植物营养物质plantnutrition.ppt

Chapter 37 Lecture,Plant nutrition,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section A: Nutritional Requirements of Plants,1.The chemical composition of plants provides clues to their nutritional requirements 2. Plants require nine macronutrients and at least eight micronutrients 3. The symptoms of a mineral deficiency depend on the function and mobility of the element,CHAPTER 37PLANT NUTRITION,Every organism is an open system connected to its environment by a continuous exchange of energy and materials. In the energy flow and chemical cycling that keep an ecosystem alive, plants and other photosynthetic autotrophs perform the key step of transforming inorganic compounds into organic ones. At the same time, a plant needs sunlight as its energy source for photosynthesis and raw materials, such as CO2 and inorganic ions, to synthesize organic molecules. The root and shoot systems extensively network a plant with its environment.,Introduction,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Early ideas about plant nutrition were not entirely correct and included: Aristotles hypothesis that soil provided the substance for plant growth van Helmonts conclusion from his experiments that plants grow mainly from water Hales postulate that plants are nourished mostly by air. Plants do extract minerals from the soil.,1. The chemical composition of plants provides clues to their nutritional requirements,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Mineral nutrients are essential chemical elements absorbed from soil in the form of inorganic ions. For example, plants acquire nitrogen mainly in the form of nitrate ions (NO3-). Yet, as indicated by van Helmonts data, mineral nutrients from the soil make only a small contribution to the overall mass of a plant. About 80 - 85% of a herbaceous plant is water. Because water contributes most of the hydrogen ions and some of the oxygen atoms incorporated into organic atoms, one can consider water a nutrient too.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,However, only a small fraction of the water entering a plant contributes to organic molecules. Over 90% is lost by transpiration. Most of the water retained by a plant functions as a solvent, provides most of the mass for cell elongation, and helps maintain the form of soft tissues by keeping cells turgid. By weight, the bulk of the organic material of a plant is derived not from water or soil minerals, but from the CO2 assimilated from the atmosphere.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The uptake of nutrients occurs at both the roots and the leaves. Roots, through mycorrhizae and root hairs, absorb water and minerals from the soil. Carbon dioxide diffuses into leaves from the surrounding air through stomata.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.1,Of the 15-20% of a herbaceous plant that is not water, about 95% of the dry weight is organic substances and the remaining 5% is inorganic substances. Most of the organic material is carbohydrate, including cellulose in cell walls. Thus, carbon, hydrogen, and oxygen are the most abundant elements in the dry weight of a plant. Because some organic molecules contain nitrogen, sulfur, and phosphorus, these elements are also relatively abundant in plants.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,More than 50 chemical elements have been identified among the inorganic substances present in plants. However, it is unlikely that all are essential. Roots are able to absorb minerals somewhat selectively, enabling the plant to accumulate essential elements that may be present in low concentrations in the soil. However, the minerals in a plant reflect the composition of the soil in which the plant is growing. Therefore, some of the elements in a plant are merely present, while others are essential.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,A particular chemical element is considered an essential nutrient if it is required for a plant to grow from a seed and complete the life cycle. Hydroponic cultures have identified 17 elements that are essential nutrients in all plants and a few other elements that are essential to certain groups of plants.,2. Plants require nine macronutrients and at least eight micronutrients,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Hydroponic culture can determine which mineral elements are actually essential nutrients. Plants are grown in solutions of various minerals dissolved in known concentrations. If the absence of a particular mineral, such as potassium, causes a plant to become abnormal in appearance when compared to controls grown in a complete mineral medium, then that element is essential.,Fig. 37.2,Elements required by plants in relatively large quantities are macronutrients. There are nine macronutrients in all, including the six major ingredients in organic compounds: carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. The other three are potassium, calcium, and magnesium.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Elements that plants need in very small amounts are micronutrients. The eight micronutrients are iron, chlorine, copper, zinc, manganese, molybdenum, boron, and nickel. Most of these function as cofactors of enzymatic reactions. For example, iron is a metallic component in cytochromes, proteins that function in the electron transfer chains of chloroplasts and mitochondria. While the requirement for these micronutrients is so modest (only one atom of molybdenum for every 16 million hydrogen atoms in dry materials), a deficiency of a micronutrient can weaken or kill a plant.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The symptoms of a mineral deficiency depend partly on the function of that nutrient in the plant. For example, a magnesium deficiency, an ingredient of chlorophyll, causes yellowing of the leaves, or chlorosis.,3. The symptoms of a mineral deficiency depend on the function and mobility of the element,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.3,The relationship between a mineral deficiency and its symptoms can be less direct. For example, chlorosis can also be caused by iron deficiency because iron is a required cofactor in chlorophyll synthesis.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Mineral deficiency symptoms depend also on the mobility of the nutrient within the plant. If a nutrient moves about freely from one part of a plant to another, then symptoms of the deficiency will appear first in older organs. Young, growing tissues have more “drawing power” than old tissues for nutrients in short supply. For example, a shortage of magnesium will lead to chlorosis first in older leaves. If a nutrient is relatively immobile, then a deficiency will affect young parts of the plant first. Older tissue may have adequate supplies which they retain during periods of shortage.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The symptoms of a mineral deficiency are often distinctive enough for a plant physiologist or farmer to diagnose its cause. This can be confirmed by analyzing the mineral content of the plant and the soil. Deficiencies of nitrogen, potassium, and phosphorus are the most common problems. Shortages of micronutrients are less common and tend to be geographically localized because of differences in soil composition. The amount of micronutrient needed to correct a deficiency is usually quite small, but an overdose can be toxic to plants.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,One way to ensure optimal mineral nutrition is to grow plants hydroponically on nutrient solutions that can be precisely regulated. This technique is practiced commercially, but the requirements for labor and equipment make it relatively expensive compared with growing crops in soil.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.4,Mineral deficiencies are not limited to terrestrial ecosystems, nor are they unique to plants among photosynthetic organisms. For example, populations of planktonic algae in the southern oceans are restrained by deficiencies of iron in seawater. In a limited trial in the relatively unproductive seas between Tasmania and Antarctica, researchers demonstrated that dispersing small amounts of iron produced large algal blooms that pulled carbon dioxide out of the air. Seeding the oceans with iron may help slow the increase in carbon dioxide levels in the atmosphere, but it may also cause unanticipated environmental effects.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section B: The Role of the Soil in Plant Nutrition,1.Soil characteristics are key environmental factors in terrestrial ecosystems 2. Soil conservation is one step toward sustainable agriculture,CHAPTER 37PLANT NUTRITION,The texture and chemical composition of soil are major factors determining what kinds of plants can grow well in a particular location. Plants that grow naturally in a certain type of soil are adapted to its mineral content and texture and are able to absorb water and extract essential nutrients from that soil. Plants, in turn, affect the soil. The soil-plant interface is a critical component of the chemical cycles that sustain terrestrial ecosystems.,1. Soil characteristics are key environmental factors in terrestrial ecosystems,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Soil has its origin in the weathering of solid rock. Water that seeps into crevices and freezes in winter fractures the rock, and acids dissolved in the water also help break down the rock. Organisms, including lichens, fungi, bacteria, mosses, and the roots of vascular plants, accelerate the breakdown by the secretion of acids and as the expansion of their roots in fissures cracks rocks and pebbles.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,This activity eventually results in topsoil, a mixture of rock, living organisms, and humus, a residue of partially decayed organic material. Topsoil and other distinct soil layers, called horizons, are often visible in vertical profile.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.5,The texture of topsoil depends on the size of its particles, which are classified from coarse sand to microscopic clay particles. The most fertile soils are usually loams, made up of roughly equal amounts of sand, silt (particles of intermediate size), and clay. Loamy soils have enough fine particles to provide a large surface area for retaining minerals and water, which adhere to the particles. Loams also have enough course particles to provide air spaces that supply oxygen to the root for cellular respiration.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Inadequate drainage can dramatically impact survival of many plants. Plants can suffocate if air spaces are replaced by water. Roots can also be attacked by molds favored by the soaked soil. Some plants are adapted to waterlogged soil. For example, mangroves, that inhabit swamps and marshes, have some roots modified as hollow tubes that grow upward and function as snorkels.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Topsoil is home to an astonishing number and variety of organisms. A teaspoon of soil has about 5 billion bacteria that cohabit with various fungi, algae and other protists, insects, earthworms, nematodes, and the roots of plants. The activities of these organisms affect the physical and chemical properties of the soil. For example, earthworms aerate soil by their burrowing and add mucus that holds fine particles together. Bacterial metabolism alters mineral composition of soil. Plant roots extract water and minerals but also affect soil pH and reinforce the soil against erosion.,Humus is the decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse. Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for the adequate aeration of roots. Humus is also a reservoir of mineral nutrients that are returned to the soil by decomposition.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,After a heavy rainfall, water drains away from the larger spaces of the soil, but smaller spaces retain water because of its attraction for the soil particles, which have electrically charged surfaces. Some water adheres so tightly to hydrophilic particles that it cannot be extracted by plants, but some water bound less tightly to the particles can be absorbed by roots.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.6,Many minerals, especially those with a positive charge, such as potassium (K+), calcium (Ca2+), and magnesium (Mg2+), adhere by electrical attraction to the negatively charged surfaces of clay particles. Clay in soil prevents the leaching of mineral nutrients during heavy rain or irrigation because of the large surface area for binding minerals. Minerals that are negatively charges, such as nitrate (NO3-), phosphate (H2PO4-), and sulfate (SO42-), are usually not bound tightly to soil particles and thus tend to leach away more quickly.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Positively charged mineral ions are made available to the plant when hydrogen ions in the soil displace the mineral ions from the clay particles. This process, called cation exchange, is stimulated by the roots which secrete H+ and compounds that form acids in the soil solution.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,It takes centuries for a soil to become fertile through the breakdown of soil and the accumulation of organic material. However, human mismanagement can destroy soil fertility within just a few years. Soil mismanagement has been a recurring problem in human history.,2. Soil conservation is one step toward sustainable agriculture,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,For example, the Dust Bowl was an ecological and human disaster that occurred in the southwestern Great Plains of the United States in the 1930s. Before the arrival of farmers, the region was covered with hardy grasses the held the soil in place in spite of long recurrent droughts and torrential rains. In the 30 years before World War I, homesteaders planted wheat and raised cattle, which left the soil exposed to wind erosion.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Several years of drought resulted in the loss of centimeters of topsoil that were blown away by the winds. Millions of hectares of farmland became useless, and hundreds of thousands of people were forced to abandon their homes and land.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.7,To understand soil conservation, we must begin with the premise that agriculture is unnatural. In natural ecosystems, mineral nutrients are usually recycled by the decomposition of dead organic material. In contrast, when we harvest a crop, essential elements are diverted from the chemical cycles in that location. In general, agriculture depletes minerals in the soil. To grow a ton of wheat, the soil gives up 18.2 kg of nitrogen, 3.6 kg of phosphorus, and 4.1 kg of potassium. The fertility of the soil diminishes unless replaced by fertilizers, and most crops require far more water than the natural vegetation for that area.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Farmers have been using fertilizers to improve crop yields since prehistory. Historically, these have included animal manure and fish carcasses. In developed nations today, most farmers use commercial fertilizers containing minerals that are either mined or prepared by industrial processes. These are usually enriched in nitrogen, phosphorus, and potassium, often deficient in farm and garden soils. A fertilizer marked “10-12-8” is 10% nitrogen (as ammonium or nitrate), 12% phosphorus (as phosphoric acid), and 8% potassium (as the mineral potash).,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Manure, fishmeal, and compost are “organic” fertilizers because they are of biological origin and contain material in the process of decomposing. The organic material must be decomposed to the inorganic nutrients that roots can absorb. In the end, the minerals that a plant extracts from the soil are in the same form whether they came from organic fertilizer or from a chemical factory. Compost releases nutrients gradually, while minerals in commercial fertilizers are available immediately. Excess fertilizers are often leached from the soil by rainwater or irrigation and may pollute groundwater, streams, and lakes.,To fertilize judiciously, the soil pH must be appropriate because pH affects cation exchange and influences the chemical form of all minerals. Even though an essential element may be abundant in the soil, plants may be starving for that element because it is bound too tightly to clay or is in a chemical form that the plant cannot absorb. Because a change in pH may make one mineral more available, but another less available, adjustments to pH of soil is tricky. The pH of the soil must be matched to the specific mineral needs of the crop. Sulfate lowers the pH, liming increases the pH.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,A major problem with acid soils, particularly in tropical areas, is that aluminum dissolves in the soil at low pH and becomes toxic to roots. Some plants can cope with high aluminum levels in the soil by secreting certain organic ions that bind the aluminum and render it harmless.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Even more than mineral deficiencies, the unavailability of water most often limits the growth of plants. Irrigation can transform a desert into a garden, but farming in arid regions is a huge drain on water resources. Another problem is that irrigation in an arid region can gradually make the soil so salty that it becomes completely infertile because salts in the irrigation water accumulate in the soil as the water evaporates. Eventually, the salt makes the water potential of the soil solution lower than that of root cells, which then loose water instead of absorbing it.,Valuable topsoil is lost to wind and water erosion each year. This can be reduced by planting rows of trees between fields as a windbreak and terracing a hillside to prevent topsoil from washing away. Some crops such as alfalfa and wheat provide good ground cover and protect soil better than corn and other crops that are usually planted in rows.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 37.8,Soil is a renewable resources in which farmers can grow food for generations to come. The goal is sustainable agriculture, a commitment embracing a variety of farming methods that are conservation-minded, environmentally safe, and profitable.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Some areas have become unfit for agriculture or wildlife as the result of human activities that contaminate the soil or groundwater with toxic heavy metals or organic pollutants. In place of costly and disruptive remediation technologies, such as removal and storage of contaminated soils, phytoremediation takes advantage of the remarka