In the present study the analysis of the relation between the excited state population in the photosystem II (PSII) antenna and photoinactivation has been extended from an in vitro system, isolated thylakoids, to an in vivo system, Chlamydomonas reinhardtii cells. The results indicate that the excited state quenching by an added singlet quencher induces maximal protection against photoinhibition of about 30% of that expected on the basis of the observed light intensity-treatment time reciprocity rule. Similar results, obtained previously with thylakoids, have been interpreted in terms of damaged or incorrectly assembled complexes that play an important role in photoinhibition in the thylakoid membranes (Santabarbara, S., K. Neverov, F. M. Garlaschi, G. Zucchelli and R. C. Jennings [2001] Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids. FEBS Lett. 491, 109-113.). In an attempt to better define this aspect, the photoinhibition action spectra were determined for mutant barley thylakoids, lacking the chlorophyll (Chl) a-b complexes of the outer antenna, and for its wild type. The results indicate that in both systems the action spectra are significantly blueshifted (2-4 nm) and are broader than the PSII absorption in the membranes. These data are interpreted in terms of a heterogeneous population of outer and inner antenna pigment-protein complexes that contain significant levels of uncoupled Chl.
In the present study the analysis of the relation between the excited state population in the photosystem II (PSII) antenna and photoinactivation has been extended from an in vitro system, isolated thylakoids, to an in vivo system, Chlamydomonas reinhardtii cells. The results indicate that the excited state quenching by an added singlet quencher induces maximal protection against photoinhibition of about 30% of that expected on the basis of the observed light intensity–treatment time reciprocity rule. Similar results, obtained previously with thylakoids, have been interpreted in terms of damaged or incorrectly assembled complexes that play an important role in photoinhibition in the thylakoid membranes (Santabarbara, S., K. Neverov, F. M. Garlaschi, G. Zucchelli and R. C. Jennings [2001] Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids. FEBS Lett.491, 109–113.). In an attempt to better define this aspect, the photoinhibition action spectra were determined for mutant barley thylakoids, lacking the chlorophyll (Chl) a–b complexes of the outer antenna, and for its wild type. The results indicate that in both systems the action spectra are significantly blueshifted (2–4 nm) and are broader than the PSII absorption in the membranes. These data are interpreted in terms of a heterogeneous population of outer and inner antenna pigment–protein complexes that contain significant levels of uncoupled Chl.
To investigate the light-harvesting properties of the Photosystem II chlorophyll (chl) a-b complexes (major light-harvesting complex of Photosystem II [LHCII], CP24, CP26, CP29) in a mature leaf under natural "daylight" illumination, the absorption spectra of the isolated complexes were converted into the photon absorption spectrum (1-T) within a leaf, using the approach of Rivadossi et al. ([1999] Photosynth. Res. 60, 209-215). In the Qy region, significant enhancement of light harvesting by the chl b electronic transitions, with respect to the absorption spectra (optical density [OD]), as well as a large and generalized increase (between two- and four-fold) associated with the vibrational bands of both chl a and b, was observed, which acquires an important light-harvesting role (approximately 30-40% of total). In the Soret region, a small increase in light harvesting by chl b was indicated. To gain more detailed information on these aspects the light harvesting of LHCII in a leaf was investigated. This required describing the pigment absorption (chl a and b, carotenoids) in the LHCII OD spectrum in terms of spectral subbands, which were subsequently used to estimate the relative light harvesting of each pigment type in LHCII of a leaf. When the entire visible spectral interval between 400 and 730 nm is considered, the chl a light harvesting is essentially unchanged with respect to the absorption spectrum (OD) of isolated LHCII, whereas the chl b contribution is 20% higher and the carotenoids are 33% lower. The relative enhancement of the chl b absorption is principally associated with the Qy electronic transition region, the light-harvesting contribution of which becomes prominent in the leaf.
To investigate the light‐harvesting properties of the Photsystem II chlorophyll (chl) a‐b complexes (major light‐harvesting complex of Photosystem II [LHCII], CP24, CP26, CP29) in a mature leaf under natural “daylight” illumination, the absorption spectra of the isolated complexes were converted into the photon absorption spectrum (1‐T) within a leaf, using the approach of Rivadossi et al. ([1991] Photosynth. Res. 60, 209–215). In the Qy region, significant enhancement of light harvesting by the chl b electronic transitions, with respect to the absorption spectra (optical density [od]), as well as a large and generalized increase (between two‐ and four‐fold) associated with the vibrational bands of both chl a and b, was observed, which acquires an important light‐harvesting role (approximately 30–40% of total). In the Soret region, a small increase in light harvesting by chlb was indicated. To gain more detailed information on these aspects the light harvesting of LHCII in a leaf was investigated. This required describing the pigment absorption (chla and b, carotenoids) in the LHCII OD spectrum in therms of spectral subbands, which were subsequently used to estimate the relative light harvesting of each pigment type in LHCLL in leaf. When the entire visible spectral interval between 400 and 730nm is considered, the chl a light harvesting is essentially unchanged with respect to the absorption spectrum (OD) of isolated LHCII, whereas the chl b contribution is 20% higher and the carotenoids are 33% lower. The relative enhancement of chl b absorption is principally associated wiht the Qy electronic transition region, the light‐harvesting contribution of which becomes prominent in the leaf.
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