Competitive interactions between cultivated and red rice as a function of recent and projected increases in atmospheric carbon dioxide

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Because wild lines of the same species often represent a weedy constraint to cultivated crops in the field, any differential response to atmospheric carbon dioxide concentration, [CO2], may alter weed–crop competition and seed yield. We evaluated the growth and reproduction of cultivated rice (Oryza sativa L.; Clearfield, CL161) and red or weedy rice (Stuttgart, StgS) in monoculture, and at two competitive densities (8 and 16 plants m–2) using [CO2] that corresponded to the 1940s, current levels, and that projected for the middle for the current century (300, 400, and 500 µmol mol–1, respectively). Competition was determined using plant relative yield (PRY) for biomass and seed yield as a function of [CO2]. At maturity, PRY significantly increased for wild rice, but decreased for the cultivated line as a function of competitive density. Increases in [CO2] resulted in significant decreases in the PRY of seed yield at both competitive densities for CL161; while significant increases in seed yield and aboveground biomass were noted for StgS, but only at the higher competitive density. In addition, the relative increase in seed yield for CL161–StgS, determined at the same competitive density, increased as a function of [CO2] suggesting a greater amount of seed rain from the weedy rice. Overall, these are the first data to indicate that for rice, weedy lines may have been favored as a function of rising carbon dioxide, with greater relative impact on the seed yield and biomass of cultivated rice lines.

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