Evidence of Chemical Interactions on Germination and Seedling Growth in Annual Forage Species

Abstract

Poor establishment of forage legumes may result when they are sown with annual grasses in mixtures. Our objective was to determine the importance of allelopathy as a mechanism of interference between seed germination and seedling growth. Laboratory studies were conducted using two grasses (donor plants): oat (Avena sativa L.) and Italian ryegrass (Lolium multiflorum Lam.) and five annual legumes (target plants)-burr medic (Medicago polymorpha L.), berseem (Trifolium alexandrinum L.), crimson clover (Trifolium incarnatum L.), squarrosum clover (Trifolium squarrosum L.), and common vetch (Vicia sativa L.). The technique used legumes seeded into 7-day-old grass plants in a Petri dish with perlite, where they grow together for 10 d under controlled conditions. Measurements were taken to determine the effect of root exudates on root and shoot lengths, total germination, rate of germination, and seedling vigor. The root was found to be the most sensitive plant organ for inhibition by grass root exudates. The legume species studied differed in their response to the root exudates of oat and ryegrass. Berseem clover, squarrosum clover, and common vetch were more inhibited than the other legume species. Total percent germination and rate of germination (82.6 and 8.5% on average, respectively) were not significantly lower than the control. Inhibition of germination was not a good measure of phytotoxicity since seedling growth was inhibited even when germination was not. By contrast, seedling vigor was significantly lower for several legume species: squarrosum clover showed a marked reduction in early seedling vigor (-54%) in the presence of oat root exudates. Generally, responses of berseem clover were similar to that of squarrosum clover. These results suggested that grass exudates might affect seedling growth and development of some legume species owing to the inhibitory effects of allelochemicals found in grass live roots. However, further research is necessary to verify our laboratory results under field conditions.