Plant 'sight' from pores and pumps.

To a plant, breathing involves a built-in cost-benefit analysis. The wider the gas-exchanging pores on the leaf surface are open, the greater the supply of carbon dioxide for photosynthesis. But wide-open pores allow evaporation of water, so the plant must balance the benefits of increased carbon dioxide against the cost of water loss.

Key to this balancing operation is a light-sensing process, one of several factors that stimulate the opening of the plant pores, called stomata. It allows plants to perceive their environment, in a manner similar to the way animals see, says Eduardo Zeiger of Stanford Universiy. This mechanism provides a plant with an increased supply of carbon dioxide only when light is available to power photosynthesis. Ziegier and his colleagues now report unexpected characteristics of central components of this process, and they are beginning to apply their findings to impoving crop yields.

The light-sensing operation that opens pores is distinct from the absorption of light used directly for photosynthesis. Blue light directly triggers the pore response, whereas both blue and red light are absorbed by chloroplasts, the structures that carry out photosynthesis. This difference has allowed Zieger to characterize the stomatal response.

Under some conditions, blue light can dramatically stimulate plant growth, Zeiger reports. He has worked with orchids, which grow slowly because they open their pores only slightly. With scientists at the Smithsonian Environmental Research Center in Rockville, Md., Zeiger grew orchids in either artificial sunlight or light having double the natural amount of blue. The stomata did open wider under the blue-light-enriched condition, and those plants grew 50 to 70 percent more in three weeks.

A surprising mechanism underlies the stomatal response to blue light. The light turns on a pump that removes protons--positively charged hydrogen ions--from a cell. Such pumps had previously been observed only in certain bacterial (SN: 2/25/78, p. 119).

The light-sensitive pump occurs in the guard cells, which surround the gas-exchange pore. The pump, when active, creates an electrical gradient across the cell membrane. This gradient pulls potassium ions and water into the cells. The cells then swell, opening the pore.

The proton pump of guard cells was reported in the Nov. 21 NATURE by Sally M. Assmann, a colleague of Zeiger's at Stanford, and Luciana Simoncini and Julia I. Schroeder of the Max Planck Institute for Biophysical Chemistry in Gottingen, West Germany.

In addition, the guard cells of most plants contain rudimentary chloroplasts, which also absorb light. Scientists disagree over whether these function or are simply evolutionary vestiges of primitive plants. Zeiger and his co-workers now have isolated guard cell chloroplasts and report that they convert light into a cellular fuel, ATP, as do the functional chloroplasts elsewhere in the plants. Zeiger hypothesizes that the guard cell chloroplasts fuel the light-activated proton pump. "This is very controversial," he says.

Zeiger and Paul Moore of the U.S. Department of Agriculture in Aeia, Hawaii, are applying their new knowledge about how stomata function to the breeding of sugar cane. They hope to increase the carbon dioxide supply and reduce water loss.