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Transcript
CHAPTER 35 Plant Nutrition Peter J. Russell • Paul E. Hertz • Beverly McMillan www.cengage.com/biology/russell Spanish Moss, an Epiphyte Why It Matters… • Tropical rainforests are among the most biologically diverse ecosystems on Earth, containing countless thousands of species of animals, fungi, protists, prokaryotes, and plants • Tropical rainforests are demanding places for plants to survive – the soil is chronically deficient in nutrients due to incessant rain and high acidity • In acid soil, minerals vital to plant metabolism (potassium, calcium, magnesium, phosphorus) are subject to leaching – “acid rain” exacerbates the leaching problem Tropical Rain Forest Figure 35-1 p794 35.1 Plant Nutritional Requirements • The tissues of most plants are more than 90% water • By growing plants in hydroponic culture, Julius von Sachs deduced a list of six essential plant nutrients: nitrogen, potassium, calcium, magnesium, phosphorus, and sulfur • Many more essential plant nutrients were identified using a modern hydroponic apparatus, in which the nutrient solution is refreshed regularly, and air is bubbled into it to supply oxygen to the roots Research Method: Hydroponic Culture A. Basic components of a hydroponic apparatus Plant support Air pumped into bubbling system Nutrient solution B. Procedure for identifying elements essential for proper plant nutrition Plant thrives; test element may not be essential Transplantation or Lettuce plant growing in complete nutrient solution Solution lacking one element Plant grows abnormally; test element may be essential Figure 35-2 p795 Essential Nutrients • An essential element is necessary for normal growth and reproduction, cannot be functionally replaced by a different element, and has one or more roles in plant metabolism • With enough sunlight and 17 essential elements, plants can synthesize all the compounds they need • Nine essential elements are macronutrients that plants incorporate into their tissues in relatively large amounts • The rest are micronutrients, required only in trace amounts Macronutrients • Carbon, hydrogen, and oxygen, the main components of lipids and carbohydrates, make up 96% of a plant’s dry mass • Nitrogen is essential to proteins and nucleic acids • Phosphorus is used in nucleic acids, ATP, and phospholipids • Potassium functions include enzyme activation and mechanisms that control the opening and closing of stomata • Calcium, sulfur, and magnesium are also macronutrients Micronutrients • The essential micronutrients include copper, chlorine, nickel, iron, boron, manganese, zinc, and molybdenum • Some species of plants may require additional micronutrients: • Many C4 plants require sodium • A few plant species require selenium • Horsetails and some grasses (such as wheat) require silicon Minerals • All essential elements (except carbon, oxygen, and hydrogen) are minerals – elements or compounds that are formed by geological processes and have a crystalline structure • Minerals are available to plants through the soil as ions dissolved in water – most are derived from the weathering of rocks and inorganic particles in the Earth’s crust • Many act as cofactors or coenzymes in protein synthesis, starch synthesis, photosynthesis, and aerobic respiration Essential Plant Nutrients DO NOT MEMORIZE Figure 35-2a p795 Essential Plant Nutrients DO NOT MEMORIZE Figure 35-2b p795 Nutrient Deficiencies • The nutrient content of soils is an important factor in determining which plants will grow well in a given location • Plants differ in the quantity of each nutrient they require – an adequate amount for one plant may be harmful to another • Plants that are deficient in one or more essential elements develop characteristic symptoms such as stunted growth, abnormal leaf color, dead spots, or abnormal stems 35.2 Soil • Soil anchors plant roots and is the main source of inorganic nutrients • Soil is the source of water for most plants, and of oxygen for respiration in root cells • The physical texture of soil is a factor in whether root systems have access to sufficient water and dissolved oxygen • Physical and chemical properties of soils have a major impact on the ability of plants to grow, survive, and reproduce Properties of a Soil • Soil is a complex mix of mineral particles, chemical compounds, ions, decomposing organic matter, air, water, and assorted living organisms • Soils develop from physical or chemical weathering of rock • Soil particles range in size from sand (2.0–0.02 mm) to silt (0.02–0.002 mm) to clay (diameter less than 0.002 mm) Properties of a Soil (cont.) • Organic components of soil include humus – decomposing parts of plants and animals, animal droppings, and other organic matter • Humus has a loose texture and retains water well – organic molecules in humus are reservoirs of nutrients, including nitrogen, phosphorus, and sulfur • Well-aerated soils that contain roughly equal proportions of humus, sand, silt, and clay are loams – the soils in which most plants do best Influences of Living Organisms • A square meter of fertile soil contains trillions of bacteria, hundreds of millions of fungi, several million nematodes, and other worms and insects • Bacteria and fungi decompose dead plant parts and other organic matter, and earthworms aerate the soil • Plant roots and other tissues help shape the characteristics and composition of soil, including the abundance of soildwelling organisms Water Availability • Gravity pulls water down through spaces between soil particles into deeper soil layers • The soil solution (water and dissolved substances) coats soil particles and partially fills pore spaces • Clay particles and organic components in soil (especially proteins) bear negatively charged ions on their surfaces that attract polar water molecules, which form hydrogen bonds with the soil particles Soil Solution Water film around soil particles Clay particle Air space in soil (pore) Sand particle Figure 35-5 p799 Water Availability (cont.) • The amount of water in soil solution depends largely on the amount and pattern of precipitation (rain or snow) in a region • Water available to plants depends on soil composition: • Sandy soil has large air spaces, so water drains rapidly • Humus-rich soil contains air spaces and holds water • Clay has few air spaces and tends to hold water tightly • Good agricultural soils tend to be sandy or silty loams, which contain a mix of humus and coarse and fine particles Water Potential • Differences in water potential govern the osmotic movement of water through root hairs into plant roots • Soil solution usually has fewer dissolved solutes than water in root cells – water tends to move from wet soil (higher water potential) into roots (lower water potential) • Plants in deserts or salty soils have adaptations that allow roots to absorb water under unfavorable conditions • Water potential in clay soils is lower than in other soil types, even when clay is wet Mineral Availability • Mineral nutrients enter roots as cations (positively charged ions) or anions (negatively charged ions) • Although both cations and anions are present in soil solutions, they are not equally available to plants • Cations (Mg2+, Ca2+, K+) are reversibly bound by net negative charges on the surfaces of soil particles (adsorption), and can’t easily enter roots Mineral Availability (cont.) • Roots acquire cations through cation exchange, in which one cation (usually H+) replaces a soil cation • Hydrogen ions come from two main sources: • CO2 from root cells dissolves in soil solution, yielding carbonic acid (H2CO3), which is ionized to HCO3– + H+ • Reactions involving organic acids inside roots also produce H+, which is excreted • H+ in the soil solution displaces adsorbed mineral cations attached to clay and humus, freeing them to move into roots Cation Exchange A. Adsorption of cations to a clay particle B. Adding gypsum to the soil Root hair Clay particle Figure 35-6 p800 Mineral Availability (cont.) • Anions in the soil solution (NO3–, SO42–, PO4–) are only weakly bound to soil particle – they move freely into root hairs • However, because anions are so weakly bound compared with cations, they are more subject to loss by leaching • pH of soil also affects the availability of some mineral ions (e.g. formation of calcium phosphate in alkaline soil): • In areas with heavy rainfall, soils tend to become acidic • In arid regions, soil is often alkaline Mineral Availability (cont.) • The mineral ions that plants take up from the soil must be replenished naturally or artificially • Over the long run, some mineral nutrients enter soil from ongoing weathering of rocks – in the short term, minerals, carbon, and other nutrients are returned to the soil by the decomposition of organisms and their wastes • Nitrogen and phosphorus also enter soil in agricultural chemicals (fertilizers) 35.3 Obtaining and Absorbing Nutrients • In natural habitats, wide variations in soil minerals, humus, pH, the presence of other organisms, and other factors influence the availability of essential elements • Roots continue to branch and grow as long as a plant lives – extensive root systems allow plants to locate nutrients in their immediate environment • Root hairs and ion-specific transport proteins are two major adaptation for the uptake of mineral ions and water Root Secretions • Roots of various plant species release organic compounds into soil – including carbohydrates, amino acids, organic and fatty acids, enzymes and other proteins • These “root exudates” may improve a plant’s access to particular nutrients • Example: Roots of A. thaliana secrete organic compounds that help determine which species of symbiotic soil fungi live near the roots Nutrients Move by Several Routes • Some mineral ions enter root cells immediately – others travel in the apoplast and are actively transported into endodermal cells and xylem for transport throughout the plant • Inside cells, most mineral ions enter vacuoles or remain in the cytoplasm where they are immediately available for metabolic reactions • Some nutrients, such as nitrogen-containing ions, move in phloem from site to site in the plant as dictated by growth and seasonal needs Mycorrhizae • Mycorrhizae are symbiotic associations between a fungus and the roots of a plant that promote the uptake of water and ions (especially phosphate) • Hyphal filaments of the fungal partner grow around or into the plant’s roots, and provide a huge surface area for absorption – aided by hyphal transport proteins • Some of the plant’s sugars and nitrogenous compounds nourish the fungus and, as the root grows, it uses some of the minerals obtained by the fungus Hyphae in a Leek Root Hyphae p803 Nitrogen: A Limiting Factor • Lack of nitrogen is the single most common limit to plant growth – there usually is not enough nitrogen available in usable (ionic) forms to meet plants’ ongoing needs • Plants can’t extract gaseous nitrogen (N2) from air because they lack the enzyme necessary to break apart the three covalent bonds in the N2 molecule (N=N) • Plants can absorb nitrogen from the soil in the form of nitrate (NO3–) or ammonium (NH4+), which are produced by bacteria as part of the nitrogen cycle Nitrogen Fixation • The incorporation of atmospheric nitrogen into compounds that plants can take up is called nitrogen fixation • Nitrogen-fixing bacteria in soil add hydrogen to N2, producing two molecules of NH3 (ammonia) and one H2 (requires ATP, catalyzed by nitrogenase) • H2O and NH3 react, forming NH4+ (ammonium) and OH– Ammonification and Nitrification • Ammonification produces ammonium (NH4+) when ammonifying bacteria break down decaying organic matter • Nitrification, in which NH4+ is oxidized to nitrate (NO3–), is carried out by nitrifying bacteria • Because of ongoing nitrification, nitrate is more abundant than ammonium in most soils • Plant roots may take up ammonium directly in highly acidic soils (e.g. bogs) where low pH is toxic to nitrifying bacteria Nitrogen Assimilation • In root cells, absorbed NO3– is converted by a multistep process back to NH4+, which is used to synthesize organic molecules such as amino acids • These molecules pass into the xylem, which transports them throughout the plant • In some plants, nitrogen-rich precursors travel in xylem to leaves, where different organic molecules are synthesized, then travel to other plant cells in the phloem How Plants Obtain Nitrogen from Soil Atmospheric nitrogen (N2) Decaying organic matter Nitrogen-fixing bacteria convert N2 to ammonia (NH3) which dissolves to form ammonium (NH4+) Ammonium (NH4+) Ammonifying bacteria NO3– converted to NH4, which is moved via xylem to the shoot system Nitrifying bacteria Nitrate (NO3–) Figure 35-7 p804 Plant-Bacteria Associations • Most nitrogen is fixed by the nitrogen-fixing bacteria Rhizobium and Bradyrhizobium, which form mutualistic associations with roots of legumes • The host plant supplies bacteria with organic molecules for cellular respiration – the bacteria supply the plant with NH4+ used to produce proteins and other nitrogenous molecules • In legumes, nitrogen-fixing bacteria reside in root nodules, localized swellings on roots Plant-Bacteria Associations (cont.) • Usually, a single species of nitrogen-fixing bacteria colonizes a single legume species, drawn by chemical attractants (flavonoids) that the roots secrete • Example: Soybean plant and Bradyrhizobium japonicum • Flavonoid is released by soybean roots, bacterial nod genes are expressed, products trigger release of bacterial enzymes that break down root hair cell wall • Bacteria enter the cell, plasma membrane forms an infection thread, bacteria invade cortex cells • Bacteroids stimulate nodule development Root Nodules Root nodule Figure 35-8a p805 Experiment: Soybeans and Rhizobium Figure 35-8b p805 Bacteroids Figure 35-8c p805 Root Nodule Formation in Legumes A. Root signal and bacterial response Soil particles Root hair B. Bacterial signal and root response Effects of the nod gene Bacteria Flavonoid Root Secreted from root Root cortex Bacterial nod hair genes expressed C. Integration of bacteria D. Micrograph of a developing root nodule Infection thread Swelling bacteroid in cortex cell Root nodule Infection thread Stepped Art Figure 35-9 p806 Plant-Bacteria Associations (cont.) • Inside bacteroids, N2 is reduced to NH4+ using ATP produced by cellular respiration (catalyzed by nitrogenase) • Toxic NH4+ is moved out of bacteroids into surrounding nodule cells and converted to other compounds • Nod genes stimulate nodule cells to produce leghemoglobin, which carries oxygen from the cell surface to bacteroids • Leghemoglobin delivers just enough oxygen to maintain bacteroid respiration without shutting down nitrogenase Unusual Modes of Nutrition • “Carnivorous” plants survive in nitrogen-deficient environments such as bogs and sand through elaborate mechanisms for extracellular digestion and absorption • Venus flytrap and sundews capture and digest insects • Cobra lily and tropical pitcher plants form a “pitcher” with downward-pointing leaf hairs and a slick coating that speeds larger prey into a pool of digestive enzymes