Phenotypic plasticity of Ammannia spp. in competition with rice

The annual broadleaved weeds Ammannia auriculata Wild. and A. coccinea Rottb. are widespread and competitive in California rice (Oryza sativa L.) fields. We studied Ammannia spp. biology in a greenhouse pot experiment. Weeds were grown alone and in competition with rice (cv. M-202), and harvested six times over 122 days. Compared with growth alone, competition reduced Ammannia spp. total plant dry weight (DW), shoot DW and leaf area in all but the first harvest. However, weed height differed at only one harvest (85 days after planting). Ammannia spp, responded to competition with increased shoot:root DW ratios, increased stem:other shoot DW ratios, decreased stem diameters, fewer but elongated internodes and fewer branches. This suggests that light capture was more important than nutrient capture for maintaining Ammannia spp. growth. The weeds appear to lodge easily in rice fields because competitive growth responses make them top heavy and unstable. Weed seed DW declined by 97% in competition, which showed the importance of crop interference to Ammannia spp. control. Plasticity may help Ammannia spp, to escape common control practices (e.g. high crop plant densities or early herbicide application), and probably contributes to its widespread distribution. Thus, Ammannia spp. plasticity should be more fully considered when designing management strategies.