Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 2001
Publication Date: N/A
Interpretive Summary: Carbon dioxide is the sole source of carbon needed for plant growth. Many plants show increased growth as carbon dioxide in the atmosphere increases. However, 95 percent of all studies examine the response of plants to C02 have only looked at a single species grown in isolation. Not all plants respond in the same way to the increase in atmospheric C02. Consequently, competition between plants will be affected. In this study we examined the response to rising atmospheric carbon dioxide of one crop plant, grain sorghum, and one weedy species, common cocklebur, a troublesome weed in grain sorghum fields in the southern United States. Under today's atmospheric carbon dioxide concentration, competition between these two species as determined by changes in photosynthesis, leaf area, and plant growth was approximately equal. However, at future levels of carbon dioxide (about 700ppm, which is expected to occur by the middle of this century), cocklebur was favored. Consequently, sorghum growth and potential grain production would be reduced even with increases in atmospheric carbon dioxide. This information would be invaluable for farmers, scientists, and modelers seeking to predict the impact of C02 on crop productivity.
Although studies on weed/crop competition and rising carbon dioxide (C02) have examined potential interactions between C4 weeds and C3 crops, there are many agricultural situations where C4 crops compete with C3 weeds. At present no data exists in evaluating competition between a C4 and a cooccurring C3 weed in response to rising C02. The growth of grain sorghum (Sorghum bicolor L. Moench) was evaluated with and without the presence of common cockleburs (Xanthium strumarium L.), at current and future atmospheric concentrations of C02. In monoculture, increased C02 stimulated photosynthesis rate, leaf area, and above ground dry weight of cockleburs more than that of sorghum. Photosynthesis rates declined for both species in competition, but increasing C02 reduced the percent decline in cockleburs and increased it in sorghum by 35 days after sowing (DAS). The relative stimulation of aboveground biomass at elevated C02 increased for cockleburs in competition with sorghum by 41 DAS. Overall, at present atmosphere C02, plant relative yield (PRY) was approximately equal between the two species but increased significantly for cockleburs and decreased significantly for sorghum for above ground biomass and leaf area, respectively, at elevated C02. These data indicate that vegetative growth, competition and potential yields are reduced in economically important C4 crops by cooccurring C3 weeds as atmospheric carbon dioxide increases.