- ions can be taken up safely at low concentrations. Soil bacte- ule, the bacteria take the form of bacteroids within membra- ria called nitrifiers, which we described in Chapter 27, oxidize nous vesicles. Bacteroids are swollen, deformed bacteria that ammonia to nitrate ions (NO3–)—another form that plants can fix nitrogen—in effect, nitrogen-fixing organelles. can take up—by the process of nitrification (Figure 37.8). Soil The partnership between bacterium and plant in nitrogen- pH affects the uptake of nitrogen: Nitrate ions are taken up fixing nodules is not the only case in which plants depend on preferentially under more acidic conditions, ammonium ions other organisms for assistance with their nutrition. Another under more basic ones. example is that of mycorrhizae, root–fungus associations in The steps that we have followed so far are carried out by which the fungus greatly increases the absorption of water bacteria: N2 is reduced to ammonia in nitrogen fixation and and minerals (especially phosphorus) by the plant (see Fig- ammonia is oxidized to nitrate in nitrification. The next steps ure 31.16). A growing body of evidence suggests that nodule are carried out by plants, which reduce the nitrate they have formation depends on some of the same genes and mecha- taken up all the way back to ammonia. All the reactions of nisms that allow mycorrhizae to develop. nitrate reduction are carried out by the plant’s own en- zymes. The later steps, from nitrite (NO2–) to ammonia, take place in the chloroplasts, but this conversion is not part of Biological nitrogen fixation does not always meet photosynthesis. The plant uses the ammonia thus formed to agricultural needs Bacterial nitrogen fixation is not sufficient to support the needs of agriculture. Traditional farmers used to plant dead 37.8 The Nitrogen Cycle Nitrogen fixation, nitrification, fish along with corn so that the decaying fish would release nitrate reduction, and denitrification are the components of an essential chemical cycle that converts atmospheric nitrogen gas fixed nitrogen that the developing corn could use. Industrial into ammonium ions and nitrate ions—forms of nitrogen that nitrogen fixation is becoming ever more can be taken up by plants—and returns N2 to the atmosphere. important to world agriculture because of the degradation of soils and the need Some denitrifying bacteria can oxidize ammonia back to feed a rapidly expanding population. N2 to nitrogen gas, which Most industrial nitrogen fixation is returns to the atmosphere. done by a chemical process called the Haber process, which requires a great DENITRIFICATION deal of energy. An alternative is urgently needed because of the rising cost of en- Plants reduce ergy. At present, in the United States, the nitrate ions back to manufacture of nitrogen-containing fer- ammonia, the form in which nitrogen NH4+ tilizer takes more energy than does any is incorporated Re other aspect of crop production. In bio- into proteins. cy Nitrogen-fixing NITROGEN c li logical systems, nitrogen fixation re- NITRATE ng bacteria FIXATION to s REDUCTION o il quires a great deal of ATP. Research on biological nitrogen fixa- NH3 NH4+ tion is being vigorously pursued, with NO3– commercial applications very much in Denitrifying Bacteria fix N2 from bacteria the atmosphere mind. One line of investigation centers Oxidation producing ammonia on recombinant DNA technology as a and ammonium ions. NO2– means of engineering new plant–bac- Nitrobacter Nitrosomonas, terium associations that produce their NITRIFICATION Nitrosococcus own nitrogenase. Currently there are at- tempts to transfer genes from Rhizobium into bacteria that already live in the Nitrifying bacteria oxidize ammonia to nitrate ions. roots of cereal plants.
  • 726 CHAPTER THIRT Y-SEVEN manufacture amino acids, from which the plant’s proteins The Venus flytraps have specialized leaves with two and all its other nitrogen-containing compounds are formed. halves that fold together. When an insect trips trigger hairs Animals cannot reduce nitrogen, and they depend on plants on a leaf, its two halves come together, their spiny margins to supply them with reduced nitrogenous compounds. interlocking and imprisoning the insect. The leaf then se- Bacteria called denitrifiers return nitrogen from animal cretes enzymes that digest its prey. The leaf absorbs the wastes and dead organisms to the atmosphere as N2. This products of digestion, especially amino acids, and uses them process is called denitrification (see Chapter 27). In combi- as a nutritional supplement. nation with leaching and the removal of crops, denitrifica- Pitcher plants produce pitcher-shaped leaves that collect tion keeps the level of available nitrogen in soils low. small amounts of rainwater. Insects are attracted into the This global nitrogen cycle is complex. It is also essential for pitchers either by bright colors or by scent and are prevented life on Earth: Nitrogen-containing compounds constitute 5 to from getting out again by stiff, downward-pointing hairs. 30 percent of a plant’s total dry weight. The nitrogen content The insects eventually die and are digested by a combination of animals is even higher, and all the nitrogen in the animal of enzymes and bacteria in the water. Even rats have been world arrives there by way of the plant kingdom. found in large pitcher plants. Sundews have leaves covered with hairs that secrete a clear, sticky, sugary liquid. An insect touching one of these Carnivorous and Heterotrophic Plants hairs becomes stuck, and more hairs curve over the insect Some plants that are found primarily in nitrogen-deficient and stick to it as well. The plant secretes enzymes to digest soils augment their nitrogen and phosphorus supply by cap- the insect and eventually absorbs the carbon- and nitrogen- turing and digesting flies and other insects. There are about containing products of digestion. 450 of these carnivorous species, the best-known of which are None of the carnivorous plants must feed on insects to Venus flytraps (genus Dionaea; Figure 37.9a), sundews (genus survive. They can grow adequately without insects, but in Drosera; Figure 37.9b), and pitcher plants (genus Sarracenia). their natural habitats they grow faster and are a darker green Carnivorous plants are normally found in boggy regions when they succeed in capturing insects. They use the addi- where the soil is acidic. Most decomposing organisms require tional nitrogen from the insects to make more proteins, a less acidic pH to break down the bodies of dead organisms, chlorophyll, and other nitrogen-containing compounds. so relatively little nitrogen is recycled into these acidic soils. Thus far in this chapter we have considered the mineral Accordingly, the carnivorous plants have adaptations that al- nutrition of plants. As you already know, another crucial as- low them to augment their supply of nitrogen by capturing pect of plant nutrition is photosynthesis—the principal animals and digesting their proteins. source of energy and carbon for plants them- selves and for the biosphere as a whole. Not (b) all plants, however, are photosynthetic au- (a) totrophs. A few, in the course of their evolu- tion, have lost the ability to sustain them- selves by photosynthesis. How do these plants get their energy and carbon? A few plants are heterotrophic parasites that obtain their nutrients directly from the living bodies of other plants. Perhaps the most familiar parasitic plants are the mistle- toes and dodders (Figure 37.10). Mistletoes are green and carry on some photosynthesis, but they parasitize other plants for water and mineral nutrients and may derive photosyn- thetic products from them as well. Mistletoes and dodders extract nutrients from the vas- cular tissues of their hosts by forming ab- Dionaea muscipula Drosera rotundifolia sorptive organs called haustoria, which invade the host plant’s tissues. Another parasitic 37.9 Carnivorous Plants Some plants have adapted to nitrogen-poor environments plant, the Indian pipe, once was thought to by becoming carnivorous. (a) The Venus flytrap obtains nitrogen and phosphorus from the bodies of insects trapped inside the plant when its hinges snap shut. (b) Sundews obtain its nutrients from dead organic matter. trap insects on sticky hairs. Secreted enzymes will digest the carcass externally. It is now known to get its nutrients, with the
  • PLANT NUTRITION 727 Deficiency symptoms suggest what essential element a plant lacks. Review Table 37.2 Tendrils of Biologists discovered the requirement for each essential ele- dodder ment by growing plants on hydroponic solutions lacking that element. Review Figure 37.1. See Web/CD Tutorial 37.1 Soils and Plants Soils are complex systems with living and nonliving compo- Dodder flowers nents. They contain water, air, and inorganic and organic sub- stances. They typically consist of two or three horizontal zones called horizons. Review Figures 37.2, 37.3 Soils form by mechanical and chemical weathering of rock. Plants obtain some mineral nutrients through ion exchange between the soil solution and the surface of clay particles. Review Figure 37.4 Farmers use fertilizers to make up for deficiencies in soil min- Host stem eral nutrient content, and they apply lime to raise low soil pH. Plants affect soils in various ways, such as by adding organic material, removing nutrients (especially in agriculture), and changing pH. Nitrogen Fixation A few species of soil bacteria are responsible for almost all nitrogen fixation. Some nitrogen-fixing bacteria live free in the 37.10 A Parasitic Plant Tendrils of dodder wrap around other soil; others live symbiotically as bacteroids within the roots of plants. This parasitic plant (genus Cuscuta) obtains water, sugars, and plants. other nutrients from its host through tiny, rootlike protuberances that penetrate the surface of the host. In nitrogen fixation, nitrogen gas (N2) is reduced to ammonia (NH3) or ammonium ions (NH4+) in a reaction catalyzed by nitrogenase. Review Figure 37.6 Nitrogenase requires anaerobic conditions, but the bacteroids in root nodules require oxygen for their respiration. Leghemo- help of fungi, from nearby actively photosynthesizing plants. globin helps maintain the oxygen supply to the bacteroids at the Hence it, too, is a parasite. proper level. The formation of a nodule requires an interaction between Dwarf mistletoe is a serious parasite in forests of the west- the root system of a legume and Rhizobium bacteria. Review ern United States, destroying more than 3 billion board feet Figure 37.7 of lumber per year. However, parasitic plants are a much Nitrogen-fixing bacteria reduce atmospheric N2 to ammonia, more urgent problem in developing countries. Striga (witch- but most plants take up both ammonium ions and nitrate ions. Nitrifying bacteria oxidize ammonia to nitrate. Plants take up weed) imperils more than 300 million sub-Saharan Africans nitrate and reduce it back to ammonia, a feat of which animals by attacking their cereal and legume crops. In the Middle are incapable. Review Figure 37.8. See Web/CD Activity 37.1 East and North Africa, Orobanche (broomrape) ravages many Denitrifying bacteria return N2 to the atmosphere, completing crops, especially vegetables and sunflowers. the global nitrogen cycle. Review Figure 37.8 Carnivorous and Heterotrophic Plants Chapter Summary Carnivorous plant species are autotrophs that supplement their nitrogen supply by feeding on insects. The Acquisition of Nutrients A few heterotrophic plants are parasitic on other plants. Some Plants are photosynthetic autotrophs that can produce all the parasitic plants have major effects on crops, especially in devel- organic compounds they need from carbon dioxide, water, and oping countries. minerals, including a nitrogen source. They obtain energy from sunlight, carbon dioxide from the atmosphere, and nitrogen- containing ions and mineral nutrients from the soil. Self-Quiz Plants explore their surroundings by growing rather than by movement. 1. Macronutrients a. are so called because they are more essential than Mineral Nutrients Essential to Plants micronutrients. b. include manganese, boron, and zinc, among others. Plants require 14 essential mineral elements, all of which c. function as catalysts. come from the soil solution. Several of these essential elements d. are required in concentrations of at least 1 gram per fulfill multiple roles. Review Table 37.1 kilogram of plant dry matter. The six mineral nutrients required in substantial amounts are e. are obtained by the process of photosynthesis. called macronutrients; the eight required in much smaller amounts are called micronutrients. Review Table 37.1
  • 728 CHAPTER THIRT Y-SEVEN 2. Which of the following is not an essential mineral element 8. Nitrate reduction for plants? a. is performed by plants. a. Potassium b. takes place in mitochondria. b. Magnesium c. is catalyzed by the enzyme nitrogenase. c. Calcium d. includes the reduction of nitrite ions to nitrate ions. d. Lead e. is known as the Haber process. e. Phosphorus 9. Which of the following is a parasite? 3. Fertilizers a. Venus flytrap a. are often characterized by their N-P-O percentages. b. Pitcher plant b. are not required if crops are removed frequently enough. c. Sundew c. restore needed mineral nutrients to the soil. d. Dodder d. are needed to provide carbon, hydrogen, and oxygen to e. Tobacco plants. 10. All carnivorous plants e. are needed to destroy soil pests. a. are parasites. 4. In a typical soil, b. depend on animals as a source of carbon. a. the topsoil tends to lose mineral nutrients by leaching. c. are incapable of photosynthesis. b. there are four or more horizons. d. depend on animals as their sole source of phosphorus. c. the C horizon consists primarily of loam. e. obtain supplemental nitrogen from animals. d. the dead and decaying organic matter gathers in the B horizon. e. more clay means more air space and thus more oxygen For Discussion for roots. 1. Methods for determining whether a particular element is 5. Which of the following is not an important step in soil essential have been known for more than a century. Since formation? these methods are so well established, why was the essen- a. Removal of bacteria tiality of some elements discovered only recently? b. Mechanical weathering c. Chemical weathering 2. If a Venus flytrap were deprived of soil sulfates and hence d. Clay formation made unable to synthesize the amino acids cysteine and e. Hydrolysis of soil minerals methionine, would it die from lack of protein? Explain. 6. Nitrogen fixation is 3. Soils are dynamic systems. What changes might result when a. performed only by plants. land is subjected to heavy irrigation for agriculture after b. the oxidation of nitrogen gas. being relatively dry for many years? What changes in the c. catalyzed by the enzyme nitrogenase. soil might result when a virgin deciduous forest is cut down d. a single-step chemical reaction. and replaced by crops that are harvested each year? e. possible because N2 is a highly reactive substance. 4. We mentioned that important positively charged ions are 7. Nitrification is held in the soil by clay particles, but other, equally impor- a. performed only by plants. tant, negatively charged ions are leached deeper into the b. the reduction of ammonium ions to nitrate ions. soil’s B horizon. Why doesn’t leaching cause an electrical c. the reduction of nitrate ions to nitrogen gas. imbalance in the soil? (Hint: Think of the ionization of d. catalyzed by the enzyme nitrogenase. water.) e. performed by certain bacteria in the soil. 5. The biosphere of Earth as we know it depends on the exis- tence of a few species of nitrogen-fixing prokaryotes. What do you think might happen if one of these species were to become extinct? If all of them were to disappear?