Physiology, History of

The history of physiology—the discipline concerned with the functioning of plants—can be organized around the discovery of several key processes.

One of the first physiological questions to be studied scientifically was how plants obtain food. Although we now know that plants manufacture carbohydrates from carbon dioxide and water via photosynthesis, the ancient Greeks reasoned that a plant's food must come from the soil. This idea persisted until the 1600s, when Jean Baptiste van Helmont performed an experiment in which he carefully weighed a pot of soil and planted a willow seedling in it. Over a period of five years he added nothing but water to the pot, and the willow grew into a tree weighing over one hundred pounds. When he cut down the tree he found that the soil weighed the same, less about two ounces, as when he began the experiment. Thus, the soil could not be the source of the plant's food. Van Helmont concluded it could have come only from the water he added.

The idea that air could be utilized by plants was first suggested by Stephen Hales in the early 1700s. Hales noticed bubbles exuding from the cut ends of stems and reasoned that air might enter the plant through its leaves and circulate to other organs. At that time air was considered a uniform substance, and it was not until the late 1700s that Joseph Priestley found that air in a closed container could be altered by a burning candle or a living animal such that the flame would be extinguished and the animal would die. However, the presence of a plant in the container kept the candle burning and the animal alive. Priestley's results were the first to demonstrate that plants produce oxygen, now known to be a product of photosynthesis. Consequently, Jan Ingenhousz showed that oxygen is produced only by green parts of plants (and not roots, for example) and only in the light.

The remainder of the photosynthetic equation was elucidated largely by Nicholas de Saussure, who showed that during photosynthesis carbon dioxide is converted to organic matter, approximately equal amounts of carbon dioxide and oxygen are exchanged, and water is a reactant. In addition, Julius von Sachs, considered the founder of modern plant physiology, demonstrated that chlorophyll, located in chloroplasts , is involved. Thus, by the late 1800s photosynthesis could be summarized as follows:

In the 1930s C. B. van Neil suggested that the oxygen released in photosynthesis came from water rather than from carbon dioxide, and this was verified in the 1940s using radioisotopes . Details concerning the role of light were worked out by Robin Hill, Robert Emerson, and Daniel Arnon, and the reactions by which carbon dioxide is converted to carbohydrate were elucidated by Melvin Calvin and his colleagues in the early 1950s.

Mineral Nutrition and the Transport of Water, Minerals, and Sugars

It had long been known that water, along with dissolved minerals, enters a plant through its roots. Sachs demonstrated that plants do not require soil and can be grown in an entirely liquid medium as long as the medium contains the minerals required for survival. This technique of hydroponics facilitated studies of the mechanisms for mineral uptake by the roots.

Another contribution of Hales was to demonstrate how water is transported in the plant. Hales established that water passes upward from the roots to the leaves, where it is lost to the atmosphere by the process of transpiration . But it was not until 1895 that Henry Dixon and John Joly proposed the cohesion theory to explain how transpiration causes water and dissolved minerals to be pulled upward through the xylem.

The transport of carbohydrates was found to take place by a different mechanism. In the late 1600s Marcello Malpighi noticed that when the bark was removed in a ring around a tree the portion of the bark above the ring increased in thickness while the portion below the ring did not. Because ringed trees continue to transpire, the ringing process apparently did not hinder water transport but instead prevented the transport of other substances necessary for growth. Later it was shown that bark contains phloem tissue, which transports sugars from the leaves to other plant parts. The mechanism of sugar transport, termed translocation, was a mystery until 1926, when E. Münch proposed the pressure-flow model, in which the osmotic entry of water into the phloem generates a hydrostatic pressure that pushes the dissolved carbohydrates both upward to the shoot tip and downward to the roots.

Plant Hormones, Environmental Physiology, and Molecular Genetics

In the late 1800s Sachs suggested that the formation of roots and shoots was controlled by internal factors that moved through the plant. The first such factor, the plant hormone auxin, was discovered in 1928 by Fritz Went, building on experiments with phototropism by Charles and Francis Darwin, Peter Boysen-Jensen, and Arpad Paál. Went found that phototropism, the process by which stems bend toward the light, is the result of auxin migrating from the illuminated side of a coleoptile to the shaded side, where it stimulates growth. Over the next decades other plant hormones—most notably the gibberellins, cytokinins, ethylene, and abscisic acid—were discovered. Together with auxin, they regulate almost every aspect of plant growth and development.

In the 1950s emphasis shifted to biochemical mechanisms underlying physiological and developmental processes. Particularly important was the discovery by Harry Borthwick and Sterling Hendricks in 1952 of phytochrome, a pigment involved in a variety of developmental responses including flowering, seed germination, and stem elongation. In addition, there was a trend toward environmental physiology, a discipline in which the methods of plant physiology are applied to the problems of ecology, including plant responses to extremes of cold, salt, or drought.

The 1970s introduced the era of molecular genetics. Plant physiologists use molecular genetics to localize and identify the genes on a chromosome, understand the mechanisms by which genes are expressed, and elucidate the processes involved in coordinating the expression of genes in response to environmental signals.

see also Calvin, Melvin; Darwin, Charles; de Saussure, Nicholas; Genetic Mechanisms and Development; Hales, Stephen; Hormones; Hydroponics; Ingenhousz, Jan; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Physiologist; Physiology; Phytochrome; Sachs, Julius von; Translocation; Tropisms; van Helmont, Jan Baptiste; van Niel, C. B.; Water Movement.

Robert C. Evans