Competition by barley and pea against mustard: Effects on resource acquisition, photosynthesis and yield

One laboratory and three field experiments were conducted to determine physiological mechanisms through which barley (Hordeum vulgare L.) monocrops and barley-pea (Pisum sativum L.) intercrops suppressed growth and seed production of two weedy species of mustard (Brassica kaber (DC) L.C. Wheeler and B. hirta Moench). The study quantified the negative effects of nitrogen (N) and light deficits on mustard's photosynthetic performance, and the reduction in mustard's access to N and light resources that resulted from the presence of barley and pea. The work demonstrated that crop competition for resources was paralleled by reductions in mustard's photosynthetic performance, above-ground biomass and seed production. It also showed that the negative effects of the crops on mustard's resource status, photosynthetic performance, and yield could be overcome, at least partially, by application of N fertilizer or use of a pea cultivar that intercepted less light. Mustard suffered most from crop competition when N fertilizer was not applied and when a long-vined pea cultivar that produced abundant leaf area was used in the intercrop mixtures. Instantaneous measurements of net photosynthesis, made on individual leaves early in the growing season, predicted 59-74% of the variation in mustard's final above-ground biomass and seed production.