Allelopathic effects of Raphanus raphanistrum and interactions of effects on wheat and oat

Abstract

Raphanus raphanistrum L. (wild radish) is an important weed of grain crops worldwide. Allelopathic activity against several species has been reported in association with wild radish shoots and wild radish-amended soils. Therefore, assuming that allelopathy is an important and primary trait of wild radish weediness, two hypotheses can be derived: 1) high levels of phytoactivity of wild radish against crops where it ranks high as a weed are to be expected and 2) synergy of effects between compounds extracted from leaves and stems should also be widespread given the advantage provided by the lesser investment needed to attain the same result. Wild radish leaves (L), stems (S), and leaves plus stems (LS) were grounded, extracted with distilled water and bioassayed in wheat and oat germination and early growth to test these hypotheses. In wheat, germination was reduced by the three extracts, root growth by L- and LS-extract, leaf growth was unaffected, and leaf/root length ratio increased by S- and LS-extract. In oat, germination was unaffected, root growth reduced by the three extracts, leaf growth was unaffected, and leaf/root length ratio increased by the three extracts. Germination is only reduced in wheat, by as much as 61%, while early root growth of wheat and oat is strongly reduced by as much as 76% in wheat and 81% in oat, strongly supporting the first hypothesis. Conversely, no support was found for the second hypothesis. Synergy between leaf and shoot extracts of wild radish never occurred, only antagonism and zero interactions were found, with antagonism more frequent and more intense in wheat than in oat. Therefore, important as it may be for its success as a weed, allelopathy is very likely to be a by-product of other primary and more important roles of secondary metabolites produced by wild radish.