Photoinactivation of photosystem II in leaves of Capsicum annuum

Leaf discs of Capsicum annuum L. were illuminated in air enriched with 1% CO₂ in the absence or presence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. The loss of functional photosystem (PS) II complexes with increase in cumulative light dose (photon exposure), assessed by the O₂ yield per single-turnover flash, was greater in leaves of plants grown in low light than those in high light; it was also exacerbated in the presence of lincomycin. A single exponential decay can describe the relationship between the loss of functional PSII and increase in cumulative photon exposure. From this relationship we obtained both the maximum quantum yield of photoinactivation of PSII at limiting photon exposures and the coefficient k, interpreted as the probability of photoinactivation of PSII per unit photon exposure. Parallel measurements of chlorophyll fluorescence after light treatment showed that 1/F_o−1/F_m was linearly correlated with the functionality of PSII, where F_o and F_m are the chlorophyll fluorescence yields corresponding to open and closed PSII reaction centers, respectively. Using 1/F_o−1/F_m as a convenient indicator of PSII functionality, it was found that PSII is present in excess; only after the loss of about 40% functional PSII complexes did PSII begin to limit photosynthetic capacity in capsicum leaves.