A rapid, whole-tissue determination of the functional fraction of PSII after photoinhibition of leaves based on flash-induced P700 redox kinetics

Accounting for photosystem I photoinhibition sheds new light on seasonal acclimation strategies of boreal conifers

The photosynthetic acclimation of boreal evergreen conifers is controlled by regulatory and photoprotective mechanisms that allow conifers to cope with extreme environmental changes. However, the underlying dynamics of photosystem II (PSII) and photosystem I (PSI) remain unresolved. Here, we investigated the dynamics of PSII and PSI during the spring recovery of photosynthesis in Pinus sylvestris and Picea abies using a combination of chlorophyll a fluorescence, P700 difference absorbance measurements, and quantification of key thylakoid protein abundances. In particular, we derived a new set of PSI quantum yield equations, correcting for the effects of PSI photoinhibition. Using the corrected equations, we found that the seasonal dynamics of PSII and PSI photochemical yields remained largely in balance, despite substantial seasonal changes in the stoichiometry of PSII and PSI core complexes driven by PSI photoinhibition. Similarly, the previously reported seasonal up-regulation of cyclic electron flow was no longer evident, after accounting for PSI photoinhibition. Overall, our results emphasize the importance of considering the dynamics of PSII and PSI to elucidate the seasonal acclimation of photosynthesis in overwintering evergreens. Beyond the scope of conifers, our corrected PSI quantum yields expand the toolkit for future studies aimed at elucidating the dynamic regulation of PSI.

The Protective Role of Non-Photochemical Quenching in PSII Photo-Susceptibility: A Case Study in the Field

Non-photochemical quenching (NPQ) has been regarded as a safety valve to dissipate excess absorbed light energy not used for photochemistry. However, there exists no general consensus on the photoprotective role of NPQ. In the present study, we quantified the Photosystem II (PSII) photo-susceptibilities (mpi) in the presence of lincomycin, under red light given to five shade-acclimated tree species grown in the field. Photosynthetic energy partitioning theory was applied to investigate the relationships between mpi and each of the regulatory light-induced NPQ [Y(NPQ)], the quantum yield of the constitutive nonregulatory NPQ [Y(NO)] and the PSII photochemical yield in the light-adapted state [Y(PSII)] under different red irradiances. It was found that in the low to moderate irradiance range (50-800 μmol m-2 s-1) when the fraction of open reaction centers (qP) exceeded 0.4, mpi exhibited no association with Y(NPQ), Y(NO) and Y(PSII) across species. However, when qP < 0.4 (1,500 μmol m-2 s-1), there existed positive relationships between mpi and Y(NPQ) or Y(NO) but a negative relationship between mpi and Y(PSII). It is postulated that both Y(NPQ) and Y(NO) contain protective and damage components and that using only Y(NPQ) or Y(NO) metrics to identify the photo-susceptibility of a species is a risk. It seems that qP regulates the balance of the two components for each of Y(NPQ) and Y(NO). Under strong irradiance, when both protective Y(NPQ) and Y(NO) are saturated/depressed, the forward electron flow [i.e. Y(PSII)] acts as the last defense to resist photoinhibition.

The Role of Alternative Electron Pathways for Effectiveness of Photosynthetic Performance of Arabidopsis thaliana, Wt and Lut2, under Low Temperature and High Light Intensity

A recent investigation has suggested that the enhanced capacity for PSI-dependent cyclic electron flow (CEF) and PSI-dependent energy quenching that is related to chloroplast structural changes may explain the lower susceptibility of lut2 to combined stresses—a low temperature and a high light intensity. The possible involvement of alternative electron transport pathways, proton gradient regulator 5 (PGR5)-dependent CEF and plastid terminal oxidase (PTOX)-mediated electron transfer to oxygen in the response of Arabidopsis plants—wild type (wt) and lut2—to treatment with these two stressors was assessed by using specific electron transport inhibitors. Re-reduction kinetics of P₇₀₀⁺ indicated that the capacity for CEF was higher in lut2 when this was compared to wt. Exposure of wt plants to the stress conditions caused increased CEF and was accompanied by a substantial raise in PGR5 and PTOX quantities. In contrast, both PGR5 and PTOX levels decreased under the same stress conditions in lut2, and inhibiting PGR5-dependent pathway by AntA did not exhibit any significant effects on CEF during the stress treatment and recovery period. Electron microscopy observations demonstrated that under control conditions the degree of grana stacking was much lower in lut2, and it almost disappeared under the combined stresses, compared to wt. The role of differential responses of alternative electron transport pathways in the acclimation to the stress conditions that are studied is discussed.

The Responses of Light Reaction of Photosynthesis to Dynamic Sunflecks in a Typically Shade-Tolerant Species Panax notoginseng

Light is highly heterogeneous in natural conditions, and plants need to evolve a series of strategies to acclimate the dynamic light since it is immobile. The present study aimed to elucidate the response of light reaction of photosynthesis to dynamic sunflecks in a shade-tolerant species Panax notoginseng and to examine the regulatory mechanisms involved in an adaptation to the simulated sunflecks. When P. notoginseng was exposed to the simulated sunflecks, non-photochemical quenching (NPQ) increased rapidly to the maximum value. Moreover, in response to the simulated sunflecks, there was a rapid increase in light-dependent heat dissipation quantum efficiency of photosystem II (PSII) (Φ_NPQ), while the maximum quantum yield of PSII under light (F _v'/F _m') declined. The relatively high fluorescence and constitutive heat dissipation quantum efficiency of PSII (Φ_f,d) in the plants exposed to transient high light (400, 800, and 1,600 μmol m^-2 s^-1) was accompanied by the low effective photochemical quantum yield of PSII (Φ_PSII) after the dark recovery for 15 min, whereas the plants exposed to transient low light (50 μmol m^-2 s^-1) has been shown to lead to significant elevation in Φ_PSII after darkness recovery. Furthermore, PSII fluorescence and constitutive heat dissipation electron transfer rate (J _f,d) was increased with the intensity of the simulated sunflecks, the residual absorbed energy used for the non-net carboxylative processes (J _NC) was decreased when the response of electron transfer rate of NPQ pathway of PSII (J _NPQ) to transient low light is restricted. In addition, the acceptor-side limitation of PSI [Y(NA)] was increased, while the donor-side limitation of photosystems I (PSI) [Y(ND)] was decreased at transient high light conditions accompanied with active cyclic electron flow (CEF). Meanwhile, when the leaves were exposed to transient high light, the xanthophyll cycle (V cycle) was activated and subsequently, the J _NPQ began to increase. The de-epoxidation state [(Z + A)/(V + A + Z)] was strongly correlated with NPQ in response to the sunflecks. In the present study, a rapid engagement of lutein epoxide (Lx) after the low intensity of sunfleck together with the lower NPQ contributed to an elevation in the maximum photochemical quantum efficiency of PSII under the light. The analysis based on the correlation between the CEF and electron flow devoted to Ribulose-1, 5-bisphosphate (RuBP) oxygenation (J _O) indicated that at a high light intensity of sunflecks, the electron flow largely devoted to RuBP oxygenation would contribute to the operation of the CEF. Overall, photorespiration plays an important role in regulating the CEF of the shade-tolerant species, such as P. notoginseng in response to transient high light, whereas active Lx cycle together with the decelerated NPQ may be an effective mechanism of elevating the maximum photochemical quantum efficiency of PSII under light exposure to transient low light.

A theoretical framework of the hybrid mechanism of photosystem II photodamage

Photodamage of photosystem II is a significant physiological process that is prevalent in the fields of photobiology, photosynthesis research and plant/algal stress. Since its discovery, numerous efforts have been devoted to determine the causes and mechanisms of action of photosystem II photodamage. There are two contrasting hypotheses to explain photodamage: (1) the excitation pressure induced by light absorption by the photosynthetic pigments and (2) direct photodamage of the Mn cluster located at the water-splitting site, which is independent of excitation pressure. While these two hypotheses seemed mutually exclusive, during the last decade, several independent works have proposed an alternative approach indicating that both hypotheses are valid. This was termed the dual hypothesis of photosystem II photodamage, and it postulates that both excess excitation and direct Mn photodamage operate at the same time, independently or in a synergic manner, depending on the type of sample, temperature, light spectrum, or other environmental stressors. In this mini-review, a brief summary of the contrasting hypotheses is presented, followed by recapitulation of key discoveries in the field of photosystem II photodamage of the last decade, and a synthesis of how these works support a full hybrid framework (operation of several mechanisms and their permutations) to explain PSII photodamage. All these are in recognition of Prof. Wah Soon Chow (the Australian National University), one of the key proposers of the dual hypothesis.

Intra-leaf gradients of photoinhibition induced by different color lights: implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers

• We studied how different color lights cause gradients of photoinhibition within a leaf, to attempt to resolve the controversy of whether photon absorption by chlorophyll or by manganese (Mn) is the primary cause of photoinhibition, as suggested by the excess-energy hypothesis or the two-step hypothesis, respectively. • Lincomycin-treated leaf discs were photoinhibited by white, blue, green or red light. Combining a microfiber fluorometer, a fiber-thinning technique and a micro-manipulator enabled us to measure the chlorophyll fluorescence signals within a leaf. Photoinhibition gradients were also compared with results from various conventional fluorometers to estimate their depth of signal detection. • The severity of photoinhibition was in the descending order of blue, red and green light near the adaxial surface, and in the descending order of blue, green and red light in the deeper tissue, which correlated with the chlorophyll and the Mn absorption spectrums, respectively. These results cannot be explained by either hypothesis alone. • These data strongly suggest that both the excess-energy and the two-step mechanisms occur in photoinhibition, and fluorometers with red or blue measuring light give overestimated or underestimated F(v)/F(m) values of photoinhibited leaves compared with the whole tissue average, respectively; that is, they measured deeper or shallower leaf tissue, respectively.