PHOTOSYNTHETIC PERFORMANCE OF MSMA-RESISTANT AND MSMA-SUSCEPTIBLE MISSISSIPPI BIOTYPES OF COMMON COCKLEBUR

Common cocklebur biotypes resistant to monosodium methanearsonate (MSM A) have been reported in South Carolina, Mississippi, and other locati ons. The resistance at the whole plant level in Mississippi biotypes w as 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 ai ha(-1); howeve r, inhibition of carbon assimilation was more rapid and of greater mag nitude in the susceptible (S) biotype than the resistant (R) biotype. MSMA affected C-i/C-a (the ratio of intercellular CO2 concentration to the external CO2 level) of the S biotype more than the R biotype and increased stomatal conductance to water vapor at 48 hr after treatment . There were no rapid effects of MSMA on variable chlorophyll fluoresc ence in either biotype, indicating that MSMA had no direct effect on p hotosynthetic electron transport. Furthermore, later reductions in var iable fluorescence were not pronounced in either biotype. These data i ndicate that, at an early stage during the development of herbicide in jury, MSMA affects photosynthetic carbon fixation in the S biotype mor e than in the R biotype. (C) 1995 Academic Press, Inc.