Herbicide Resistant Weeds

Photosynthetic performance of MSMA-resistant and -susceptible Mississippi biotypes of common cocklebur.

Nimbal, C. I., J. J. Heitholt, D. R. Shaw, and S. O. Duke. 1995. Photosynthetic performance of MSMA-resistant and -susceptible Mississippi biotypes of common cocklebur. Pesticide Biochemistry and Physiology. 53: 129-137.

Common cocklebur [Xanthium strumarium] biotypes resistant to monosodium methanearsonate (MSMA) have been reported in South Carolina, Mississippi and other locations. The resistance at the whole plant level in Mississippi biotypes was found to be low under growth chamber and greenhouse conditions. The CO2 exchange rate (CER), as measured by infrared gas analysis of both biotypes, was lowered by MSMA application at 0.5 kg a.i./ha; however, inhibition of carbon assimilation was more rapid and of greater magnitude in the susceptible (S) biotypes than the resistant (R) biotype. MSMA affected Ci/Ca (the ratio of intercellular CO2 concn to the external CO2 level) of the S biotype more than the R biotype and increased stomatal conductance to water vapour at 48 h after treatment. There were no rapid effects of MSMA on variable chlorophyll florescence in either biotype, indicating that MSMA had no direct effect on photosynthetic electron transport. Furthermore, later reductions in variable fluorescence were not pronounced in either biotype. These data indicate that, at an early stage during the development of herbicide injury, MSMA affects photosynthetic carbon fixation in the S biotype more than in the R biotype.