The effect of CO2 enrichment on the growth of a C3 weed (Parthenium hysterophorus L.) and its competitive interaction with a C4 grass (Cenchrus ciliaris L.)

S.C. NavieA, R.E. McFadyenB, F.D. PanettaC and S.W. AdkinsA

A School of Land and Food Sciences, The University of Queensland, St. Lucia, Queensland 4072, Australia.
B CRC for Australian Weed Management, Block B, 80 Meiers Road, Indooroopilly, Queensland 4068, Australia.

C Queensland Department of Natural Resources and Mines, Alan Fletcher Research Station, PO Box 36, Sherwood, Queensland 4075, Australia.


The growth of parthenium weed (Parthenium hysterophorus L.), an introduced annual weed species with a C3 photosynthesis mechanism, was stimulated when the atmospheric CO2 concentration was increased from 360 (ambient) to 480 (enriched) ppmv, regardless of whether competition from buffel grass (Cenchrus ciliaris L.) was present or absent. Seventy days after sowing a significant increase in the height, stem base diameter, above-ground biomass, capitula production and seed production was observed in P. hysterophorus plants grown at the higher CO2 concentration. In addition, these plants showed greater phenological development than those grown at the ambient CO2 concentration.

The large magnitude of the increases detected is probably due to the time period over which the experiment was conducted, the fact that this weed is a C3 broad-leaf species, and the highly adaptable nature of this aggressive weed. The growth of C. ciliaris, a perennial pasture species with a C4 photosynthesis mechanism, was not significantly affected by elevated CO2 concentration, but some differences in its growth were detected (i.e. reduced tiller production and changes in its appearance). When P. hysterophorus was grown in competition with C. ciliaris, the ratio of C. ciliaris to P. hysterophorus above-ground biomass was much lower in the pots from the enriched CO2 cabinet. This is an indication that the C3 weed species gained a competitive advantage over the C4 pasture species at the higher CO2 concentration.

The stimulation of growth, and increased competitiveness, of P. hysterophorus at an elevated CO2 concentration indicates that this weed could become more aggressive in the future if atmospheric CO2 levels continue to rise, especially in areas dominated by C4 species such as the semi-arid sub-tropical and tropical pastures of northern Australia.

Keywords: parthenium weed, buffel grass, carbon dioxide, competition.


Plant Protection Quarterly (2005) 20 (2) 61-66.