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Air pollution and seed
by Bob Hartzler
1997 - A common characteristic of almost all annual weeds is seed dormancy. This trait insures that once a weed becomes established in a field, it is unlikely that it will be eradicated. Although this has been an intense area of research, our understanding of the factors that break dormancy in the field is limited. An article in the 23 May issue of Science provides an interesting perspective on this complex issue.
There are numerous plant species that quickly invade areas after the site has been burned. These species are best adapted to bare areas free of competition from other species, thus fire creates an ideal envirnonment for these plants. Many of these species have hard seed coats that are impervious to water. Heat from fire cracks the seedcoat, allowing moisture to seep into the seed and initiate growth. However, in the late 1970's scientists recognized that chemicals present in smoke could trigger germination in the absence of heat. Since that time dozens of smoke-germinated species have been identified around the world.
J. Keeley and C. Fotheringham of Occidental College in Los Angeles report that nitrogen dioxide, a common substance in smoke, triggers germination of yellow whispering bells seeds. Exposure to the compound for as little as a minute can trigger germination. The researchers found that exposure to nitrogen dioxide increased the permeability of a subdermal membrane, which might be involved in triggering germination.
Weed scientists have searched, largely unsuccessfully, for dormancy-breaking chemicals. If found, these chemicals could be used to aid in depleting weed seed banks, therefore simplifying weed management. Ethylene has been used in the southeast U.S. as a management tool for witchweed, a parasitic plant that attacks corn. Ethylene is injected in infested fields during years non-host crops are planted. Seedlings that emerge due to the ethylene treatment quickly die due to the lack of host corn plants.
The sensitivity of the seeds to nitrous oxides has raised questions about the potential impact of air pollution on native plants. Nitrous oxides found in air pollution could trigger emergence of sensitive seeds during conditions which are unfavorable for their survival. Over time the seed bank might be depleted of these species, thus when an area was burned the plants would not be regenerated. This could leave the burned areas bare for a longer period, increasing erosion and other risks.
Keeley, J. E. and C. J. Fotheringham. 1997. Trace gas emissions and smoke-induced seed germination. Science 276:1248-1250.
Malakoff, D.A. 1997. Nitrogen oxide pollution may spark seed's growth. Science 276:1199.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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