Photoinactivation of Photosystem II in wild-type and chlorophyll b-less barley leaves: which mechanism dominates depends on experimental circumstances

Weaker photosynthetic acclimation to fluctuating than to corresponding steady UVB radiation treatments in grapevines

The effects of transient increases in UVB radiation on plants are not well known; whether cumulative damage dominates or, alternately, an increase in photoprotection and recovery periods ameliorates any negative effects. We investigated photosynthetic capacity and metabolite accumulation of grapevines (Vitis vinifera Cabernet Sauvignon) in response to UVB fluctuations under four treatments: fluctuating UVB (FUV) and steady UVB radiation (SUV) at similar total biologically effective UVB dose (2.12 and 2.23 kJ m-2 day-1), and their two respective no UVB controls. We found a greater decrease in stomatal conductance under SUV than FUV. There was no decrease in maximum yield of photosystem II (Fv/Fm) or its operational efficiency (ɸPSII) under the two UVB treatments, and Fv/Fm was higher under SUV than FUV. Photosynthetic capacity was enhanced under FUV in the light-limited region of rapid light-response curves but enhanced by SUV in the light-saturated region. Flavonol content was similarly increased by both UVB treatments. We conclude that, while both FUV and SUV effectively stimulate acclimation to UVB radiation at realistic doses, FUV confers weaker acclimation than SUV. This implies that recovery periods between transient increases in UVB radiation reduce UVB acclimation, compared to an equivalent dose of UVB provided continuously. Thus, caution is needed in interpreting the findings of experiments using steady UVB radiation treatments to infer effects in natural environments, as the stimulatory effect of steady UVB is greater than that of the equivalent fluctuating UVB.

Exposed anthocyanic leaves of Prunus cerasifera are special shade leaves with high resistance to blue light but low resistance to red light against photoinhibition of photosynthesis

BACKGROUND AND AIMS

The photoprotective role of foliar anthocyanins has long been ambiguous: exacerbating, being indifferent to or ameliorating the photoinhibition of photosynthesis. The photoinhibitory light spectrum and failure to separate photo-resistance from repair, as well as the different methods used to quantify the photo-susceptibility of the photosystems, could lead to such a discrepancy.

METHODS

We selected two congeneric deciduous shrubs, Prunus cerasifera with anthocyanic leaves and Prunus triloba with green leaves, grown under identical growth conditions in an open field. The photo-susceptibilities of photosystem II (PSII) and photosystem I (PSI) to red light and blue light, in the presence of lincomycin (to block the repair), of exposed leaves were quantified by a non-intrusive P700+ signal from PSI. Leaf absorption, pigments, gas exchange and Chl a fluorescence were also measured.

KEY RESULTS

The content of anthocyanins in red leaves (P. cerasifera) was >13 times greater than that in green leaves (P. triloba). With no difference in maximum quantum efficiency of PSII photochemistry (Fv/Fm) and apparent CO2 quantum yield (AQY) in red light, anthocyanic leaves (P. cerasifera) showed some shade-acclimated suites, including lower Chl a/b ratio, lower photosynthesis rate, lower stomatal conductance and lower PSII/PSI ratio (on an arbitrary scale), compared with green leaves (P. triloba). In the absence of repair of PSII, anthocyanic leaves (P. cerasifera) showed a rate coefficient of PSII photoinactivation (ki) that was 1.8 times higher than that of green leaves (P. triloba) under red light, but significantly lower (-18 %) under blue light. PSI of both types of leaves was not photoinactivated under blue or red light.

CONCLUSIONS

In the absence of repair, anthocyanic leaves exhibited an exacerbation of PSII photoinactivation under red light and a mitigation under blue light, which can partially reconcile the existing controversy in terms of the photoprotection by anthocyanins. Overall, the results demonstrate that appropriate methodology applied to test the photoprotection hypothesis of anthocyanins is critical.

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.

Investigating the impact of light quality on macromolecular composition of Chaetoceros muelleri

Diatoms (Bacillariophyceae) are important to primary productivity of aquatic ecosystems. This algal group is also a valuable source of high value compounds that are utilised as aquaculture feed. The productivity of diatoms is strongly driven by light and CO2 availability, and macro- and micronutrient concentrations. The light dependency of biomass productivity and metabolite composition is well researched in diatoms, but information on the impact of light quality, particularly the productivity return on energy invested when using different monochromatic light sources, remains scarce. In this work, the productivity return on energy invested of improving growth rate, photosynthetic activity, and metabolite productivity of the diatom Chaetoceros muelleri under defined wavelengths (blue, red, and green) as well as while light is analysed. By adjusting the different light qualities to equal photosynthetically utilisable radiation, it was found that the growth rate and photosynthetic oxygen evolution was unchanged under white, blue, and green light, but it was lower under red light. Blue light improved the productivity return on energy invested for biomass, total protein, total lipid, total carbohydrate, and in fatty acids production, which would suggest that blue light should be used for aquaculture feed production.

Intrinsic Photosensitivity of the Vulnerable Seagrass Phyllospadix iwatensis: Photosystem II Oxygen-Evolving Complex Is Prone to Photoinactivation

Phyllospadix iwatensis, a foundation species of the angiosperm-dominated marine blue carbon ecosystems, has been recognized to be a vulnerable seagrass. Its degradation has previously been reported to be associated with environmental changes and human activities, while there has been a limited number of studies on its inherent characteristics. In this study, both the physiological and molecular biological data indicated that the oxygen-evolving complex (OEC) of P. iwatensis is prone to photoinactivation, which exhibits the light-dependent trait. When exposed to laboratory light intensities similar to typical midday conditions, <10% of the OEC was photoinactivated, and the remaining active OEC was sufficient to maintain normal photosynthetic activity. Moreover, the photoinactivated OEC could fully recover within the same day. However, under harsh light conditions, e.g., light intensities that simulate cloudless sunny neap tide days and continual sunny days, the OEC suffered irreversible photoinactivation, which subsequently resulted in damage to the photosystem II reaction centers and a reduction in the rate of O₂ evolution. Furthermore, in situ measurements on a cloudless sunny neap tide day revealed both poor resilience and irreversible photoinactivation of the OEC. Based on these findings, we postulated that the OEC dysfunction induced by ambient harsh light conditions could be an important inherent reason for the degradation of P. iwatensis.

Genetic architecture of photosynthesis energy partitioning as revealed by a genome-wide association approach

The photosynthesis process is determined by the intensity level and spectral quality of the light; therefore, leaves need to adapt to a changing environment. The incident energy absorbed can exceed the sink capability of the photosystems, and, in this context, photoinhibition may occur in both photosystem II (PSII) and photosystem I (PSI). Quantum yield parameters analyses reveal how the energy is managed. These parameters are genotype-dependent, and this genotypic variability is a good opportunity to apply mapping association strategies to identify genomic regions associated with photosynthesis energy partitioning. An experimental and mathematical approach is proposed for the determination of an index which estimates the energy per photon flux for each spectral bandwidth (Δ_λ) of the light incident (QI index). Based on the QI, the spectral quality of the plant growth, environmental lighting, and the actinic light of PAM were quantitatively very similar which allowed an accurate phenotyping strategy of a rice population. A total of 143 genomic single regions associated with at least one trait of chlorophyll fluorescence were identified. Moreover, chromosome 5 gathers most of these regions indicating the importance of this chromosome in the genetic regulation of the photochemistry process. Through a GWAS strategy, 32 genes of rice genome associated with the main parameters of the photochemistry process of photosynthesis in rice were identified. Association between light-harvesting complexes and the potential quantum yield of PSII, as well as the relationship between coding regions for PSI-linked proteins in energy distribution during the photochemical process of photosynthesis is analyzed.

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.

Singlet oxygen damages the function of Photosystem II in isolated thylakoids and in the green alga Chlorella sorokiniana

Singlet oxygen (¹O₂) is an important damaging agent, which is produced during illumination by the interaction of the triplet excited state pigment molecules with molecular oxygen. In cells of photosynthetic organisms ¹O₂ is formed primarily in chlorophyll containing complexes, and damages pigments, lipids, proteins and other cellular constituents in their environment. A useful approach to study the physiological role of ¹O₂ is the utilization of external photosensitizers. In the present study, we employed a multiwell plate-based screening method in combination with chlorophyll fluorescence imaging to characterize the effect of externally produced ¹O₂ on the photosynthetic activity of isolated thylakoid membranes and intact Chlorella sorokiniana cells. The results show that the external ¹O₂ produced by the photosensitization reactions of Rose Bengal damages Photosystem II both in isolated thylakoid membranes and in intact cells in a concentration dependent manner indicating that ¹O₂ plays a significant role in photodamage of Photosystem II.

The effect of different spectral light quality on the photoinhibition of Photosystem I in intact leaves

Light energy causes damage to Photosystem I (PSI) and Photosystem II (PSII). The majority of the previous photoinhibition studies have been conducted with PSII, which shows much larger photoinhibition than PSI; therefore, relatively little is known about the mechanism of PSI photoinhibition so far. A previous report showed that the photoinhibition action spectrum measured with PSI activity of isolated thylakoid is similar to the absorption spectrum of chlorophyll. However, it is known that the extent of PSI photoinhibition is much smaller in vivo compared to in vitro. It is also possible that the different extent of PSII photoinhibition, caused by different spectral light qualities, can affect the photoinhibition of PSI in vivo because PSI receives electrons from PSII. In the present research, to study the effect of light quality and the effect of the extent of PSII photoinhibition on the PSI photoinhibition in vivo, intact leaves were photoinhibited under four different light qualities. The rate coefficient of PSI photoinhibition was significantly higher in blue and red light compared to white light. The rate of PSI photoinhibition at the same photon-exposure was the largest in blue and red light and followed by white and green light. These results support the notion that light absorption by chlorophyll is responsible for the PSI photoinhibition, even in intact leaves. The variation among light colors in the relationships between the extent of photoinhibition of PSII and that of PSI indicate that PSI and PSII are independently photoinhibited with different mechanisms in the early stage of in vivo photoinhibition.

My precarious career in photosynthesis: a roller-coaster journey into the fascinating world of chloroplast ultrastructure, composition, function and dysfunction

Despite my humble beginnings in rural China, I had the good fortune of advancing my career and joining an international community of photosynthesis researchers to work on the 'light reactions' that are a fundamental process in Nature. Along with supervisors, mentors, colleagues, students and lab assistants, I worked on ionic redistributions across the photosynthetic membrane in response to illumination, photophosphorylation, forces that regulate the stacking of photosynthetic membranes, the composition of components of the photosynthetic apparatus during acclimation to the light environment, and the failure of the photosynthetic machinery to acclimate to too much light or even to cope with moderate light due to inevitable photodamage. These fascinating underlying mechanisms were investigated in vitro and in vivo. My career path, with its ups and downs, was never secure, but the reward of knowing a little more of the secret of Nature offset the job uncertainty.

A magic red coat on the surface of young leaves: anthocyanins distributed in trichome layer protect Castanopsis fissa leaves from photoinhibition

The presence of anthocyanins in young leaves plays an important role in mitigation against photodamage and allows leaves to grow and develop normally. Many studies have reported that foliar anthocyanins are distributed within the vacuoles of mesophyll cells, so we explored the novel defence style of anthocyanin-coated young leaves of Castanopsis fissa, a dominant subtropical forest tree species, via removable trichomes. Anthocyanins were distributed in C. fissa leaf trichomes, which produced a red coating for the young leaves. As young leaves developed and then matured, the thickness and density of the anthocyanin trichomes progressively decreased, the coating finally disappearing, allowing greater utilization of light by mature leaves. In addition to anthocyanins, the trichomes contained a remarkably high amount of phenolics, which enable the red coating to be more efficient in screening ultraviolet light. Compared with mature leaves, the young leaves exhibited lower photosynthetic ability, which was attributable to the reduced chlorophyll and Rubisco contents. Removal of the red coating had little effect on the photosynthetic capacity of young leaves. However, the young leaves without the coating suffered greater light-induced photoinhibition due to greater excess light entering the chloroplast and the production of H₂O₂ Our results suggest that the anthocyanin coating is photoprotective and this anthocyanin defence style may be a metabolically cost-effective way of adjusting the anthocyanin content in response to demand.

Light environment within a leaf. II. Progress in the past one-third century

Studies directly related to light environments within a leaf, conduced mainly in the past one-third century, are reviewed. In particular, studies that revealed the profiles of light absorption and photosynthetic capacity are highlighted. Progress in this research field has been accelerated by devising innovative techniques. Roles of the main photosynthetic tissues, the palisade and spongy tissues, as the light guide and diffuser, respectively, are discussed. When the leaf is illuminated with diffuse light, light is absorbed more by the chloroplasts located near the illuminated surface. The meanings of the occupation of the mesophyll surfaces facing the intercellular spaces by chloroplasts and chloroplast movement are also discussed. The discrepancy between the light absorption profile and that of photosynthetic capacity is examined most intensively.