Nicolas-Théodore de Saussure’s research helped establish the basis of plant metabolism. He discovered that the gain in dry matter by growing plants far exceeds the weight of the carbon dioxide that they absorb, and that water is the major contributor to the gain. He also showed that plants obtain nitrogen and various minerals from the soil and demonstrated that minerals are essential for plant growth.
Swiss chemist and plant physiologist Nicolas-Théodore de Saussure built his work on seventeenth and eighteenth century research and experimentation, leading to a series of insights into plant nutrition and respiration. Earlier scientists had learned that the green tissues of a plant use two simple, inorganic raw materials, water and carbon dioxide, and, in the presence of light, assimilate carbon into the plant’s dry matter. (It was later learned that the carbon is fixed as carbohydrate.) Scientists also found that, in the process of carbon assimilation, oxygen gas is released as a waste product. This understanding of photosynthesis replaced the belief that plants obtain all their nutrition from the soil, in a manner analogous to an animal’s ingestion of food.
Along with this insight came an understanding of the countervailing process called respiration. In a modern, cellular sense, during respiration an organism uses oxygen to break down organic compounds to meet its needs for energy and building blocks for other organic molecules. In respiration, carbon dioxide is released. The processes of photosynthesis and respiration are opposed.
The development of this new understanding of plant physiology began in the early seventeenth century, when Jan Baptista van Helmont concluded from an experiment that water rather than soil was the source of the gain in dry weight by growing plants. He was incorrect in concluding that water was the sole source, however.
During the early eighteenth century, Stephen Hales correctly surmised that plants derive some of their nutrition from “air” (in modern terms, from atmospheric carbon dioxide). Then, in 1771, Joseph Priestley showed that growing plants release oxygen. In 1779, Jan Ingenhousz
The contributions by Saussure to advancing the understanding of plant physiology owed much to his use of, to a greater extent than his predecessors, the quantitative physical and chemical methods pioneered by eighteenth century French chemist Antoine-Laurent Lavoisier.
Saussure was the first to use direct chemical methods to show that, in a photosynthesizing leaf, fixed carbon appears at the same time that carbon dioxide is lost from the surrounding atmosphere and replaced by oxygen. He demonstrated that, even though the atmosphere contains only a trace of carbon dioxide, plant leaves assimilate large amounts of carbon directly from that source. Saussure thereby disproved Senebier’s
In plant respiration, the carbon dioxide formed by the combination of oxygen with carbon is emitted by green parts of the plant in the dark and by nongreen parts all the time, in contrast to the release of oxygen by illuminated green parts. Saussure demonstrated that plants fix carbon dioxide photosynthetically with an efficiency greatly exceeding that of the countervailing process of respiration and that, consequently, the net effect of growing plants on the atmosphere is the fixation of carbon dioxide and the evolution of oxygen.
Saussure also discovered that the dry weight gained by a growing plant equals roughly twice the weight of carbon fixed. He then reasoned correctly—like van Helmont—that water is a plant nutrient. It is now known that water contributes the large amount of hydrogen present in plant dry matter. (Water is the basic medium of living cells, but the meaning here is of incorporation into the structural and functional building blocks, not the watery matrix, of the cell.)
Saussure also identified the route by which plants obtain nitrogen, which is as essential a constituent of plants as it is of animals. Previous researchers such as Priestley
In 1804, Saussure published his findings in plant metabolism as Recherches chimiques sur la végétation (chemical research on vegetation). This work, which consists of research papers, established the theoretical basis of plant nutrition and the methodology of plant physiology. He had previously published three articles on carbon dioxide and its fixation in plant tissues in the journal Annales de Chimie (annals of chemistry).
Saussure’s book Recherches chimiques sur la végétation culminated a revolution in the understanding of plant nutrition and physiology that had taken two centuries to achieve. He identified the source and route of supply of nearly all the major elements that had been shown to occur in mature plants. Although knowledge was still rudimentary, his publication marked the establishment of plant physiology as a science.
Saussure’s discoveries were fundamental to later work in plant physiology, but, immediately after him, there was a thirty-year lull in interest in the quantitative study of carbon assimilation and the synthesis of organic compounds by plants. Then, during the mid-nineteenth century, there were advances in understanding of the steps in carbohydrate formation by photosynthetic activity. Toward the end of the nineteenth century, the idea that carbohydrate formation is the starting point for the synthesis of the other organic constituents of a plant began to gain adherents.
During the early twentieth century came the insight that the oxygen released in photosynthesis is derived from the splitting of water, not from carbon dioxide as Saussure and other previous researchers had thought. As of the early twenty-first century, at least fifty intermediate steps in photosynthesis had been identified, and the discovery of many more was fully anticipated. Scientists had determined that cellular respiration, too, is complex, involving numerous chemical steps.
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Epstein, Emanuel, and Arnold J. Bloom. Mineral Nutrition of Plants: Principles and Perspectives. 2d ed. Sunderland, Mass.: Sinauer Associates, 2005. Describes how plants acquire and use mineral nutrients. -
Galston, Arthur W. Life Processes of Plants. New York: Scientific American Library, 1994. An easy-to-read overview of plant physiological processes. Chapter 1 explores photosynthesis. -
Govindjee, J. T. Beatty, H. Gest, and J. F. Allen, eds. Discoveries in Photosynthesis. Berlin: Springer, 2005. An encyclopedic volume detailing the history of photosynthesis research. -
Hall, David O., and Krishna Rao. Photosynthesis. 6th ed. New York: Cambridge University Press, 1999. A concise introduction to the complexities of photosynthesis. Chapter 2 provides a historical overview. -
Morton, A. G. History of Botanical Science: An Account of Botany from Ancient Times to the Present Day. London: Academic Press, 1981. Chapter 8 of this comprehensive volume includes details about Saussure’s work. -
Nash, Leonard K. Plants and the Atmosphere. Harvard Case Histories in Experimental Science. Cambridge, Mass.: Harvard University Press, 1952. A well-written, authoritative account of the basic discoveries in photosynthesis from about 1650 to 1804. -
Reed, Howard S. A Short History of the Plant Sciences. Waltham, Mass.: Chronica Botanica, 1942. Includes a detailed account of the development of the understanding of plant nutrition.
Dalton Formulates the Atomic Theory of Matter
Schwann and Virchow Develop Cell Theory
Liebig Advocates Artificial Fertilizers
Darwin Publishes On the Origin of Species
Luther Burbank; Ferdinand Julius Cohn; Charles Darwin; Joseph-Louis Gay-Lussac; Asa Gray; Justus von Liebig.