Chapter 37 LecturePlant nutritionCopyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsSection A: Nutritional Requirements of Plants1. 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 elementCHAPTER 37 PLANT 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. IntroductionCopyright 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 requirementsCopyright 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 CummingsFig. 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 micronutrientsCopyright 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 miner