PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Influence of the outer antenna on the photochemical trapping rate in PSII of barley thylakoids. Enrico Engelmann^*,1, Giuseppe Zucchelli¹, Anna Paola Casazza¹, Flavio Garlaschi¹, Robert Jennings, ¹ Dipartimento di Biologia, Milano, MI, Italy

ABSTRACT- We have investigated the previous suggestion of Briantais et al. 1996 and Gilmore et al. 1996, that the effective primary photochemical trapping rate in barley is not influenced by the outer antenna of photosystem II. By steady state fluorescence measurements at the F0 fluorescence level of wild type and the chlorina f2 mutant, using the chlorophyll b as a marker, it was possible to verify that the outer antenna is thermally equilibrated with the core pigments at room temperature, under conditions of photochemical trapping. This is in contrast with the conclusions of the earlier studies in which it was suggested that energy was transferred rapidly and irreversibly from the outer antenna to the photosystem II core.
Furthermore we have determined the effective trapping time by single photon counting time resolved measurements. It was was shown that it increases from 0.17 plus minus 0.017ns to a value within the range 0.42 plus minus 0.036 and 0.47 plus minus 0.044ns. This 2.2-2.6 fold increase in the effective trapping time is however significantly less than that expected for a fully thermalised system. The data can be explained in terms of the outer antenna increasing the primary charge rate by about 50 per cent.

KEY WORDS: barley, trapping time, chlorina, fluorescence