In photosynthetic light-harvesting systems carotenoids and chlorophylls jointly absorb light and transform its energy within about a picosecond into electronic singlet excitations of the chlorophylls only. This paper investigates this process for the light-harvesting complex II of the purple bacterium Rhodospirillum molischianum, for which a structure and, hence, the exact arrangement of the participating bacteriochlorophylls and carotenoids have recently become known. Based on this structure and on CI expansions of the electronic states of individual chromophores ~bacteriochlorophylls and carotenoids! as well as on an exciton description of a circular aggregate of bacteriochlorophylls, the excitation transfer between carotenoids and bacteriochlorophylls is described by means of Fermi’s golden rule. The electronic coupling between the various electronic excitations is determined for all orders of multipoles ~Coulomb mechanism! and includes the electron exchange ~Dexter mechanism! term. The rates and efficiencies for different pathways of excitation transfer, e.g., 1 1 B u (carotenoid)!bacteriochlorophyll aggregate and 2 1 A g (carotenoid)! bacteriochlorophyll aggregate, are compared. The results show that in LH-II the Coulomb mechanism is dominant for the transfer of singlet excitations. The 1 1 B u !Qx pathway appears to be partially efficient, while the 2 1 A g !Qy pathway, in our description, which does not include vibrational levels, is inefficient. An improved treatment of the excitation transfer from the 2 1 Ag state is required to account for observed transfer rates. Exciton splitting of bacteriochlorophyll Qy excitations slightly accelerates the excitation transfer from the 2 1 Ag state, while it plays a crucial role in accelerating the transfer from the B800 BChl Qy state. Photoprotection of bacteriochlorophylls through triplet quenching is investigated, too. The results suggest that eight of the 16 B850 bacteriochlorophylls in LH-II of Rhodospirillum molischianumare protected well by eight carotenoids observed in the x-ray structure of the protein. The remaining eight B850 bacteriochlorophylls can transfer their triplet excitation energy efficiently to their neighboring protected bacteriochlorophylls. Eight B800 bacteriochlorophylls appear not to be protected well by the observed carotenoids. @S1063-651X~98!11007-3#

There are many, often relatively anecdotal, observations suggesting that agricultural activities affect lichens. However, the nature of the ' agricultural effect' involved is much less clear. Agriculture practices can involve anything from the use of pesticides, to inorganic fertilisers, to ammonia volatilised from animal waste, or pollution from farm vehicles. The nature, scale and extent of the effect will depend on the differential sensitivity of lichens to the cocktail of chemicals involved and the extent of their dispersal under the particular conditions. The result is a fascinating field of research to separate the different factors involved. Surprisingly little has been written on this subject. Currently, much of the published work has sought to demonstrate the impact of farms or farming without identifying the agents responsible. This is a necessary prelude to further investigation. Valuable advances have been made by categorising lichen species as ' acidophilous ' or ' nitrophilous ', even though the nature of the chemicals responsible are still unknown. Moreover, there is little comparative work reported. In northern Europe, the assemblage of epiphytic species observed around an active farm may, superficially, resemble that of a dust-impregnated mediterranean tree. But how close is the resemblance, and is it related to nitrogen enrichment? In order to try to develop this field of study further, Dennis Brown and Han van Dobben organised a small international workshop that met in Wageningen, the Netherlands, between 16th and 18th April 1993. The meeting was supported by the European Environmental Research Organisation and attended by researchers from ten countries. Although judged a successful exchange of ideas, for various reasons, no proceedings were produced. Some speakers' work was already in press, while others appeared, for example, in publications linked to the IAL symposium ' Progress and Problems in Lichenology in the Nineties ', held in Sweden. More recently, it was proposed that, to further develop this field of study, the original participants at the Wageningen workshop should be invited to submit manuscripts to form the nucleus of an issue of The Lichenologist devoted to ' agricultural effects '. This issue is the result and shows part of the range of studies and approaches reported at the earlier meeting. Some articles have been developed further than the original reports, whereas others remain relatively preliminary studies but ones that were judged to have the potential to stimulate further research. If this issue stimulates more discussion, debate and even disagreement, which finally helps clarify how farming may influence lichens, then the original workshop will be judged to have had a wider influence than just an enjoyable meeting, skilfully organised by Han van Dobben.