Ch37WordLectureOutline - CHAPTER 37 PLANT NUTRITION...

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CHAPTER 37 PLANT NUTRITION Introduction 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 CO 2 and inorganic ions, to synthesize organic molecules. The root and shoot systems extensively network a plant with its environment. A. Nutritional Requirements of Plants 1. The chemical composition of plants provides clues to their nutritional requirements Early ideas about plant nutrition were not entirely correct and included: Aristotle’s hypothesis that soil provided the substance for plant growth. Van Helmont’s conclusion from his experiments that plants grow mainly from water Hale’s postulate that plants are nourished mostly by air. Plants do extract minerals from the soil. 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 (NO 3 - ). However, as indicated by van Helmont’s data, mineral nutrients from the soil make only a small contribution to the overall mass of a plant.
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About 80 - 85% of a herbaceous plant is water. Because water contributes most of the hydrogen ions and some of the oxygen atoms that are incorporated into organic atoms, one can consider water a nutrient too. 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 CO 2 assimilated from the atmosphere. 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. Of the 15-20% of a herbaceous plant that is not water, about 95% of the dry weight is organic substance and the remaining 5% is inorganic substance. 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. More than 50 chemical elements have been identified among the
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Ch37WordLectureOutline - CHAPTER 37 PLANT NUTRITION...

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