Hydroxamic Acid Content and Toxicity of Rye at Selected Growth Stages

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 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 previously. (2R)-2-beta-D-Glucopyranosyloxy-4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA-G), 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA), benzoxazolin-2(3H)-one (BOA), and the methoxy-substituted form of these compounds, (2R)-2-beta-D-glucopyranosyloxy-4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA glucose), 2,4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), and 6-methoxy-benzoxazolin-2(3H)-one (MBOA), were all detected in rye tissue. DIBOA and BOA were prevalent in shoot tissue, whereas the methoxy-substituted compounds, DIMBOA glucose and MBOA, were 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 a 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 the phytoxicity of extracts derived from rye at early growth stages, but other compounds are probably responsible in later growth stages.

Cover crop residue management for optimizing weed control

“…The ED 50 was higher in the one-week older residues used in experiment 3b, indicating a lower allelopathic potential of these residues. This coincides with several publications that state that the hydroxamic acid concentration in rye decreases with age (Reberg Horton et al 2005;Rice et al 2005;Kruidhof et al 2008a). It also coincides with the observations in the field, where lettuce emergence in the treatments with soil-incorporated residues was, on average, less inhibited in experiment 3b than in experiment 3a.…”

Short-term plant species impact on microbial community structure in soils with long-term agricultural history

“…Rye, for example, has been shown to produce alleopathic hydroxamic acids (Rice et al 2005). Related compounds release by legume roots have also been shown to detrimentally impact plant microbe interactions related to symbiosis and pathogenesis (Carlsen and Fomsgaard 2008).…”

The Allelopathic Potential of Hairy Vetch (Vicia villosa Roth.) Mulch

“…Contrary to hairy vetch, vegetative stage fall rye and winter wheat had the greatest allelopathic effect on germination when compared to the corresponding reproductive stage extracts; likely related to higher hydroxamic acid concentration in vegetative tissue [36]. This shows potential for fall rye or winter wheat to be used as a cover crop incorporated at early growth stages for utilization of allelopathy for weed suppression.…”

“…We did not evaluate known allelochemicals in fall rye or winter wheat because these putative benzoxazinoid allelochemicals and their derivatives are relatively unstable and not readily available aside from chemical synthesis [9] [12] [37]. Interestingly, reproductive stage fall rye had the greatest stimulatory effect on radicle elongation of response species; likely due to hormesis caused by the decrease of hydroxamic acids in fall rye tissue throughout its development [4] [9] [36]. Reproductive stage fall rye and winter wheat mulches expressed potential for utilization for species-specific growth promotion.…”