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.Skip to next paragraph
Subscribe Today to the Monitor
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.