Light absorption by the chlorophyll a-b complexes of photosystem II in a leaf with special reference to LHCII

To investigate the light-harvesting properties of the Photosystem 11 chlorophyll (chl) a-b complexes (major light-harvesting complex of Photosystem 11 [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 (I - T) within a leaf, using the approach of Rivadossi et al. ([1999] Photosynth. Res. 60, 209-215). In the Q(y) region, significant enhancement of light harvesting by the chl b electronic transitions, with respect to the absorption spectra (optical density [01)]), as well as a large and generalized increase (between two- and four-fold) associated with the vibrational bands of both chi 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 chi 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 (chi 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 chi a light harvesting is essentially unchanged with respect to the absorption spectrum (01)) of isolated LHCII, whereas the chi b contribution is 20% higher and the carotenoids are 33% lower. The relative enhancement of the chi b absorption is principally associated with the Q(y) electronic transition region, the light-harvesting contribution of which becomes prominent in the leaf.