ATRAZINE-RESISTANT BIOTYPES OF CONYZA-CANADENSIS HAVE ALTERED FLUORESCENCE QUENCHING AND XANTHOPHYLL CYCLE PATTERN

The light sensitivity of the photosynthetic apparatus was investigated as a function of successive short-term (15 min) adaptation to increas ed actinic light(AL) intensity in intact attached leaves of atrazine-( AtrR), paraquat-(PQR), paraquat/atrazine-resistant (PQAtrR) and suscep tible (S) biotypes of Conyza canadensis grown under high-light conditi ons. The response to the AL intensity of CO2 fixation, the xanthophyll cycle activity and the modulated fluorescence quenching parameters we re investigated under steady-state conditions. There was no significan t difference between biotypes as concerns the optimal quantum yield (F v/Fm) of PS iI. The effective quantum yield of photochemistry (Delta F /Fm') and the CO2 fixation capacity were lower at any light intensity in atrazine-resistant (AR: AtrR and PQAtrR) biotypes. The photochemica l quenching coefficient (qP) was lower at all light intensities and de creased more sensitively with the light intensity in AR than in atrazi ne-sensitive (AS: S and PQR) biotypes. The non-photochemical fluoresce nce quenching parameter (qN) and the rate of nonradiative energy dissi pation (NPQ) under saturating light intensities were lower in AR bioty pes than in AS ones. The energy-dependent fluorescence quenching (qE) was strongly reduced in AR biotypes. The conversion of the xanthophyll cycle component violaxanthin to zeaxanthin was also limited in AR bio types. The higher photosensitivity of AR plants and the lowered zeaxan thin formation in these biotypes suggest that, besides D1 protein muta tion, the limited conversion of xanthophyll cycle components may contr ibute to the higher susceptibility to photoinhibition of AR plants.