Science: Rabinowitch’s Nightmare

The mainspring of life is photosynthesis, the process by which plants manufacture food out of carbon dioxide-and water under the influence of sunlight. So one of the problems of biology is to learn as much as possible about photosynthesis. If the process could be made more efficient, the world’s food supply would take a large jump. Since photosynthesis depends on the energy of sunlight, it stops when a plant is in darkness. In fact it runs backward. A plant respires (breathes) like an animal, absorbing oxygen and giving off carbon dioxide, and biologists have assumed that the plant respires in sunlight, too. No one could prove it, however, because the effect of respiration (CO2 given off) is masked by the effect of photosynthesis (CO2 absorbed). The difficulty of measuring the daytime respiration rate is called “Rabinowitch’s nightmare.”* For years it haunted biologists, who compared it to the problem of finding out if the refrigerator light is shining after the door is closed. Last week Dr. John Decker of the Brooklyn Botanic Garden announced that he has punched a hole in Rabinowitch’s problem. He enclosed one leaf of a tobacco plant in a glass chamber fitted with sensitive instruments for measuring CO2 in the air. The amount of CO2 decreases when the leaf is photosynthesizing under a strong electric light. It increases when the plant is respiring in darkness. Dr. Decker’s experiment: measuring what happens after the light is turned off. For seconds, he found, the leaf gave off a large amount of C02. Then the figure fell to the normal amount given off in darkness. Dr. Decker believes that photosynthesis stops when the light fails, but that the daytime respiration that accompanies photosynthesis does not stop quite so suddenly. “It is like catching the leaf,” he says, “with its pants down before it can adjust to darkness.” Dr. Decker’s method with Rabino-witch’s nightmare may have considerable implications. A plant’s production of food is the net result of its photosynthesis minus its respiration. Dr. Decker hopes to breed hybrid plants that breathe only slightly in sunlight, permitting photosynthesis to manufacture more food.

* From Eugene Rabinowitch, authority on photosynthesis, now editor of the Bulletin of the Atomic Scientists.