Photosynthetic light harvesting in excess light is regulated by a process known as feedback deexcitation. Femtosecond transient absorption measurements on thylakoid membranes show selective formation of a carotenoid radical cation upon excitation of chlorophyll under conditions of maximum, steady-state feedback deexcitation. Studies on transgenic Arabidopsis thaliana plants confirmed that this carotenoid radical cation formation is correlated with feedback deexcitation and requires the presence of zeaxanthin, the specific carotenoid synthesized during high light exposure. These results indicate that energy transfer from chlorophyll molecules to a chlorophyllzeaxanthin heterodimer, which then undergoes charge separation, is the mechanism for excess energy dissipation during feedback deexcitation.

Oxygenic photosynthesis in plants involves highly reactive intermediates and byproducts that can damage the photosynthetic apparatus and other chloroplast constituents. The potential for damage is exacerbated when the amount of absorbed light exceeds the capacity for light energy utilization in photosynthesis, a condition that can lead to decreases in photosynthetic efficiency. A feedback de-excitation mechanism (qE), measured as a component of nonphotochemical quenching of chlorophyll fluorescence, regulates photosynthetic light harvesting in excess light in response to a change in thylakoid lumen pH. qE involves de-excitation of the singlet excited state of chlorophyll in the light-harvesting antenna of photosystem II, thereby minimizing the deleterious effects of high light via thermal dissipation of excess excitation energy. While the physiological importance of qE has been recognized for many years, a description of its physical mechanism remains elusive. We summarize recent biochemical and spectroscopic results that have brought us closer to the goal of a mechanistic understanding of this fundamental photosynthetic regulatory process.

Nonphotochemical quenching (NPQ) refers to a process that regulates photosynthetic light harvesting in plants as a response to changes in incident light intensity. By dissipating excess excitation energy of chlorophyll molecules as heat, NPQ balances the input and utilization of light energy in photosynthesis and protects the plant against photooxidative damage. To understand the physical mechanism of NPQ, we have performed femtosecond transient absorption experiments on intact thylakoid membranes isolated from spinach and transgenic Arabidopsis thaliana plants. These plants have well defined quenching capabilities and distinct contents of xanthophyll (Xan) cycle carotenoids. The kinetics probed in the spectral region of the S 1 3 S n transition of Xans (530 -580 nm) were found to be significantly different under the quenched and unquenched conditions, corresponding to maximum and no NPQ, respectively. The lifetime and the spectral characteristics indicate that the kinetic difference originated from the involvement of the S 1 state of a specific Xan, zeaxanthin, in the quenched case.G reen plants live with a continual paradox: They have evolved to both use and dissipate solar energy with high efficiency. Highly reactive, photooxidative intermediates are inevitable byproducts of photosynthesis. An excess photon flux can exacerbate the damage caused by these intermediates, leading to problems ranging from reversible decreases in photosynthetic efficiency, to, in the worst case, death of the plant. Nonphotochemical quenching (NPQ) is a process that thermally dissipates the absorbed light energy in photosystem (PS) II that exceeds a plant's capacity for CO 2 fixation, minimizing the deleterious effects of high light. Although NPQ has been phenomenologically documented for years, a fundamental understanding of its physical mechanism remains elusive (1-3).Feedback deexcitation or energy-dependent quenching (qE) (2, 3) is the major, rapidly reversible component of NPQ in a variety of plants, including spinach and Arabidopsis thaliana (4,5), and is the focus of this study. qE is characterized by a light-induced absorbance change at 535 nm (⌬A 535 ) (6) and the shortening of specific components of chlorophyll (Chl) fluorescence lifetimes, the exact numerical value for the shortened lifetime depending on the specific form of the photosynthetic system and the measurement conditions. For isolated thylakoid systems with closed reaction centers, a Chl lifetime component is reduced from Ϸ2.0 to Ϸ0.4 ns (4). It requires the buildup of a pH gradient (⌬pH) under high light conditions (2, 3), which triggers the enzymatic conversion of carotenoids, violaxanthin (Vio) to zeaxanthin (Zea), by means of the xanthophyll (Xan) cycle (Fig. 1a).Currently two hypotheses concerning the mechanism of qE exist, one in which the effect of Zea is solely structural (termed indirect quenching) and the other in which Zea acts as an energy acceptor for excitation transfer from the first Chl singlet excited state (termed direct quenching). The direct ...