Photosystem II (PSII) initiates photosynthesis in plants through the absorption of light and subsequent conversion of excitation energy to chemical energy via charge separation. The pigment binding proteins associated with PSII assemble in the grana membrane into PSII supercomplexes and surrounding light harvesting complex II trimers. To understand the high efficiency of light harvesting in PSII requires quantitative insight into energy transfer and charge separation in PSII supercomplexes. We have constructed the first structure-based model of energy transfer in PSII supercomplexes. This model shows that the kinetics of light harvesting cannot be simplified to a single rate limiting step. Instead, substantial contributions arise from both excitation diffusion through the antenna pigments and transfer from the antenna to the reaction center (RC), where charge separation occurs. Because of the lack of a rate-limiting step, fitting kinetic models to fluorescence lifetime data cannot be used to derive mechanistic insight on light harvesting in PSII. This model will clarify the interpretation of chlorophyll fluorescence data from PSII supercomplexes, grana membranes, and leaves.

Oxygen-evolving photosynthetic organisms possess nonphotochemical quenching (NPQ) pathways that protect against photoinduced damage. The majority of NPQ in plants is regulated on a rapid timescale by changes in the pH of the thylakoid lumen. In order to quantify the rapidly reversible component of NPQ, called qE, we developed a mathematical model of pH-dependent quenching of chlorophyll excitations in Photosystem II. Our expression for qE depends on the protonation of PsbS and the deepoxidation of violaxanthin by violaxanthin deepoxidase. The model is able to simulate the kinetics of qE at low and high light intensities. The simulations suggest that the pH of the lumen, which activates qE, is not itself affected by qE. Our model provides a framework for testing hypothesized qE mechanisms and for assessing the role of qE in improving plant fitness in variable light intensity.regulation of photosynthesis | nonlinear differential equations | biological feedback | chlorophyll fluorescence | photoprotection P hotosynthetic organisms are highly efficient at absorbing photons and transferring energy to a reaction center, where charge separation takes place. However, when the rate of energy consumption by the reaction center is slower than the rate of energy transfer to the reaction center, long-lived chlorophyll excited states build up in the Photosystem II (PSII) antenna. These longlived states present a significant hazard to the organism because the energy contained in excited chlorophyll is sufficient to generate singlet oxygen, which is highly reactive and can break bonds in the proteins essential for photosynthesis (1). Because sufficient light harvesting is necessary for fueling growth, but too much is harmful, plants face a challenge in balancing light harvesting and photoprotection, especially when light intensity rapidly fluctuates between levels that limit photosynthesis and levels that exceed the plant's capacity for photosynthesis (2).The mechanisms of regulated dissipation of excess absorbed energy in the PSII antenna are collectively known as nonphotochemical quenching (NPQ) (3). NPQ mechanisms dissipate excitation energy harmlessly as heat, reducing the extent of photoinhibition (4). There are multiple mechanisms for NPQ and these mechanisms respond on different timescales (3). The most rapid component of NPQ is called qE, and it responds to fluctuations in light intensity on the timescale of seconds to minutes (5, 6).The qE quenching pathway is activated by a decrease in the pH of the thylakoid lumen (3). The low pH of the lumen activates qE by protonating the proteins PsbS (7) and violaxanthin deepoxidase (VDE) (8, 9), and possibly other light harvesting complexes (10, 11). VDE goes on to convert the carotenoid violaxanthin to zeaxanthin in the xanthophyll cycle, which includes the intermediate antheraxanthin (12). The presence of zeaxanthin and the xanthophyll lutein, along with PsbS, is necessary for full expression of qE in vivo. In addition to the protonation of PsbS and the formation of zeaxanthin...

Photosynthetic organisms are capable of harvesting solar energy with near unity quantum efficiency. Even more impressively, this efficiency can be regulated in response to the demands of photosynthetic reactions and the fluctuating light-levels of natural environments. We discuss the distinctive design principles through which photosynthetic light-harvesting functions. These emergent properties of photosynthesis appear both within individual pigment-protein complexes and in how these complexes integrate to produce a functional, regulated apparatus that drives downstream photochemistry. One important property is how the strong interactions and resultant quantum coherence, produced by the dense packing of photosynthetic pigments, provide a tool to optimize for ultrafast, directed energy transfer. We also describe how excess energy is quenched to prevent photodamage under high-light conditions, which we investigate through theory and experiment. We conclude with comments on the potential of using these features to improve solar energy devices.

The first step of photosynthesis in plants is the absorption of sunlight by pigments in the antenna complexes of photosystem II (PSII), followed by transfer of the nascent excitation energy to the reaction centers, where long-term storage as chemical energy is initiated. Quantum mechanical mechanisms must be invoked to explain the transport of excitation within individual antenna. However, it is unclear how these mechanisms influence transfer across assemblies of antenna and thus the photochemical yield at reaction centers in the functional thylakoid membrane. Here, we model light harvesting at the several-hundred-nanometer scale of the PSII membrane, while preserving the dominant quantum effects previously observed in individual complexes. We show that excitation moves diffusively through the antenna with a diffusion length of 50 nm until it reaches a reaction center, where charge separation serves as an energetic trap. The diffusion length is a single parameter that incorporates the enhancing effect of excited state delocalization on individual rates of energy transfer as well as the complex kinetics that arise due to energy transfer and loss by decay to the ground state. The diffusion length determines PSII's high quantum efficiency in ideal conditions, as well as how it is altered by the membrane morphology and the closure of reaction centers. We anticipate that the model will be useful in resolving the nonphotochemical quenching mechanisms that PSII employs in conditions of high light stress.excitation energy transfer | quantum coherence | structure−function relationships | photosynthesis | fluorescence lifetime T he first step of photosynthesis is light harvesting, the absorption and conversion of sunlight into chemical energy. In photosynthetic organisms, the functional units of light harvesting are self-assembled arrays of pigment-protein complexes called photosystems. Antenna complexes absorb and transfer the nascent excitation energy to reaction centers, where long-term storage as chemical energy is initiated (1). In plants, photosystem II (PSII) flexibly responds to changes in sunlight intensity on a seconds to minutes time scale. In dim light, under ideal conditions, PSII harvests light with a >80% quantum efficiency (2), whereas, in intense sunlight PSII dissipates excess absorbed light safely as heat via nonphotochemical quenching pathways (3). The ability of PSII to switch between efficient and dissipative states is important for optimal plant fitness in natural sunlight conditions (4). Understanding how PSII's function arises from the structure of its constituent pigment−protein complexes is a prerequisite for systematically engineering the light-harvesting apparatus in crops (5-7) and could be useful for designing artificial materials with the same flexible properties (8, 9).Recent advances have established structure−function relationships within individual pigment−protein complexes, but not how these relationships affect the functioning of the dynamic PSII (grana) membrane (10). Electron micros...

Energy-dependent quenching (qE) in photosystem II (PSII) is a pH-dependent response that enables plants to regulate light harvesting in response to rapid fluctuations in light intensity. In this review, we aim to provide a physical picture for understanding the interplay between the triggering of qE by a pH gradient across the thylakoid membrane and subsequent changes in PSII. We discuss how these changes alter the energy transfer network of chlorophyll in the grana membrane and allow it to switch between an unquenched and quenched state. Within this conceptual framework, we describe the biochemical and spectroscopic measurements and models that have been used to understand the mechanism of qE in plants with a focus on measurements of samples that perform qE in response to light. In addition, we address the outstanding questions and challenges in the field. One of the current challenges in gaining a full understanding of qE is the difficulty in simultaneously measuring both the photophysical mechanism of quenching and the physiological state of the thylakoid membrane. We suggest that new experimental and modeling efforts that can monitor the many processes that occur on multiple timescales and length scales will be important for elucidating the quantitative details of the mechanism of qE.

We describe a technique to measure the fluorescence decay profiles of intact leaves during adaptation to high light and subsequent relaxation to dark conditions. We show how to ensure that photosystem II reaction centers are closed and compare data for wild type Arabidopsis thaliana with conventional pulse-amplitude modulated (PAM) fluorescence measurements. Unlike PAM measurements, the lifetime measurements are not sensitive to photobleaching or chloroplast shielding, and the form of the fluorescence decay provides additional information to test quantitative models of excitation dynamics in intact leaves.

SignificancePlants’ photosynthetic mechanism adjusts to fluctuations in light intensity. Intermittent bright sunlight can damage light-harvesting proteins; to preempt this, plants dissipate excess absorbed excitation energy as heat. Energy-dependent quenching (qE) of excitations occurs on the seconds to minutes timescale through conformational changes in antenna proteins. Using a multiscale model of photosystem II, we show that changes in light harvesting due to qE can be explained using a single parameter, the excitation diffusion length, which decreases as qE activates. These findings have implications for the interpretation of pulse amplitude-modulated fluorescence, a common noninvasive measurement of photosynthetic activity in leaves.

Photosynthetic organisms avoid photodamage to photosystem II (PSII) in variable light conditions via a suite of photoprotective mechanisms called nonphotochemical quenching (NPQ), in which excess absorbed light is dissipated harmlessly. To quantify the contributions of different quenching mechanisms to NPQ, we have devised a technique to measure the changes in chlorophyll fluorescence lifetime as photosynthetic organisms adapt to varying light conditions. We applied this technique to measure the fluorescence lifetimes responsible for the predominant, rapidly reversible component of NPQ, qE, in living cells of Chlamydomonas reinhardtii. Application of high light to dark-adapted cells of C. reinhardtii led to an increase in the amplitudes of 65 ps and 305 ps chlorophyll fluorescence lifetime components that was reversed after the high light was turned off. Removal of the pH gradient across the thylakoid membrane linked the changes in the amplitudes of the two components to qE quenching. The rise times of the amplitudes of the two components were significantly different, suggesting that the changes are due to two different qE mechanisms. We tentatively suggest that the changes in the 65 ps component are due to charge-transfer quenching in the minor light-harvesting complexes and that the changes in the 305 ps component are due to aggregated light-harvesting complex II trimers that have detached from PSII. We anticipate that this technique will be useful for resolving the various mechanisms of NPQ and for quantifying the timescales associated with these mechanisms.photosynthesis | in vivo spectroscopy | time-resolved fluorescence | feedback de-excitation | pH-dependent regulation