Growth, reproduction and potential competition between rice (Oryza sativa L.) and its associated weeds under elevated carbon dioxide and high temperature

In the first experiment, exposure to increased CO2 resulted in a significant increase in the percent emergence of Echinochloa glabrescens seedlings but not for Ludwigia octovalvis. Exposure to increased CO2 and temperature significantly increased the percent emergence for rice relative to E. glabrescens. In a separate field experiment, elevated CO2 (Ambient + 300 ppm) stimulated the emergence and number of weed seedlings particularly Fimbristylis miliacea (L.) Vahl (C4 weed). Total dry weight of the weed seedlings that emerged in the open-top field chambers increased with increasing CO2 (Ambient + 300 ppm) and temperature (Ambient + 4 deg C). In the second experiment, elevated CO2 at the optimal temperature of 29/21 deg C increased the total dry weight, leaf area and plant height of rice, E. glabrescens, and L. octavalvis at maturity. However, interaction between high CO2 and temperature also promoted greater total dry weight, tiller number and leaf area for all the three species. Overall, rice produced more grain yield than the other two species. Differences in reproductive characteristics were also observed among the three species with high CO2 and temperature. Rice and L. octavalvis were more responsive to increasing CO2 and temperature than E. glabrescens. Overall, L. octavalvis showed greater growth than rice and E. glabrescens with increasing CO2 and temperature. In the third experiment, increased CO2 concentration promoted greater biomass, tiller number, grain yield, panicle weight and percent filled spikelets for rice relative to E. glabrescens. Comparison between the two species showed that the ratio of rice/E. glabrescens for total above ground biomass, leaf area, tiller number and grain yield increased with increasing CO2 concentration indicating a potential competitive advantage for rice under elevated CO2. Overall, these experiments suggest that increasing atmospheric CO2 could potentially increase the growth of C3 plants with differential growth anticipated for C3 crops such as rice and C3 weeds such as L. octavalvis. While competition was not directly measured between those two C3 species, it appears based on relative enhancement of growth that at least in some circumstances C3 weeds could outcompete a C3 crop