A last-ditch defence against high-light stress: photoinactivated photosystem II complexes protect functional neighbours in Capsicum annuum L. leaves

Abstract

Leaf segments from Capsicum annuum plants grown in 100 (low light) or 500 (high light) µmol m¿2 s¿1 were illuminated at three irradiances and three temperatures for several hours. At various times, the remaining fraction (f ) of functional photosystem II (PS II) complexes was measured by a chlorophyll fluorescence parameter (1/Fo ¿ 1/Fm, where Fo and Fm are the fluorescence yields corresponding to open and closed PS II traps, respectively), which was in turn calibrated by the oxygen yield per saturating single-turnover flash. During illumination of leaf segments in the presence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis, the decline of f from 1.0 to about 0.3 was mono-exponential. Thereafter, f declined much more slowly, the remaining fraction (~0.2) being able to survive prolonged illumination. The results can be interpreted as being in support of the hypothesis that photoinactivated PS II complexes photoprotect functional neighbours [Öquist et al. (1992) Planta 186: 450-460]. In the absence of lincomycin, photoinactivation and repair of PS II occur in parallel, allowing f to reach a steady-state value. The results obtained in the presence and absence of lincomycin are analysed according to a simple kinetic model which formally incorporates a conversion from weak to strong quenchers, yielding the ("intrinsic") rate coefficient of photoinactivation and the rate coefficient of repair, as well as gaining an insight into the influence of f on the operational rate coefficient of photoinactivation.