Global Warming Puts the Freeze on Seed Yield (to a Degree)

While research on the effects of greenhouse gasses continues, studies by Agricultural Research Service scientists and others show that higher carbon dioxide levels increase forage crop yields over a range of temperatures--and seed crop yields when temperatures are optimal. But research also shows that the global change some are predicting could leave seed crops partly sterile.

The threat to cereal grains and legumes is the potential rise in temperatures, according to L. Hartwell Allen, Jr., a soil scientist in the ARS Crop Genetics and Environmental Research Unit, Gainesville, Fla.

Seed productivity decreases about 10 percent for every 2 degrees Fahrenheit the temperature increases above ideal levels, according to Allen. But photosynthesis and plant size are little affected until much higher temperatures are reached. Even when seed development fails, the plant may still grow to its typical size.

In elevated-temperature studies, pollination of individual flowers either fails completely or, when fertilization is successful, seeds of some crops develop poorly. For example, soybean yields are reduced because fewer seeds are produced, and individual seeds weigh less.

Allen and ARS plant physiologist Joseph Vu, University of Florida crop physiologist Kenneth Boote, and postdoctoral scientist Vara Prasad found that several physiological functions related to reproduction and pollination fail as temperatures increase. Abnormally high temperatures during the 2- to 3-week pollination season could affect seed development at exactly the wrong time in a plant's life.

Traditional breeding could help if scientists locate wild relatives--or even current cultivars--that produce high seed yields in very hot environments and incorporate their tolerance into otherwise productive varieties. Cultivars that shed pollen earlier in the day, when temperatures are cooler, would be more likely to flourish, according to Allen. Recent ongoing research indicates that some high-temperature tolerance does exist in certain rice cultivars. Genetic engineering could help scientists introduce desirable genes from other plants.

ARS is the U.S. Department of Agriculture's chief scientific research agency.