Photoinactivation of photosystem II complexes and photoprotection by non-functional neighbours in Capsicum annuum L. leaves

Leaf segments from Capsicum annuum plants grown at 100 mu mol photons m(-2) s(-1) (low light) or 500 mu mol photons m(-2)s(-1) (high light) were illumi nated at three irradiances and three temperatures for several hours. At var ious times, the remaining fraction CR of functional photosystem II (PS II) complexes was measured by a chlorophyll fluorescence parameter (1/F-o - 1/F -m, where F-o and F-m are the fluorescence yields corresponding to open and closed PS II traps, respectively), which was in turn calibrated by the oxy gen yield per saturating single-turnover flash. During illumination of leaf segments in the presence of lincomycin, an inhibitor of chloroplast-encode d protein synthesis, the decline off from 1.0 to about 0.3 was mono-exponen tial. Thereafter, f declined much more slowly, the remaining fraction (appr oximate to0.2) being able to survive prolonged illumination. The results ca n be interpreted as being in support of the hypothesis that photoinactivate d PS II complexes photoprotect functional neighbours (G. Oquist et al. 1992 , Planta 186: 450-460), provided it is assumed that a photoinactivated PS I I is initially only a weak quencher of excitation energy, but becomes a muc h stronger quencher during prolonged illumination when a substantial fracti on of PS II complexes has also been photoinactivated. In the absence of lin comycin, photoinactivation and repair of PS II occur in parallel, allowing f to reach a steady-state value that is determined by the treatment irradia nce, temperature and growth irradiance. The results obtained in the presenc e and absence of lincomycin are analysed according to a simple kinetic mode l which formally incorporates a conversion from weak to strong quenchers, y ielding the rate coefficients of photoinactivation and of repair for variou s conditions, as well as gaining an insight into the influence off on the r ate coefficient of photoinactivation. They demonstrate that the method is a convenient alternative to the use of radiolabelled amino acids for quantif ying photoinactivation and repair of PS II in leaves.