Photoinactivation of photosystem II in leaves of Capsicum annuum

Leaf discs of Capsicum annuum L. were illuminated in air enriched with 1% C O2 in the absence or presence of lincomycin, an inhibitor of chloroplast-en coded protein synthesis. The loss of functional photosystem (PS) II complex es with increase in cumulative light dose (photon exposure), assessed by th e O-2 yield per single-turnover flash, was greater in leaves of plants grow n in low light than those in high light; it was also exacerbated in the pre sence of lincomycin. A single exponential decay can describe the relationsh ip between the loss of functional PSII and increase in cumulative photon ex posure. From this relationship we obtained both the maximum quantum yield o f photoinactivation of PSII at limiting photon exposures and the coefficien t k, interpreted as the probability of photoinactivation of PSII per unit p hoton exposure. Parallel measurements of chlorophyll fluorescence after lig ht treatment showed that 1/F-o--1/F-m was linearly correlated with the func tionality of PSII, where F-o and F-m are the chlorophyll fluorescence yield s corresponding to open and closed PSII reaction centers, respectively. Usi ng 1/F-o--I/F-m as a convenient indicator of PSII functionality, it was fou nd that PSII is present in excess; only after tbe loss of about 40% functio nal PSII complexes did PSII begin to limit photosynthetic capacity in capsi cum leaves.