Emerson, Robert

Emerson, Robert

(b. New York, N.Y., 4 November 1903; d. New York, 4 February 1959)

plant physiology.

After studying at Harvard, Emerson received his doctorate under Otto Warburg at Berlin in 1927 and joined the California Institute of Technology biology department in 1930. From 1937 to 1940 he worked at the Carnegie Laboratory of Plant Biology at Stanford, California. After returning to Cal Tech, Emerson spent the World War II years working with Japanese deportees from the West Coast, attempting to develop rubber production from guayule, a Mexican desert shrub. In 1947 he became director of the newly founded photosynthesis research laboratory associated with the botany department of the University of Illinois in Urbana, which he built into one of the leading research laboratories in this field. He died, at the height of his research career, in a plane crash in the East River, off New York City. In 1949 Emerson received the Stephen Hales Award of the American Society of Plant Physiologists, and in 1950 he was elected to membership in the National Academy of Sciences.

In appearance and character Emerson was a typical New Englander; tall, lean long-headed, self-denying, hard-working, expecting (and appreciating) hard work in others. He exerted great influence on his co-workers and students. A pacifist and believer in democratic socialism, he was always defending the underdog—working with deported Japanese, fighting housing discrimination, befriending students from Africa and India. Emerson was a perfectionist in experimental research and skillful in manual work, including cabinetmaking and gardening. He combined pride in the quality of his own work and critical rejection of less careful work with great modesty and deep respect for the achievements of others.

Emerson’s lifelong concern was the precise, quantitative study of photosynthesis—the basic process of life on earth by which organic matter is synthesized by plants from water and carbon dioxide with the aid of light absorbed by plant pigments (of which the green pigment chlorophyll is the most important and ubiquitous).

In 1937 Emerson set out to check the conclusions of his teacher Otto Warburg that plants can synthesize sugar (glucose), using only four light quanta (photons) for each molecule of carbon dioxide utilized and of oxygen liberated. This suggested a remarkable efficiency of the process—conversion of up to 30 percent of the absorbed light energy into chemical energy of the products. Steadily improving the measurement techniques and systematically determining the quantum requirement of photosynthesis in green, brown, red, and blue-green algal cells, in monochromatic light of widely different wavelengths, Emerson arrived at a number of important conclusions.

1. The minimum quantum requirement of photosynthesis for all plants is not four but eight. This conclusion led to a drawn-out controversy with Warburg, in which Emerson’s conclusions were gradually accepted as correct by most workers in the field, although not by Warburg himself.

2. Quanta absorbed in chlorophyll a, chlorophyll b, and the red and blue phycobilin pigments of certain algae are about equally effective in producing photosynthesis. Light quanta absorbed by the yellow pigments (the carotenoids) have a much smaller efficiency—with the exception of a special carotenoid, fucoxanthol, present in brown algae and diatioms.

3. At the longwave end of the absorption band of chlorophyll a (above 680 nm in green cells and above 650 nm in red algae) the yield of photosynthesis drops sharply (“red drop”); it can be restored to normal by additional illumination with shortwave light (the “Emerson effect”).

This last result has become one of two main foundations of the now widely accepted theory according to which photosynthesis involves two successive photochemical processes brought about by two pigment systems. Light absorption in one system oxidizes water, liberating oxygen and reducing an intermediate product (perhaps a cytochrome); light absorbed in the other system reduces carbon dioxide (sugar is the ultimate product), oxidizing the intermediate product that had been reduced by the first system. Each of the two steps requires four quanta (to move four hydrogen atoms “uphill,” that is, with an increase in chemical energy), which explains the total quantum requirement of eight. In the region of the red drop, too many quanta are absorbed in one pigment system and not enough in the other; this can be corrected by supplementary shortwave illumination.

Another important work of Emerson’s (together with William Arnold) dealt with photosynthesis in flashing light. Since photosynthesis involves one or two photochemical steps, preceded and followed (and also separated) by nonphotochemical, enzymecatalyzed “dark” reactions, the study of photosynthesis in flashing light permits the separation of the light stage from the dark stage. By varying the dark interval between flashes, Emerson proved that the dark stage needs about 0.01 second for its completion at room temperature. Another important result was to show that a single, intense flash of light can produce, in normal healthy plants, only one molecule of oxygen (and reduce one molecule of carbon dioxide) per approximately 2,500 chlorophyll molecules present. This finding became the starting point of the theory of the photosynthetic unit, which postulates the association in plant cells of about 300 chlorophyll molecules (2,500 divided by 8) with a single reaction center (an enzyme molecule) to which the light energy absorbed in any one of the 300 associated pigment molecules is conveyed by a special physical mechanism (resonance energy migration). This is one of the basic concepts of the present-day theory of photosynthesis.

Emerson carried out all his experiments himself, alone or with a trusted assistant. Among his students and co-workers were William Arnold, Charleton Lewis, Shimpe Nishimura, Mrs. Marcia Brody, Carl Cederstrand, and Mrs. Ruth Chalmers.

BIBLIOGRAPHY

I. Original Works. Emerson’s principal writings include “A Separation of the Reactions in Photosynthesis by Means of Intermittent Light,” in Journal of General Physiology, 15, no. 4 (1932), 391–420, written with W. Arnold; “The Photochemical Reaction in Photosynthesis,” ibid., 16, no. 2 (1932), 191–205, with W. Arnold; “Photosynthesis,” in Annual Review of Biochemistry, 6 (1937), 535–556; “Carbon Dioxide Exchange and the Measurement of the Quantum Yield of Photosynthesis,” in American Journal of Botany, 28, no. 9 (1941), 789–804, with C. M. Lewis; “The Dependence of the Quantum Yield of Chlorella Photosynthesis on Wave Length of Light,” ibid., 30, no. 3 (1943), 165–178, with C. M. Lewis; “Some Factors Influencing the Long-wave Limit of Photosynthesis,” in Proceedings of the National Academy of Sciences of the United States of America, 43 (1957), 133–143, with R. Chalmers and C. Cederstrand; “The Quantum Yield of Photosynthesis,” in Annual Review of Plant Physiology, 9 (1958), 1–24; and “Red Drop and Role of Auxiliary Pigments in Photosynthesis,” in Plant Physiology, 35 (1960), 377–485, with E. Rabinowitch.

II. Secondary Literature. On Emerson and his work, see E. Rabinowitch and Govindjee, Photosynthesis (New York, 1969).

Eugene Rabinowitch