Metal toxicity in marine ecosystems is a growing issue owing to terrestrial runoff and anthropogenic pollution. Heterocapsa cf. bohaiensis, a newly isolated dinoflagellate from New Caledonia, was cultivated in photobioreactors operating continuously with high concentrations (10^-3 M) of nickel (Ni²⁺) and/or iron (Fe²⁺) and their photosynthetic efficiency was assessed. The photosynthetic measurements indicated that H. cf. bohaiensis was tolerant to Ni²⁺ but sensitive to Fe²⁺ high concentrations. In the presence of Fe²⁺, maximum quantum efficiency and maximal relative electron transport rate decreased from 0.62 to 0.47 and from 156 to 102, respectively. The JIP-tests suggested a reduction of the photosynthesis in response to Fe²⁺ due to a disruption in the electron transport chain rather than a defect in the light absorption and trapping capacity which were on the contrary enhanced by Fe²⁺. These results bring new knowledge on the impact of nickel and iron on microalgae photosynthetic pathways.

We present here a tribute to George E. Hoch, one of the top leaders in deciphering the primary steps of oxygenic photosynthesis. After providing his academic background, we discuss his selected research on the photoreactions of photosynthesis. We end this tribute with reminiscences by Robert S. Knox, Jerome Kaye, and Doris Teichler-Zallen.

Christa Critchley is a distinguished researcher in basic and applied photosynthesis research. Her research has centered on the structure and function of chloroplasts and the application of chlorophyll a fluorescence to understanding the way PSII works. In her research, she used two biophysical tools, Nuclear Magnetic Resonance (NMR) and Chlorophyll (Chl) a fluorescence, as well as several other biochemical and plant physiological methods. Later in her career, she pioneered research in artificial photosynthesis (AP) focusing on the process of light-mediated water splitting and oxygen evolution. Here, only a glimpse of her life and some selected research is included.

A 50-year-old solitary, sun-exposed ginkgo tree had strongly been pruned in the fall of 2021. Very few buds for the formation of new leaves, twigs, and branches were left over. In spring 2022, these few remaining buds responded with the formation of a different leaf type. These leaves were 2.7 times larger and also thicker than in the years before. In addition, the mean content of total chlorophylls [Chl (a+b)] per leaf area unit of dark-green leaves was 1.45, those of green leaves two times higher as compared to the years before pruning and the two other ginkgo trees which had been investigated in parallel. A comparable increase was also found for the level of total carotenoids (x+c). The mean content for Chl (a+b) were 1,118 mg m^-2 for dark-green and 898 mg m^-2 for green leaves as compared to 435 to 770 mg m^-2 in leaves of other trees. The higher values for Chl (a+b) and total carotenoid content showed up also on a fresh and dry mass basis. Thus, with the formation of a new, larger leaf type by changes in morphology (leaf size and thickness) and the increase of photosynthetic pigments, the pruned ginkgo tree was able to compensate for the much lower number of leaves and photosynthetic units.

Diatom thylakoids contain much higher amounts of sulfoquinovosyl diacylglycerol (SQDG) than vascular plants and the hypothesis was brought forward that this relates to their special thylakoid structure. To test this hypothesis we created knock-down mutants in Thalassiosira pseudonana that exhibited a decreased SQDG content per cell. Surprisingly, the ratio between the different lipid classes did not change, pointing to strict regulation of thylakoid lipid composition. The antenna proteins, fucoxanthin-chlorophyll proteins (FCP), were reduced and photosystem (PS) I compared to PSII was increased as judged from absorbance spectra. CD spectroscopy indicated a tighter packing of chromophores. The reduction in FCP might help to avoid diametral changes in excitation energy transfer. In contrast, the increase in PSI in the mutants might counteract the diminishment of the usually huge PSI antenna. No changes in thylakoid structure were observed since the stoichiometry between different lipid classes seems to be carefully balanced.

Active control of photosynthetic activities is important in plant physiological study. Although models of plant photosynthesis have been built at different scales, they have not been fully examined for their application in plant growth control. However, we do not have an infrastructure to support such experiments since current plant growth chambers usually use fixed control protocols. In our current paper, an open IoT-based framework is proposed. This framework allows a plant scientist or agricultural engineer, through an application programming interface (API), in a desirable programming language, (1) to gather environmental data and plant physiological responses; (2) to program and execute control algorithms based on their models, and then (3) to implement real-time commands to control environmental factors. A plant growth chamber was developed to demonstrate the concept of the proposed open framework.

At a three-leaf stage, two Fe treatments [0 mg kg^-1 (Fe-) and 20 mg.kg^-1 (Fe+) in the form of FeCl₃] were used in the soil of the pot and then two concentrations of α-ketoglutaric acid [0 mg L^-1 (A-) and 50 mg L^-1 (A+)] were sprayed to the rice plants of Meixiangzhan and Yuxiangyouzhan cultivars. We showed that seedlings exhibited an increased length and fresh and dry mass of shoots and roots with treatments Fe+A- and Fe-A+, as well as the Fe content increased greatly. Both treatments increased the morphological characteristic values of roots and promoted photosynthesis. Interestingly, Fe+A+ notably affected the photosynthesis of fragrant rice seedlings; however, it exerted no significant differences on other parameters. Overall, Fe and α-ketoglutaric acid had the potential for improving the growth of fragrant rice seedlings. The interaction between Fe and α-ketoglutaric acid regulated photosynthesis in seedling leaves, which provided evidence for further improvement of rice cultivation.

Chloroplasts and photosynthesis are the physiologically fateful arenas of salinity stress. Morphological and anatomical alterations in the leaf tissue, ultrastructural changes in the chloroplast, compromise in the integrity of the three-layered chloroplast membrane system, and defects in the light and dark reactions during the osmotic, ionic, and oxidative phases of salt stress are conversed in detail to bring the salinity-mediated physiological alterations in the chloroplast on to a single platform. Chloroplasts of salt-tolerant plants have evolved highly regulated salt-responsive pathways. Thylakoid membrane remodeling, ion homeostasis, osmoprotection, upregulation of chloroplast membrane and stromal proteins, chloroplast ROS scavenging, efficient retrograde signalling, and differential gene and metabolite abundance are the key attributes of optimal photosynthesis in tolerant species. This review throws light into the comparative mechanism of chloroplast and photosynthetic response to salinity in sensitive and tolerant plant species.

This study evaluated the side-cleft grafting in native species of the genus Piper (Piper aduncum L., Piper hispidum Sw., and Piper tuberculatum Jacq.) with BRS Kottanadan pepper cultivar as rootstocks (Experiment 1). The top-cleft grafting was evaluated in native species Piper arboreum Aubl., P. aduncum, and P. tuberculatum with Balankotta pepper cultivar as graft using black pepper cv. Bragantina (Experiment 2). Black pepper cv. Bragantina autograft was considered as control. The rootstock P. tuberculatum showed initial incompatibility with the cv. Bragantina, with survival of about 34.8% (side) and 62.5% (top) and total inhibition of graft shooting. The side and top graft on P. aduncum resulted in 84.0 and 47.5% of survival, respectively. The JIP-test parameters indicated better photochemical efficiency in the species grafted using the method of side-cleft grafting. Finally, the initial compatibility was associated with greater survival, shooting, and better energy flow through the electron transport chain.

In this historical perspective, we focus on selected discoveries that Albert Frenkel (1919-2015) made all by himself - single-handedly - which is the discovery of photophosphorylation and NAD reduction in anoxygenic photosynthetic bacteria. Then, we present various aspects of his research life through his unpublished letters with some key scientists in his research field. To give a glimpse of his personal life, we have also provided some photographs.

The investigation of spatial heterogeneity within the thylakoid membrane (TM) proteins has gained increasing attention in photosynthetic research. The recent advances in live-cell imaging have allowed the identification of heterogeneous organisation of photosystems in small cyanobacterial cells. These sub-micrometre TM regions, termed microdomains in cyanobacteria, exhibit functional similarities with granal (Photosystem II dominant) and stromal (Photosystem I dominant) regions observed in TM of higher plants. This study delves into microdomain heterogeneity using super-resolution Airyscan-based microscopy enhancing resolution to approximately ~125 nm in x-y dimension. The new data reveal membrane areas rich in Photosystem I within the inner TM rings. Moreover, we identified analogous dynamics in the mobility of Photosystem II and phycobilisomes; countering earlier models that postulated differing mobility of these complexes. These novel findings thus hold significance for our understanding of photosynthesis regulation, particularly during state transitions.

Light-induced changes in miRNAs, morphogenesis, and photosynthetic processes in phytochrome-deficient mutant plants grown under different light qualities were studied. miRNA activity in many processes is regulated by phytochromes and phytochrome-interacting factors (PIFs). The reduced content of photoreceptors in phytochrome mutants affects the PIF-microRNA interaction. In plants grown under red light (RL) and white light (WL), the phenotype of phyb mutant was distorted; however, under blue light (BL) conditions, the phyb phenotype was normalized. The photosynthetic rates of both the mutants and wild type were higher under BL than under RL and WL. The expression of most studied miRNAs increased in phyaphyb mutants under BL conditions, which is probably one of the reasons for the normalization of the phenotype, the increase in PSII activity, and the photosynthetic rate. MicroRNAs under BL can partially improve photosynthesis and phenotype of the mutants, which indicates the conjugation of the functioning of phytochromes in miRNA formation.

Natural SiO₂ nanoparticles (SiO₂-NPs) are widely distributed in the environment, and at the same time, synthetic SiO₂-NP may be applied in agriculture. Evaluations of physiological responses to SiO₂-NPs treatment of plants are controversial. They are often performed at adaxial leaf sides whereas NPs permeate leaf tissues through stomata located at the abaxial leaf side in the majority of bifacial plants. We measured coefficients of the functional dorsoventral asymmetry of NPs-stressed Chelidonium majus leaves, S, by values of the CO₂ assimilation rate (SP_N), dark respiration (SR), maximal and operating quantum yields of photosystem II (SF_v/F_m, SF_v'/F_m'; using PAM-fluorometry), and oxygen coefficients of photosynthesis (SΨ_O2; using photoacoustics). The results indicated that SP_N and SΨ_O2 were significantly influenced by SiO₂-NPs treatment, since P_N and Ψ_O2 were declining more markedly when the light was directed to the abaxial side of leaves compared to the adaxial side. Overall, SiO₂-NPs-induced stress increased 'anoxygenity' of photosynthesis.

Oxygenic photosynthesis takes place in thylakoid membranes (TM) of cyanobacteria, algae, and higher plants. It begins with light absorption by pigments in large (modular) assemblies of pigment-binding proteins, which then transfer excitation energy to the photosynthetic reaction centers of photosystem (PS) I and PSII. In green algae and plants, these light-harvesting protein complexes contain chlorophylls (Chls) and carotenoids (Cars). However, cyanobacteria, red algae, and glaucophytes contain, in addition, phycobiliproteins in phycobilisomes that are attached to the stromal surface of TM, and transfer excitation energy to the reaction centers via the Chl a molecules in the inner antennas of PSI and PSII. The color and the intensity of the light to which these photosynthetic organisms are exposed in their environment have a great influence on the composition and the structure of the light-harvesting complexes (the antenna) as well as the rest of the photosynthetic apparatus, thus affecting the photosynthetic process and even the entire organism. We present here a perspective on 'Light Quality and Oxygenic Photosynthesis', in memory of George Christos Papageorgiou (9 May 1933-21 November 2020; see notes a and b). Our review includes (1) the influence of the solar spectrum on the antenna composition, and the special significance of Chl a; (2) the effects of light quality on photosynthesis, measured using Chl a fluorescence; and (3) the importance of light quality, intensity, and its duration for the optimal growth of photosynthetic organisms.

Modulation of glycoregulation in agriculture is reviewed here with emphasis on the elucidation of previously unknown pathways involving vacuolar lectins as well as a bypass of lectins that direct free sugars toward productivity. The reversible binding sequences of the endogenous lectin cycle are compared to an induced lectin bypass, as follows. (1) In the cycle, carbohydrate ligands, with similar binding specificities that compete for binding sites on lectins, are involved in the natural cycle of sugar exchanges. (2) For the bypass, tightly bound ligands that occupy lectins prevent free sugars from binding, making them available for productivity. This bypass is induced by methyl-α-D-mannopyranoside, a biochemical plant growth regulator for photosynthesis. Integration of this novel technology, with structural elements crucial for ligand binding by the lectins and with nitrogen assimilation, provides the basis for successful modulation of glycoregulation in crops for enhancement of quality and quantity.

This study was conducted to assess whether silicon (Si) supply can alleviate the harmful effects of severe salinity in barley (Hordeum vulgare). Plants were grown on non-saline (0 mM NaCl) or saline (200 mM NaCl) nutrient media supplemented or not with 0.5 mM Si. Salinity impacted plant morphology and induced sodium and chloride accumulation within plant tissues. It significantly affected almost all measured parameters. Interestingly, Si supply alleviated salt stress effects on plant morphology, growth (up to +59%), water status (up to +74%), membrane integrity (up to +35%), pigment contents (up to +121%), and the activity of the two photosystems (PSI and PSII) by improving their yields, and by reducing their energy dissipation. Si beneficial effect was more pronounced on PSI as compared to PSII. As a whole, data inferred from the present study further confirmed that silicon application is an effective approach to cope with salinity.

Increasing the efficiency of photosynthesis in sugarcane canopies is the key for improving crop yield. Herein, we evaluated the photosynthetic performance along the canopy of ten sugarcane cultivars and three Saccharum species. Canopy morphological traits were evaluated, and leaf gas exchange was measured in the first (sun-exposed, +1) and the fourth (shaded, +4) fully expanded leaves and under low- and high-light conditions. Similar photosynthetic capacity was found in leaves +1 and +4 under high light in genotypes with a high leaf area index and a high fraction of the sky blocked by the foliage (> 85%). Interestingly, such canopy characteristics cause low light availability to leaves +4, suggesting the photosynthetic acclimation of these leaves to self-shading in some genotypes. We highlight IACCTC06-8126 and CTC4 as those genotypes with higher canopy photosynthetic capacity, presenting high leaf area, high photosynthetic rates in sun-exposed leaves, and high responsiveness of shaded leaves to increasing light availability.

We present here our adventures in research in photosynthesis with George C. Papageorgiou (1933-2020) focusing on George's initiative in the discovery of the protective effects of glycine betaine on the oxygen-evolving photosystem II complex. We end with a brief description of research on glycine betaine-synthesizing transgenic cyanobacteria. Two of us, Norio Murata (in Japan) and Kostas Stamatakis (in Greece), and all our collaborators, have the highest respect for George, and we miss him and our intense discussions with him on various topics of photosynthesis research.

Tribute to Jean Lavorel (16 March 1928–12 January 2021), a pioneer of the ‘Light Reactions of Photosynthesis’. He was known not only for his ingenuity in devising new instruments but in thoroughly analyzing all the available data theoretically and mathematically – mostly all by himself. He measured, elegantly, oxygen evolution and light given off by photosynthetic organisms, both prompt and delayed chlorophyll fluorescence. He ingeniously used these data to understand how light energy is converted to chemical energy in natural systems. We present below a summary of his life and research.

The ability to modulate photosynthesis is essential for plants to adapt to fluctuating growing conditions. Populus species show high tolerance to various and highly variable environments. To understand their response strategies against fluctuating environments, this study investigated the morphological and physiological differences of white poplar (Populus alba) leaves when grown in a phytotron, glasshouse, and field. Our results show that the palisade cells were elongated in the field, which would enhance intercellular CO₂ exchange. Photosynthetic capacity was the highest in the field leaves, as shown by higher electron transport rates (1.8 to 6.5 times) and carbon assimilation rates (2.7 to 4.2 times). The decrease of PSI acceptor-side limitation and increase of PSI donor-side limitation suggests changes in PSI redox status may contribute to photoprotection. This plasticity of white poplar allows adjusting its structure and photosynthesis under fluctuating conditions, which may partly enable its outstanding tolerance against environmental changes.

The content of chlorophylls (Chl) (a+b), total carotenoids (x+c), and the pigment ratios of Chl a/b and Chls to carotenoids (a+b)/(x+c) of green leaves of five C₄ plants were determined and compared to those of C₃ plants. The C₄ plants were: Pacific and Chinese silvergrass (Miscanthus floridulus and Miscanthus sinensis), sugar cane (Saccharum officinarum) as well as feed and sugar maize (Zea mays). The three C₃ plants were beech, ginkgo, and oak. C₄ plants possess higher values for the ratio Chl a/b (3.4-4.5) as compared to the C₃ plants (2.6-3.3). Sugar maize had the highest values for Chl a/b (4.04-4.70) and exceptionally high contents of total carotenoids and consequently lower values for the ratio of (a+b)/(x+c) (mean: 3.75 ± 0.6). During autumnal senescence also C₄ plants showed a faster decline of Chl b as compared to Chl a yielding high values for Chl a/b of 6 to 8. Chlorophylls declined faster than carotenoids yielding low (a+b)/(x+c) values below 1.0.

In stressful environments, invasive plants acclimate more efficiently than native plants and hybridization mainly contributes to this process. We examined changes in the morphological characteristics, photosynthetic characteristics, and antioxidant capacity of Sphagneticola trilobata and its hybrids in a low-light environment to explore their invasiveness, with Sphagneticola calendulacea serving as the control. The morphological plasticity of S. trilobata was not dominant, the maximal photochemical efficiency of PSII, actual quantum yield of PSII, and electron transport rate of PSⅡ increased and nonphotochemical quenching decreased, while S. calendulacea and the hybrid produced opposite results. S. trilobata showed fewer spots stained for reactive oxygen species in tissues, with an increase in superoxide dismutase activity. Although S. trilobata is a heliophilous plant, we found that the shade tolerance of S. trilobata and the hybrid were stronger than that of S. calendulacea, which may be one important mechanism of invasion.

Bacon Ke was a physical chemist and an ingenious instrument designer whose work led to an in-depth understanding of the primary photochemistry of photosynthesis.

Congratulation to Govindjee on his 90^th birthday

Many studies have been conducted on maize to study the effect of drought on yield at the flowering stage, but understanding biochemical and photosynthetic response against drought at the seedling stage needs to be well established. Thus, to understand differential changes and interaction of biochemical and photosynthetic parameters at the seedling stage under drought, a greenhouse experiment with twelve maize genotypes under severe drought (30% field capacity) and irrigated (90-100% field capacity) conditions were performed. Drought differentially altered biochemical and photosynthetic parameters in all genotypes. A sharp increase in hydrogen peroxide, malondialdehyde (MDA), and total antioxidant capacity (TAOC) were seen and a positive association between H₂O₂ and TAOC, and MDA and transpiration rate (E) was observed under drought. Nonphotochemical quenching increased under drought to avoid the photosystem damage. PCA biplot analysis showed that reducing E and increasing photosynthetic efficiency would be a better drought adaptation mechanism in maize at the seedling stage.

Two light treatments [ambient sunlight (L1) during the entire growth period and 40% shade (L2) from 40 d after sowing until 24 d after flowering] and two phosphate fertilizer treatments [no phosphate fertilizer application (P0) and a conventional phosphate fertilizer application (P1)] were used to determine how phosphate fertilizer regulates soybean [Glycine max (L.) Merr.] photosynthesis under shading. We showed that phosphorus significantly increased chlorophyll content and grain yield under shading. The light-saturated net photosynthetic rate, apparent quantum yield, maximum electron transport rate, and maximum Rubisco carboxylation rate in P1 under L2 significantly increased. Moreover, phosphate fertilizer significantly improved the electron transfer and PSII reaction center performance under shading. Therefore, phosphate fertilizer increases low light-utilization efficiency by improving PSII performance, promoting ribulose-1,5-bisphosphate regeneration, ensuring a source of carboxylate substrates, and coordinating the balance between photochemical reaction and Calvin cycle under shading.

Parameters representing leaf photosynthetic capacity, namely, the maximal carboxylation rate (V_cmax), maximal electron transport rate (J_max), and triose phosphate-utilization rate (T_p), can vary depending on various factors. The present study investigated diurnal variations in V_cmax, J_max, and T_p of uppermost leaves of soil-grown, well-watered eggplant in a greenhouse based on the simultaneous measurements of leaf gas exchange and chlorophyll fluorescence. The values of net photosynthetic rates and electron transport rates plotted against intercellular CO₂ concentrations were noticeably higher in the morning than in the afternoon. Significant differences were detected among the values of V_cmax, J_max, and T_p obtained at different times of day (08:30, 11:00, 13:30, and 16:00 h). All three parameters tended to decline as the time of day advanced; compared to the values at 08:30 h, V_cmax, J_max, and T_p declined by approximately 15% at 16:00 h. Among the three parameters, T_p appeared to be the most sensitive to time.

Evidence that the green fluorescence protein (GFP) develops a significant toxicity in plants has not been found, but it may represent a source of free radicals as a consequence of its fluorescence. In addition, green light is known to trigger the acclimatisation of the photosynthetic system towards a shady environment. Moreover, the light-harvesting system may acclimate to an increased availability of green light. Each of these effects may be induced by the GFP. Therefore, the hypothesis was tested, whether transformation of Nicotiana tabacum cv. Bursan to express the GFP could affect chlorophyll fluorescence parameters. The analysis revealed a significantly lower absorption of energy per excited cross section in GFP-transformed tobacco, a lower number of active reaction centres per excited cross section, a larger absorption and trapped energy flux leading to the reduction of the primary quinone electron acceptor of PSII per reaction centre, and a lower variable fluorescence.

Melatonin is a well-known bioactive molecule able to mitigate photooxidative damage caused by excess light. Here we have shown that mutant Arabidopsis lines with disrupted genes for melatonin putative receptor CAND2/PMTR1 and GPA1 encoding the α-subunit of heterotrimeric G-protein were partially insensitive to melatonin treatment under high light stress. They exhibited a higher degree of photodamage due to a significantly decreased photosynthetic activity and diminished expression of chloroplast and nuclear-encoded genes and the corresponding proteins. A possible mechanism for melatonin-dependent regulation of chloroplast genes is associated with a change in the activity of the genes for chloroplast RNA polymerases. We conclude that under high light stress, melatonin may act as a hormone-like signaling molecule via the CAND2/PMTR1-mediated signaling pathway.