The Influence of Nitrogen Source on the Tolerance of Holcus Lanatus L.* and Bromus Erectus Huds. To Manganese

SUMMARY Both Bromus erectus and Holcus lanatus grew better in NO3‐N than in NH4‐N. NH4‐N inhibited Mn uptake and NO3‐N enhanced it. The concentration of Mn found in both species was greater in NO3‐N than NH4‐N treated plants and increased severity of toxicity symptoms was observed in plants given NO3‐N treatments. In general, uptake and concentration of Mn was greater in Holcus than in Bromus, although Holcus displayed less severe symptoms of toxicity. Root concentrations of Mn were greater than those of shoots in Holcus. As excess Mn generally affects plant tops more severely than roots it is possible that a mechanism reducing upward translocation of Mn conferred a degree of tolerance in Holcus. Growth of the population of Holcus used was affected to a lesser extent than Bromus by large concentrations of Mn in nutrient solution which may partially account for the markedly different frequency of occurrence of these species on soils which contain high concentrations of both NH4‐N and Mn. In Bromus NH4‐N and Mn interact to reduce their combined effects but in both species the adverse effects of NH4‐N on growth for the most part masks that of Mn. Conversely with NO3‐N, itself beneficial, Mn uptake is not depressed as it is with NH4‐N and when Mn‐toxicity occurs, it is more marked. In solution culture Bromus is very sensitive to NH4‐N and also to Mn in the presence of NH4‐N or NO3‐N, whereas in the kind of habitat in which Bromus occurs N is present as NO3 and the adverse effect of any Mn present is neutralized by Ca.