PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Carotenoid radical formation and the regulation of photosynthetic light harvesting. Nancy Holt^{*,1, 2}, Donatas Zigmantas^{1, 2}, Xiao-Ping Li³, Krishna Niyogi³, Graham Fleming^{1, 2}, ¹ Department of Chemistry, Berkeley, CA, USA² Physical Biosciences Division, Berkeley, CA, USA³ Department of Plant and Microbial Biology, Berkeley, CA, USA

ABSTRACT- Feedback de-excitation quenching (qE), measured as a component of non-photochemical quenching of chlorophyll (Chl) fluorescence, is the observable associated with the process that regulates photosynthetic light harvesting under conditions in which the input light intensity exceeds a plant's capacity for carbon fixation. The mechanism of qE involves harmless thermal dissipation of Chl singlet excited states in photosystem II of green plants and algae in order to minimize the alternate reaction pathways which generate toxic, photo-oxidative intermediates. To gain insight into the identity of the quenching species and the means by which it dissipates the energy, we have performed femtosecond transient absorption measurements on active, intact thylakoids membranes. Upon excitation of the Chl Qy band in spinach thylakoids at 664 and 683 nm, the kinetics probed in the near-IR (900-1080 nm) depended on whether no qE or maximum, steady-state qE existed in the membranes. Additional studies on transgenic Arabidopsis thaliana plants confirmed that the kinetic differences are related to qE and depend on the presence of a specific carotenoid necessary for qE, zeaxanthin. The spectral characteristics of the difference and the mutant work collectively suggest that a zeaxanthin cation is formed selectively during qE. The lifetimes extracted from the kinetics, the relationship between these measurements and previous TA measurements probed in the visible (530-580 nm), and the connection between these finding and the mechanism of qE will be discussed.

KEY WORDS: feedback de-excitation quenching