PHOTOINACTIVATION OF FUNCTIONAL PHOTOSYSTEM-II AND D1-PROTEIN SYNTHESIS IN-VIVO ARE INDEPENDENT OF THE MODULATION OF THE PHOTOSYNTHETIC APPARATUS BY GROWTH IRRADIANCE

To investigate whether the in-vivo photoinhibition of photosystem II ( PSII) function by excess light is an intrinsic property of PSII, the m aximal photochemical efficiency of PSII (Fv/Fm) and the content of fun ctional PSII (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum sativum L.), grown in 50 (low light), 250 (medium light), and 650 (high light) mu mol photons . m(-2 ). s(-1). The modulation of PSII functionality in vivo was induced in 1.1% CO2 by varying either (i) the duration (0-2 h) of light treatment (fixed at 1800 mu mol photons . m(-2). s(-1)) or (ii) irradiance (0-3 200 mu mol photons . m(-2). s(-1)) at a fixed duration (1 h), after in filtration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), or a combination of lincomyci n with nigericin (an uncoupler), through the cut petioles of leaves of 22-to 24-d-old plants. The reciprocity law of irradiance and duration of illumination for PSII function in vivo (Park et al. 1995, Planta 1 96: 401-411) holds in all differently light-grown peas, demonstrating that inactivation of functional PSII depends on photon exposure (mel p hotons . m(-2)), not on the rate of photon absorption. In vivo, PSII a cts as an intrinsic ''photon counter'' and at higher photon exposures is inactivated following absorption of about 3 x 10(7) photons. There is a functional heterogeneity of PSII in vivo with 25% less-stable PSI Is that are inactivated at low photon exposure, compared to 75% more-s table PSIIs regardless of modulation of the photosynthetic apparatus. We suggest that the less-stable PSIIs represent monomers located in th e nonappressed granal margins, while the more-stable PSIIs are dimers located in the appressed grana membrane cores. The capacity for D1-pro tein synthesis was the same in all the light-acclimated peas and satur ated at low light, indicating that D1-protein repair is also an intrin sic property of PSII. This accounts for the low intensity required for recovery of photoinhibition in sun and shade plants which is independ ent of light-harvesting antennae size or PSII/PSI stoichiometries.