Drought-resistant plants: the case of the hairy soybean
Concerned about recurrent drought, some Midwestern scientists are trying to develop new crop plants that can grow with less water.
Such plants would be one of the best long-term defenses against dry spells, says Dr. Norman J. Rosenberg of the University of Nebraska. However, he explains , while varieties that use less water continue producing crops when other plants have died, such crops still must compete with today's high-yielding varieties. Farmers would be loath to trade yield for drought resistance.
The need to reduce a plant's dependence on soil moisture without losing other desirable qualities such as yield or disease resistance adds to the difficulty of developing plants for semiarid farming. However, Dr. Rosenberg says that a new variety of soybean he is developing -- a hairy soybean--may meet this standard.
Describing this potential crop plant during the recent annual meeting of the American Association for the Advancement of Science, he explained that plants use less water and conserve water better if they have certain structural differences from many commonly planted varieties.
For example, plants that have leaves with many hairs on their surface reflect more sunlight than plants without such hairs. Therefore, the hairy plants photosynthesize less and also use less soil water for manufacturing food. This reduced photosynthesis does not decrease the yield of many plants. Thus, in trying to confer drought resistance on a crop plant such as the soybean, one might try to develop a hairy-leafed variety.
Such a soybean was bred by two University of Nebraska researchers, Dr. James E. Specht and Dr. James Williams, in 1979 and now is being tested further and developed by Dr. Rosenberg and his colleagues.
Dr. Specht explains that he thought of breeding a soybean with extremely hairy leaves after reading a study that described how several hairy desert cactuses retain water. He says he felt intuitively that hairiness might be ''a morphological trait associated with drought-tolerant species.''
Locating a wild soybean variety from China which had an extreme degree of hairiness, Drs. Specht and Williams crossed it with an experimental high-yielding American soybean. The result was a new variety differing only in one gene from the American plant. Yet, because this single gene determined leaf hairiness, the new plant was four times hairier than its American parent. Also the scientists bred the new plant for white hairs because white reflects sunlight better than darker colors.
Dr. Rosenberg, an agricultural climate specialist, Shashi Verma, and Blaine Blad then joined the plant breeders to put the new hairy soybean through two years of field tests. It passed these successfully, showing that it uses 15 percent less water from the soil than the American variety. That is enough to prevent large plantings of the crop from using all the soil's water and dying during moderate droughts.
At the same time, the new soybean produces the same amount of food as does its high-yielding American parent. Because soybean plants do not require full sunlight, the crops' food value remains as high as that produced by conventional soybean plants, even though its white hairs reflect more of the sunshine.
Encouraging as this is, the hairy soybean is not yet ready to be given to farmers. Those results come from just the first two years of research in what will probably be at least a 10-year project. Now the Chinese hairy soybean is being crossed with today's most popular, highest-yielding American soybean variety. The eventual variety developed after many crossings will undergo a new series of field tests. These will determine the new plant's ability to withstand droughts of varying intensities, durations, and repetitions.
If these field tests prove successful, the result will be 10 pounds of seed to be further propagated by Nebraska's Foundation Seed Division. This will yield 6,000 pounds of seed. Next, the seed will be released to certified seed growers, who will produce the high quantity of seed that soybean farmers need. If all of these steps are taken smoothly, the seed should be available to soybean farmers who live in the driest, most drought-prone areas of the Great Plains around 1990 .
Only 15 percent of US soybeans are grown in the Great Plains. However, this still is major market for the new soybean. Moreover, drought resistance for this part of the crop should have important commercial consequences. During 1979, a record year nationwide for soybean production, the Great Plains crop netted almost $14 billion.
Also, the new variety could capture the interest of Great Plains farmers who now grow other highly water-dependent crops. Dr. Specht hopes that farmers growing such water-intensive crops as corn in the region west of Lincoln, Neb., which is drier even than the ''soybean belt,'' will replace a portion of their crops with the new soybean. On the other hand, Dr. Rosenberg says he hopes farmers will not use the drought resistance of the new soybean as a reason for tilling extremely dry rangelands not yet uncultivated. Turning over the soil of untilled ground to grow any crop increases the land's susceptibility to drought, he explains.
Meanwhile, research to confer drought resistance on other Great Plains crop plants is less advanced.
At Montana State University Drs. Hayden Ferguson, Jarvis Brown, Robert Eslic, and C. S. Cooper have changed the structure of barley leaves to decrease water use. They bred a golden-leaved barley, the color indicating that the plant contains less chlorophyll. Chlorophyll is a necessary ingredient in the photosynthesis process, and with less chlorophyll, the plant photosynthesizes less and uses less water. Dr. Brown notes that ''we were able to delay the soil's moisture depletion'' through the structural difference.
However, the golden-leaved barley's success has been sketchy. Although on several occasions of drought Dr. Brown and his associates found that the yield was 20 percent higher in golden-leaved barley than conventional barley, the result has not been consistent. Furthermore, under normal nondrought conditions the golden barley has not produced more seed than the popular variety.
From these results, the University of Montana scientists believe that further research is not warranted. The researchers do not think farmers will use a new seed type unless the new variety can produce higher yields under all conditions, whether the growing season is wet or dry. Nevertheless, it represents a possible line of barley development that could be reinvestigated should the need arise.
Dry spells recur on a more or less cyclical basis. Droughts that range from relatively moderate to severe destroy crops and affect the related economy about every two decades in the Great Plains, notes Dr. Charles W. Stockton of the University of Arizona. While the 1930s ''dust bowl'' dry spell ''is the drought that conditioned American perception,'' says Dr. Rosenberg, even the moderate drought that accompanied the 1980 summer heat wave in the Southern states severely affected crops. Aridity two summers ago cost Nebraska $600 million in agricultural losses alone, explains Dr. Donald Wilhite, meteorologist with the University of Nebraska.
Crops with built-in drought resistance could do much to prevent such loses.