sinse agriculture is both the source andsink ofgreenhouse gases, and plants show different response toelevated co2 concentration, an experiment wasconducted atthe research grenhhouse of facultyof agriculture, ferdowsi university ofmashhad. the purpose of the experiment was to examine the effects of elevated co2 onthe competition ability among varous crops and weeds in factotial arrangement based on completley randomized block design with three replication. tratments includedambient andelevated co2 concentrations and various combinations of the plantation of crops andweeds of c3 andc4 species , whether of the pure culture of intercropping . results of experiment showed that, asco2 concentrationincreased , the leaf area and root dry weight of milletincreased , wheras those of otherspesies decreased . millet rootlengthincreased, wheras those ofother soecies decreaed . root dry weight in each cultural combination , compared to the ambient co2 concentration, decreaed. theamount cholorophyl in lambsquartersincreased, whereasitdecreaedinpigweed, milletandsoybean.

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