Chapter 37 Plant Nutrition
Chapter 37 Plant Nutrition Nutrient Reservoirs Every organism continually exchanges energy and materials with its environment For plants…water and minerals come from the soil, while carbon dioxide comes from the air The branching root system and shoot system of a vascular plant ensure extensive networking with both reservoirs of inorganic nutrients Macronutrients and Micronutrients Plants derive most of their organic mass from the CO2 of air but they also depend on soil nutrients More than 50 chemical elements have been identified among the inorganic substances in plants, but not all of these are essential A chemical element is considered essential if it is required for a plant to complete a life cycle How would you identify an essential nutrient? Hydroponic culture can be used to determine which chemicals elements are essential APPLICATION In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants. TECHNIQUE Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential. Control: Solution containing all minerals Experimental: Solution without potassium RESULTS If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil. Macronutrients and Micronutrients Nine of the essential elements are called macronutrients because plants require them in relatively large amounts C, O, H, N, K, Ca, Mg, P, S The remaining eight essential elements are known as micronutrients because plants need them in very small amounts Cl, Fe, Zn, Mn, B, Cu, N, Mo Essential elements in plants Mineral Deficiency The symptoms of mineral deficiency Depend partly on the nutrient’s function Depend on the mobility of a nutrient within the plant Deficiency of a mobile nutrient Usually affects older organs more than young ones (young tissue can more efficiently draw minerals to it) Deficiency of a less mobile nutrient Usually affects younger organs more than older ones (older tissue has a store of minerals to fall back on when the mineral is in short supply) Mineral Deficiency The most common deficiencies Are those of nitrogen, potassium, and phosphorus Healthy Phosphate-deficient Reddish-purple margins esp. on young leaves Potassium-deficient “Firing”…drying along tips and margins of older leaves Nitrogen-deficient Yellowing that starts at the tip and moves along the center of older leaves Soil Characteristics Soil quality is a major determinant of plant distribution and growth Along with climate Texture…is the soil’s general structure (sandy, clayey, etc) Composition…refers to the soil’s organic and inorganic chemical components The major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil Various sizes of particles derived from the breakdown of rock are found in soil along with organic material (humus) in various stages of decomposition Topsoil… is the mixture of particles of rock and organic material Soil Horizons The topsoil and other distinct soil layers, or horizons are often visible in vertical profile where there is a road cut or deep hole The A horizon is the topsoil, a mixture of broken-down rock of various textures, living organisms, and decaying organic matter. A B The B horizon contains much less organic matter than the A horizon and is less weathered. C The C horizon, composed mainly of partially broken-down rock, serves as the “parent” material for the upper layers of soil. Availability of Soil Water After a rainfall, water drains away from the larger spaces of soil but smaller spaces retain water because of its attraction to surfaces of clay and other particles. The film of loosely bound water is usually available to plants Soil particle surrounded by film of water Root hair Water available to plant Air space Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root. Cation Exchange Acids derived from roots contribute to a plant’s uptake of minerals when H+ displaces mineral cations from clay particles Soil particle K+ – – Cu2+ – – K+ – – – Mg2+ – + K – Ca2+ H+ H2O + CO2 H2CO3 HCO3– + H+ Root hair Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca 2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairs and also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution. Agriculture Conventional agriculture In contrast to natural ecosystems agriculture depletes the mineral content of the soil, taxes water reserves, and encourages erosion Sustainable agriculture Is ecologically sound Is economically viable Is socially just Is humane. Fertilizers Commercially produced fertilizers contain minerals that are either mined or prepared by industrial processes “Organic” fertilizers are composed of manure, fishmeal, or compost Irrigation Is a huge drain on water resources when used for farming in arid regions Can change the chemical makeup of soil Salinization (salt buildup) drip Ditch…trench sprinkler Erosion Topsoil from thousands of acres of farmland Is lost to water and wind erosion each year in the United States The U.S. Soil Conservation Service reports that more than 4 million acres of cropland are being lost to erosion in this country every year. That's an area greater than the size of Connecticut. Our annual topsoil loss amounts to 7 billion tons. That is 60,000 pounds for each member of the population. Erosion on conventionally tilled field Prevention of topsoil loss Strip cropping: practice of growing field crops in narrow strips either at right angles to the direction of the prevailing wind, or following the natural contours of the terrain to prevent wind and water erosion of the soil Contour tillage (slows water runoff and erosion) Prevention of topsoil loss Terraces Conservation tillage (Min-till) A minimum tillage system may involve quicker and fewer passes at a shallower depth Cover Crops Cover crop in an orchard Cover crop in vegetable garden Soil Reclamation Some areas are unfit for agriculture Because of contamination of soil or groundwater with toxic pollutants Phytoremediation: is a biological, nondestructive technology that seeks to reclaim contaminated areas by using the ability of some plants to remove soil pollutants Nitrogen Nitrogen is often the mineral that has the greatest effect on plant growth Plants require nitrogen as a component of proteins, nucleic acids, chlorophyll, and a host of other important organic molecules Soil Bacteria and Nitrogen Availability Nitrogen-fixing bacteria convert atmospheric N2 to nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesis Atmosphere N2 N2 Atmosphere Soil N2 Nitrogen-fixing bacteria Denitrifying bacteria H+ Nitrate and nitrogenous organic compounds exported in xylem to shoot system (From soil) Soil + NH4 NH3 (ammonia) – + NH4 (ammonium) Nitrifying bacteria NO3 (nitrate) Ammonifying bacteria Organic material (humus) Root The Role of Bacteria in Symbiotic Nitrogen Fixation Symbiotic relationships with nitrogen-fixing bacteria provide some plant species with a built-in source of fixed nitrogen From an agricultural standpoint the most important and efficient symbioses between plants and nitrogen-fixing bacteria occur in the legume family (peas, beans, and other similar plants) Root Nodules Along a legumes roots are swellings called nodules composed of plant cells that have been “infected” by nitrogen-fixing Rhizobium bacteria The bacteria of a nodule obtain sugar from the plant and supply the plant with fixed nitrogen Each legume is associated with a particular strain of Rhizobium Nodules Roots Pea plant root. The bumps on this pea plant root are nodules containing Rhizobium bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant. Development of a soybean root nodule 1 Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form an infection thread by an invagination of the plasma membrane. Infection thread Infected root hair Rhizobium bacteria Dividing cells in root cortex Bacteroid Dividing cells in pericycle 1 2 2 The bacteria penetrate the cortex within the Infection thread. Cells of the cortex and pericycle begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids. Developing root nodule 3 3 Growth continues in the affected regions of the cortex and pericycle, and these two masses of dividing cells fuse, forming the nodule. 4 4 The nodule develops vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant. Bacteroid Bacteroid Nodule vascular tissue Symbiotic Nitrogen Fixation and Agriculture The agriculture benefits of symbiotic nitrogen fixation are the basis for crop rotation In this practice a non-legume such as maize is planted one year, and the following year a legume is planted to restore the concentration of nitrogen in the soil Mycorrhizae and Plant Nutrition Mycorrhizae: are modified roots consisting of mutualistic associations of fungi and roots The fungus benefits from a steady supply of sugar donated by the host plant In return, the fungus increases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plant Agricultural importance: Farmers and foresters often inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizae Ectomycorrhizae In ectomycorrhizae the mycelium of the fungus forms a dense sheath over the surface of the root Epidermis a Ectomycorrhizae. The mantle (a) of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant. Cortex Mantle (fungal sheath) 100 m Endodermis Mantle (fungal sheath) Fungal hyphae between cortical cells (colorized SEM) Endomycorrhizae In endomycorrhizae the microscopic fungal hyphae extend into the root (b) 2 Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex. Epidermis Cortex Cortical cells 10 m Endodermis Fungal hyphae Vesicle Casparian strip Root hair Arbuscules (LM, stained specimen) Epiphytes, Parasitic Plants, and Carnivorous Plants Epiphytes use a host for support but do not extract nutrients from the host EPIPHYTES Some plants have nutritional adaptations that use other organisms in nonmutualistic ways Staghorn fern, an epiphyte PARASITIC PLANTS Host’s phloem Dodder Haustoria Mistletoe, a photosynthetic parasite Dodder, a nonphotosynthetic parasite Indian pipe, a nonphotosynthetic parasite CARNIVOROUS PLANTS Venus’ flytrap Pitcher plants Sundews Carnivorous plant movie Improving the Protein Yield of Crops Plant breeding research has resulted in new varieties of maize, wheat, and rice that are enriched in protein Such research addresses the most widespread form of human malnutrition: protein deficiency Many of the projects creating GMOs (genetically modified organisms) are aimed at protein enrichment of crops. High lysine corn
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