Shoot and Root Competition in aLolium multiflorum-Wheat Association

Three outdoor pot experiments with three wheat cultivars and Lolium multiflorum were conducted at La Plata National University (Argentina), where the effects of shoot and root competition were separated using aerial partitions. Root competition (RC) for both shoot dry matter yield (SDM) and grain yield was higher (p < 0.05) than shoot competition (SC). Cv. P.I. Super was most affected by SC while cvs. K. Cacique and B. Charrua were least affected. Crop competition caused a highly significant reduction of weed variables. RC had a greater effect on both weed SDM and weed seed yield than did SC. Cvs. B. Charrua and K. Cacique registered a higher photosynthetically active radiation interception (PAR) than cv. P.I. Super and L. multiflorum in SC treatment, but cv. P.I. Super intercepted more PAR than L. multiflorum and cvs. B. Charrua and K. Cacique in RC treatment. The resources complementarity for grain yield was obtained by means of relative yield total (RYT). There was resources complementarity when wheat and weed were in SC, but not in RC and full competition (FC), showing that the species were in full competition, despite the high nitrogen availability throughout the crop growing cycle. Cvs. B. Charrua and K. Cacique were more aggressive than the weed, while cv. P.I. Super was less aggressive than the weed in the SC treatment. Conversely, in both the RC and FC treatments, higher aggressivity than wheat cvs. was shown by the weed. This result could be ascribed to the lower shoot/root ratio and the higher nitrogen uptake of L. multiflorum. Despite the high availability of nitrogen, the results demonstrate that competition was mainly for soil nutrients, and wheat cvs. with a higher competitive ability in above-ground competition may have no competitive advantage when RC occurs. Because the experimental technique in pots does not reflect field conditions, caution is needed in extrapolating the results to a field situation.