|Abdul Baki, Aref|
Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2005
Publication Date: 8/1/2005
Citation: Park, Y.B., Adam, F., Rice, C., Abdul Baki, A.A., Teasdale, J.R. 2005. Hydroxamic acid content and toxicity of rye at selected growth stages. Journal of Chemical Ecology. 31(8):1887-1905. Interpretive Summary: Rye is an important cover crop that provides many benefits to cropping systems including protecting soil from erosion, preventing losses of nutrients into surface or ground waters, and suppressing weeds and pests. Weed and pest suppression has been attributed to release of toxic compounds from rye tissue or from the decomposing residues of rye. A family of chemical compounds known as hydroxamic acids have been identified as an important class of toxic compounds in rye. This research was conducted to improve the methodology for quantifying hydroxamic acids and to determine the relationship between hydroxamic acid content and toxicity of extracts of rye root and shoot tissue harvested at selected growth stages. Improved methodology was developed that lowered the detection limits for identifying these hydroxamic acid compounds. Total hydroxamic acid concentration in rye tissue generally declined with age. Water extracts of young rye seedlings were most toxic to growth of indicator plant species, but there was no pattern to the toxicity of extracts of rye sampled at later growth stages and there was no correlation of hydroxamic acid concentration and toxicity to indicator species. Results suggested that hydroxamic acids may account for toxicity of extracts derived from rye at early growth stages but unknown compounds were probably responsible for the toxicity of extracts of rye sampled at later growth stages. These results will aid chemists by improving methodology for analyzing hydroxamic acids in plants, and will direct scientists to search for additional toxic compounds that can explain toxicity of rye at all stages of growth.
Technical Abstract: Rye (Secale cereale L.) is an important cover crop that provides many benefits to cropping systems including weed and pest suppression resulting from allelopathic substances. Hydroxamic acids have been identified as an important class of allelopathic compounds in rye. This research was conducted to improve the methodology for quantifying hydroxamic acids and to determine the relationship between hydroxamic acid content and phytotoxicity of extracts of rye root and shoot tissue harvested at selected growth stages. Detection limits for an LC/MS-MS method for analysis of hydroxamic acids from crude aqueous extracts were better than have been reported by others. DIBOA-glucose, DIBOA, BOA and the methoxy-substituted form of these compounds, DIMBOA-glucose, DIMBOA, and MBOA were all detected in rye tissue. DIBOA and BOA were most prevalent in shoot tissue whereas the methoxy-substituted compounds, DIMBOA-glucose and MBOA, were most prevalent in root tissue. Total hydroxamic acid concentration in rye tissue generally declined with age. Aqueous crude extracts of rye shoot tissue were more toxic than extracts of root tissue to lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum Mill.) root length. Extracts of rye seedlings (Feekes growth stage 2) were most phytotoxic but there was no pattern to the phytotoxicity of extracts of rye sampled at growth stages 4 to 10.5.4 and no correlation of hydroxamic acid content and phytotoxicity (I50 values). Analysis of dose-response model slope coefficients indicated lack of parallelism among models for rye extracts from different growth stages suggesting that phytotoxicity may be attributed to compounds with different modes of action at different stages. Hydroxamic acids may account for phytoxicity of extracts derived from rye at early growth stages but other compounds are probably responsible for phytotoxicity of extracts of rye sampled at later growth stages.