   Natural Herbicides from Crop Plants - April 4, 2007 Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County Some plants secrete chemicals from their leaves and roots that discourage or kill other plants growing near them. Pliny the Elder, a Roman natural science author, observed that black walnut had a toxic effect on neighboring plants and wrote about it in 77 A.D. Since then, scientists have been studying this phenomenon called allelopathy. Recent research has demonstrated that allelopathic compounds exist in many crop and ornamental plants and have the potential to reduce use of herbicides in agricultural settings, parks, and home landscapes. Fine fescue is a widely grown turfgrass. It is often mixed with Kentucky bluegrass and ryegrass for a cool season turfgrass blend. Certain cultivars (plant of a species having distinct characteristics) of fine fescue have been shown to prevent weeds from growing because they exude a form of an amino acid called m-tyrosine from their roots. Weeds growing among these fine fescue plants readily absorb m-tyrosine mistaking it for regular tyrosine (an essential amino acid). The weed’s roots soon become deformed and stunted causing death in the weed. The implication is that where theses grasses are planted, herbicides will be greatly reduced or not need to be applied. Many cereal grains, including rice, wheat, corn, and barely, also exude allelochemicals from their roots. Researchers have focused allelopathy research on rice because it is a staple in many parts of the world and often has serious weed problems. Weeds reduce crop yield by using soil nutrients and competing for water and sunlight. In addition, many of the weeds that reduce rice yields have become resistant to commercial herbicides. However, a few rice cultivars have allelopathic compounds with the ability to kill barnyard grass: a pesky weed for rice growers. There is still much work to be done as the known weed suppressing varieties have poor grain quality and low yields, but many rice varieties have yet to be tested for allelopathic qualities. This research is time consuming and expensive, but scientists are optimistic. The USDA’s Natural Products Utilization Research Unit in Mississippi is focused on allelopathic qualities of sorghum. Sorghum root hairs produce an oily secretion called sorgoleone. The sorgoleone is produced in very minute quantities just as it leaves the plant root hair. This is a touchy issue because the sorgoleone is also toxic to the parent plant. Once this was discovered, scientists genetically altered the plant for enhanced sorgoleone production. The idea is that organic farmers could plant sorghum during their normal rotation or sparingly interplant sorghum with other crops to decrease weeds. The USDA team is now considering genetic engineering of other crop plants so that they can also produce sorgoleone. Of course, this will also be controversial as there are many people that are opposed to genetic engineering. In addition to genetic engineering, scientists are thinking about synthetically producing the allelopathic compounds found in plants. This much like the processes currently used to synthesize pyrethroid insecticides with the same chemical structure as pyrethrin found in perennial chrysanthemum (Chrysanthemum cinerariaefolium). Pyrethrins and pyrethroids are used in flea collars, garden insecticides, and head lice shampoos. Some work has been done with allelopathic compounds released by wheat and researchers have discovered that by adding a short chain of carbon molecules, its weed-fighting potency increases 1,000-fold. In this case, the parent plant is immune to the allelopathic compounds because it can detoxify the compounds the same as it can the naturally produced version. Synthesis of these compounds also shows promise for agricultural applications. What do you know - Pliny the Elder was on the right track almost 2,000 years ago. The information for this article came from Science News March 17, 2007 (Vol. 171, No. 11). If you are like me and enjoy science but don’t have time (not to mention the vocabulary and patience) to read journal articles, then I highly recommend Science News (www.sciencenews.org). The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |