Trimers of light harvesting complex 11 from Arabidopsis thaliana were studied by femtosecond fluorescence up-conversion. The average lifetime of the carotenoid S-2 state was similar to57 fs for wild type trimers and similar to70 fs for trimers, from a mutant plant with a distinctly different carotenoid composition. We estimate that similar to56% of the energy transferred from carotenoids to chlorophylls proceeds via the carotenoid S, state in the wild type and similar to46% in the mutant. By comparison with the fluorescence excitation spectra, we find that similar to20% of the energy transferred in both samples proceeds through the carotenoid S, state. (C) 2003 Elsevier B.V. All rights reserved.