The light sensitivity of the photosynthetic apparatus was investigated as a function of successive short-term (15 min) adaptation to increased actinic light (AL) intensity in intact attached leaves of atrazine-(AtrR), paraquat-(PQR), paraquat/atrazine-resistant (PQAtrR) and susceptible (S) biotypes of Conyza canadensis grown under high-light conditions. The response to the AL intensity of CO2 fixation, the xanthophyll cycle activity and the modulated fluorescence quenching parameters were investigated under steady- state conditions. There was no significant difference between biotypes as concerns the optimal quantum yield (Fv/Fm) of PS II. The effective quantum yield of photochemistry (ΔF/Fm') and the CO2 fixation capacity were lower at any light intensity in atrazine-resistant (AR: AtrR and PQAtrR) biotypes. The photochemical quenching coefficient (qP) was lower at all light intensities and decreased more sensitively with the light intensity in AR than in atrazine-sensitive (AS: S and PQR) biotypes. The non-photochemical fluorescence quenching parameter (qN) and the rate of nonradiative energy dissipation (NPQ) under saturating light intensities were lower in AR biotypes 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 biotypes. The higher photosensitivity of AR plants and the lowered zeaxanthin formation in these biotypes suggest that, besides DI protein mutation, the limited conversion of xanthophyll cycle components may contribute to the higher susceptibility to photoinhibition of AR plants.
|Number of pages||10|
|Journal||Plant Physiology and Biochemistry|
|Publication status||Published - dec. 1 1996|
ASJC Scopus subject areas
- Plant Science