Photosynthetic organisms are famously adaptable to environmental variances, yet the underlying physical mechanisms are seldom known. Here, we study the mechanisms that are responsible for the very large (similar to 50-nm) blue shift of the lowest-energy absorption band in the stressed LH3 antenna form from the photosynthetic bacterium Rhodopseudomonas acidophila compared to its position in regular LH2 antenna. By analyses of the antenna absorption and polarized fluorescence excitation spectra measured at 5 K, essential modifications of the antenna excitons have been revealed that involve both relative narrowing and shifting of the exciton band. (C) 2010 Elsevier B.V. All rights reserved.