Quest for more efficient crops
Strange-looking sugar beets with a crown of yellow leaves have given Norman Terry a new insight into photosynthesis, the process that plants use to convert sunlight into food.
A plant physiologist with the University of California here, Dr. Terry has produced the first solid evidence that a plant's ability to utilize light for food production is limited by the first step in the photosynthetic process, called the ''light reactions.'' These are the steps in which the plant converts light into chemical fuel.
Through his experiments, Terry has opened an avenue for photosynthesis research that hasn't even been considered in the past. Until now, researchers have generally held that the limitations that restrict the efficiency of plants' photosynthetic processes were centered in the second half of the process, the ''dark reactions,'' where the plant uses its stored chemical energy to produce sugars from carbon dioxide and water.
Photosynthesis research is generally motivated by an assumption that making plants use sunlight more efficiently will ultimately lead to increased crop yields on the world's farms. (Currently, most crops convert only about 5 percent of the light reaching them into food for the plant.) In the last decade, increasing emphasis has been given to this and other biological approaches because of evidence that traditional methods of increasing yields - use of fertilizers, pesticides, machinery, and irrigation - are approaching the limit of their usefulness.
In pursuing the goal of more efficient plants, other researchers have been looking to genetic engineering. If successful, Terry's work holds the promise of developing such plants through traditional breeding techniques, a much simpler process.
Terry's painstaking work began more than a decade ago with the study of the effect that various nutrient deficiencies have on the photosynthetic ability of sugar beets. He discovered that starving these plants for iron selectively inhibits the production of membrane sacs, called thylakoids. These sacs hold chlorophyll molecules within the leaf. Chlorophyll molecules act as a plant's solar collectors, absorbing energy from sunlight. The thylakoids hold the ''machinery'' the plant uses to convert this energy into chemical form. By varying the amount of iron in a plant's diet, Terry found he could systematically vary the amount of chlorophyll and number of thylakoids over a wide range in order to determine the effect on the overall photosynthetic process.
To do this, Terry grew sugar beets hydroponically. For the first several weeks the beets were fed a balanced diet so they developed normal, dark-green leaves. Then he switched their diet to one deficient in iron.
''The beets then produced leaves normal in every respect, except for being deficient in chlorophyll and thylakoids,'' the scientist says. He measured the chlorophyll content of these yellow leaves and their effect on the entire plant's rate of photosynthesis. ''We found that even reducing the thylakoid/chlorophyll content slightly reduced the rate of photosynthesis,'' he says.
This flies in the face of the generally held assumption that the light reactions are not a limiting factor. If this were the case, then it would take major reductions in thylakoid/chlorophyll content to have a noticeable effect on overall photosynthesis.
Terry's work remains controversial with ''dark reaction'' researchers like Bill Ogren of the US Department of Agriculture. Based on the observation that increasing the amount of carbon dioxide in a plant's air supply increases its photosynthetic efficiency, he has pioneered efforts to understand and increase the efficiency of the largest enzyme known, carboxylase, which creates sugars out of carbon dioxide, water, and a plant's stored chemical energy. ''I've looked at many crop plants, with large variations in chlorophyll levels, and haven't seen any relation to photosynthesis,'' Dr. Ogren says.
According to Terry, it does not appear to be a leaf's chlorophyll content that limits photosynthesis, but the number of thylakoids, which is closely related. For the last two years, the scientist has been using this as the basis of an attempt to breed sugar beets with improved rates of photosynthesis. Although he is not yet ready to discuss the results, Terry reports that so far they have been encouraging.
Even if his latest efforts succeed, Terry cautions that breeding plants that make more efficient use of light does not automatically translate into plants with improved yields.
''So far, there is little indication that improving photosynthesis will actually increase yields,'' Terry warns.
This is still the hope, though, he acknowledges. And even if that hope is not realized, his research will help determine just how far mankind can tinker with the results of millions of years of plant evolution in order to feed his growing numbersmore efficiently.