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Response of C3 and C4 Weeds to Herbicide Applications at Elevated CO2
Ziska, L.H., Teasdale, J.R. and Bunce, J.A.  1999.  Future atmospheric carbon dioxide may increase tolerance to glyphosate.  Weed Science 47: 608-615.

What was done
The authors grew broad-leaved C3 (Chenopodium album) and C4 (Amaranthus retroflexus) weeds in glasshouses receiving atmospheric CO2 concentrations of 360 and 720 ppm to determine the effects of atmospheric CO2 enrichment on these weedy species.  In addition, both young and mature plants of each species were sprayed with one-tenth and full-strength recommended solutions of the chemical glyphosate ("Roundup") to determine if elevated CO2 alters weed susceptibility to this widely used herbicide.

What was learned
Elevated CO2 significantly increased photosynthesis and total dry weight of the unsprayed C3 weed, regardless of maturity stage, but had no effects on these parameters for the C4 weed.  As a general comparison, the growth response of the C3 weed to a doubling of the atmospheric CO2 concentration was approximately 51%, which is in close agreement with the average 52% plant growth response tabulated by Idso (1992), and that obtained by Poorter (1993) for rapidly-growing wild C3 species (54%).

For the C4 weed, spraying young and mature plants with full-strength herbicide resulted in their death, regardless of atmospheric CO2 concentration.  However, for the C3 weed, spraying plants with full-strength glyphosate severely reduced, but did eliminate growth in the elevated CO2 environment, whereas chemically treated plants died in the ambient CO2 environment.  Thus, atmospheric CO2 enrichment did not alter the tolerance of the C4 weed to glyphosate, but somehow slightly increased the tolerance of the C3 weed to this herbicide.

What it means
As the CO2 content of the air increases, it is not likely that the competitive interactions among these two weeds and agronomic crops will change to favor the weedy species.  At the whole-plant level, this particular C4 weed was unresponsive to atmospheric CO2 enrichment, and thus would be expected to exhibit reduced growth performance, relative to that of most agronomic crops, as the CO2 concentration of the air continues to rise.  Although the C3 weed was responsive to elevated CO2, it was not any more responsive than are most of earth's plants to a doubling of the air's CO2 content.  Thus, if this particular weed and an agronomic crop both exhibit an approximate 50% increase in growth in response to a doubling of the air's CO2 content, it is not likely that their current competitive interactions will change much in the future.

As far as the intensive agricultural sector is concerned, farm managers who use glyphosate to control the C4 weed Amaranthus retroflexus should not need to modify their current chemical practices in the future.  However, to better control the C3 weed Chenopodium album, farm managers may need to apply glyphosate earlier in the season when weeds are smaller in size, or apply it at higher concentrations, as elevated CO2 slightly increases the tolerance of this particular C3 weed to glyphosate.


Idso, K.E.  1992.  Plant Responses to Rising Levels of Carbon Dioxide: A Compilation and Analysis of the Results of a Decade of International Research into the Direct Biological Effects of Atmospheric CO2 Enrichment.  Climatological Publications Scientific Paper #23, Office of Climatology, Arizona State University, Tempe, AZ.

Poorter, H.  1993.  Interspecific variation in the growth response of plants to an elevated and ambient CO2 concentration.  Vegetatio 104/105: 77-97.

Reviewed 15 January 2000