INFLUENCE OF DIFFERENT CO2 LEVELS ON THE GROWTH AND COMPETITION OF SOME IMPORTANT WEEDS IN WHEAT (Triticum aestivum L.)

Pot experiments were conducted to investigate the effects of different CO 2 levels on the growth and competition of Avena sterilis, Phalaris minor, Galium tricornatum and Sinapis arvensis which are important weed species in wheat (Triticum aestivum) growing areas.Two CO 2 levels (500 ppm as ambient, 750-800 ppm as elevated) were used in the study.Weeds or wheat plants were sown to pots alone or in competition with each other.Six weeks after emergence above ground fresh and dry biomasses of all plants were determined and compared.Results showed that biomasses of all plants were significantly increased by elevated CO 2 when grown in monoculture.Under competition CO 2 showed no significant effect on weed biomasses.Weeds reduced wheat biomass at both CO 2 level but only significantly under elevated CO 2 .Wheat suppressed weeds also to higher degrees under elevated CO 2 levels.S. arvensis suppression by wheat was 50% at ambient but about 95% at elevated CO 2 conditions.These results show that elevated CO 2 improve the growth of weeds under noncompetitive conditions significantly.Although weed growth was not directly affected by elevated CO 2 under competitive conditions, they caused higher biomass reduction of wheat.However, the growth of wheat plants was so improved by elevated CO 2 , that they still produced as much biomass as of those grown without competition at ambient CO 2 .These results suggest that global CO 2 increase might cause increase in wheat growth, but weed control will still be important issue to maintain maximum yield.