Govindjee and Hartmut Lichtenthaler have a very similar curriculum vitae. Both chose photosynthesis as research field and actively applied chlorophyll fluorescence. Their research was overlapping and complementary. On the occasion of Govindjee's 88^th anniversary in 2020, Hartmut Lichtenthaler gives a short retrospective on interactions and joint activities with Govindjee over the past five decades.

Chlorophyll fluorescence has developed into a well-established noninvasive technique to study photosynthesis and by extension, the physiology of plants and algae. The versatility of the fluorescence analysis has been improved significantly due to advancements in the technology of light sources, detectors, and data handling. This allowed the development of an instrumention that is effective, easy to handle, and affordable. Several of these techniques rely on point measurements. However, the response of plants to environmental stresses is heterogeneous, both spatially and temporally. Beside the nonimaging systems, low- and high-resolution imaging systems have been developed and are in use as real-time, multi-channel fluorometers to investigate heterogeneous patterns of photosynthetic performance of leaves and algae. This review will revise in several paragraphs the current status of chlorophyll fluorescence imaging, in exploring photosynthetic features to evaluate the physiological response of plant organisms in different domains. In the conclusion paragraph, an attempt will be made to answer the question posed in the title.

Low temperature combined with low light (LL) is an adverse factor seriously affecting pepper productivity and quality. However, little is known about the molecular mechanisms related to LL stress responses. Therefore, transcriptome and physiological changes in Hangjiao No. 2 (H2) and Xiangtela No. 2 (X2) pepper were studied under normal conditions and LL. We found 8,392 and 8,028 differentially expressed genes in H2 and X2, respectively, significantly associated with photosynthesis, photosynthesis antenna proteins, and carotenoids, were enriched in 27 and 40 gene ontology terms in H2 and X2, respectively, and 14 and 16 Kyoto Encyclopedia of Gene and Genomes pathways. The accuracy and reliability of the RNA-Seq results were confirmed by qRT-PCR. Furthermore, carotenoid-related genes ZDS, CA1, CA2, NCED, LOC107840293, and LOC107850059 functioned in response to LL. Additionally, LL significantly decreased photosynthesis capacity, photosynthetic pigment contents, as well as maximum quantum efficiency, and changed carotenoid-related compounds, revealing photosynthesis and carotenoids were involved in LL stress response. Our findings provide insight into LL stress-induced transcriptional expression patterns.

We investigated if high irradiance imposes stress conditions and differently affects the performance of species from the early-successional light-demanding group and if the costs associated with plasticity underlie the trade-off between growth and survival. Survival, growth, chlorophyll a fluorescence, and carbon stable isotopic ratio (δ¹³C) were measured in seedlings of four tree species in a restoration area in the Atlantic Forest under full-sun exposure and partial shading treatments. The maximal quantum yield of PSII indicated stress conditions mainly in the full-sun exposure treatment (values from 0.72 to 0.81). The δ¹³C ranged from -31.9 to -30.4‰, but did not differ between treatments for three species, suggesting that C assimilation in initial restoration conditions is stressful even in shade. The survival scaled negatively with growth and plasticity adjustments. We conclude that early-successional species do not form a homogenous species group because they differ in functional responses to stress, and phenotypic plasticity, with these variations showing a strong link with the trade-off between growth and survival. This information should be taken into account for species selection in restoration programs.

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.

Bouteloua gracilis (blue grama grass) is one of the most drought and grazing tolerant plants in the short-grass ecosystem. To obtain information on their photosystem activities, we measured the fast (< 1 s) chlorophyll a fluorescence transient (the OJIP curve) from their leaves, and isolated cells grown photoautotrophically in suspension in a culture medium, or with added sucrose. One of our goals was to study the effect of different sucrose concentrations (0, 0.15, 0.3, and 3%) on PSII activity in isolated cells. Our results on cells suspended in culture medium, using the JIP-test, showed a decrease in PSII activity at increasing sucrose concentrations, while the photoautotrophic cells showed an optimal PSII activity, close to that of the leaves. Further, our data on cells grown in 0, 0.15, and 0.3% sucrose, but with added CO₂, and measured while the cells were deposited on filter paper, indicate that supplementary CO₂ can increase the PSII activity in the presence of sucrose, although further research is necessary to understand these results.

We honor here Hartmut Karl Lichtenthaler, a pioneer of plant physiology, plant biochemistry, plant biophysics, plant molecular biology, and stress physiology. His contributions to the ingenious use of chlorophyll a fluorescence imaging in understanding the physiological processes in leaves stand out. We wish him many happy and productive years of research and educating others.

Chlorophyll a (Chl a) has an asymmetrical molecular organization, which dictates its orientation and the location of the pigment in the mature photosynthetic apparatus. Although Chl a fluorescence (ChlF) is widely accepted as a proxy for plant photosynthetic performance under countless stress conditions and across species, a mechanistic understanding of this causality is missing. Since water plays a much greater role than solvent for the photosynthetic machinery, elucidating its influence on Chl a may explain the reliable reflection of plant stress response in the ChlF signal. We examine the effect of hydration from well-watered to lethal drought on ChlF imagery results across morphologically diverse species to begin testing the impact of molecular scale hydration of Chl a on ChlF. Our results support a conceptual model where water is an integral part of the photosystems' structure and directly influences Chl a behavior leading to changes in the energy partitioning and ultimately in ChlF.

The initial photochemical quantum efficiency of photosystem II (F_v/F_m) and performance index (PI), describing the ability of the photosynthetic apparatus to collect light energy, have been used to screen tolerance to drought stress by ten maize accessions, monitored by leaf relative water content (RWC) and soil water content (SWC). The F_v/F_m, PI, and RWC values were significantly reduced in drought-stressed plants. The analysis of chlorophyll a fluorescence induction rise from the basic dark-adapted fluorescence yield to the maximum (OJIP transient), distinguished accession Zea 1006 from Libya and Zea 612 from Italy, as the most tolerant and the least tolerant genotypes. The maize genotypes were classified using the Principal Component Analysis (PCA) and clustering methods, based on F_v/F_m and PI values, leaf RWC and SWC. Genotypes from Egypt and Libya were differentiated from genotypes from Europe, Russia and the USA.

Seasonal variations in photosynthesis of cassava cv. Rayong 9 (RY9) under irrigated and rain-fed conditions were evaluated at the age of three and six months after planting (MAP). Photosynthetic light-response (P_N/I) curves revealed that cassava leaves attained the highest maximum net photosynthetic rates (P_Nmax) in the rainy season, followed by the hot one, while the lowest P_Nmax was found in the cool season. Photosynthetic potential of the 3-month-old plants was mostly higher than that of the 6-month-old plants, and the seasonal variation in photosynthetic capacity was also more apparent in the younger plants. P_N/I curves were used to predict daily net photosynthetic rate (P_N) for each season based on daily average solar radiation data. The predicted P_N were considerably lower than the P_Nmax values. This indicated that solar radiation is a limiting factor for photosynthesis, particularly in the rainy season. The data provided basic information for breeding cassava genotypes with enhanced photosynthesis during the period of unfavorable environment. Furthermore, the data are potentially useful in modeling photosynthesis and crop growth as affected by environmental factors.

An experiment was performed to study gas exchange and chlorophyll fluorescence responses of rice (Oryza sativa L.) to various regimes, such as flooding-midseason drying-flooding (FDF), flooding-midseason drying-saturation (FDS), and flooding-rain-fed (FR) regimes. Compared to FDF, FR resulted in an obvious decrease in net photosynthetic rate (P_N), due to the decrease in stomatal conductance and the increase in stomatal limitation. In contrast, FDS plants did not suffer stomatal limitation and had comparable P_N with FDF plants. For diurnal light-saturated electron transport rate and saturation irradiance, FDF performed the best, which was followed by FDS and FR successively. FR and FDS plants tended to suffer from midday depression. FDS reduced irrigated water by 17.2% compared to FDF for comparable yields. The results suggested that FDS can be an effective irrigation regime to save water.

High salt concentration is a major abiotic stress limiting plant growth and productivity in many areas of the world. Elaeagnus angustifolia L. adapts to adverse environments and is widely planted in the western region of China as a windbreaker and for landscape and soil stabilization. High salt concentrations inhibited photosynthesis of E. angustifolia, but the mechanism is not known. In this paper, RNA-sequencing was used to investigate effects of salt stress on the photosynthetic characteristics of the species. In total, 584 genes were identified and involved in photosynthetic pathways. The downregulation of genes that encode key enzymes involved in photosynthesis and genes correlated to important structures in photosystem and light-harvesting complexes might be the main reason, particularly, the downregulation of the gene that encodes magnesium chelatase. This would decrease the activity of enzymes involved in chlorophyll synthesis and the downregulation of the key gene that encodes Rubisco, and thereby decreases enzyme activity and the protein content of Rubisco.

Comparative studies of the adaptation of green- and purple-leaved Ocimum basilicum varieties, which differ in carotenoid and anthocyanin contents, to high-intensity light have been carried out. The plants were grown for 21 d under white light-emitting diode providing different light intensities [300, 550, and 1,350 µmol(photon) m^-2 s^-1]. During the experiment, fresh mass and relative water content, photosynthetic pigment and anthocyanin content, amount of total free phenolic compounds, and the contents of thiobarbituric acid reactive substances and H₂O₂ were analysed. The fluorescent parameters, which characterized the PSII activity in the leaves (F_v/F_m, Y_(II)), as well as the efficiency of dissipation of absorbed energy into heat (Y_(NPQ),Y_(NO)) and the fraction of the PSII open centres (q_L), were determined. The obtained results are consistent with the concept of the important role of not only anthocyanin but also carotenoids in plant adaptation to high-intensity light.

One of the major questions in photosynthesis research is related to the role of the ultrastructure of thylakoid membranes in higher plants for their functional performance at optimal and stress environmental conditions. In this work, we compared the drought stress response of pea plants grown at moderate and low light conditions, based on our recent findings that they differ in the extent of grana stacking and in the thermal stability of the major light-harvesting complex of PSII that plays important regulatory role for plant survival. Our data demonstrate that low light-adapted plants are structurally and functionally less affected by the water deficit, strongly suggesting a major role of thylakoid structural organization for drought stress response.

Water stress is one of the most important abiotic factors affecting plants worldwide. This study compared the photochemical responses of two lineages of Crambe abyssinica (FMS CR 1307 and 1326) under water deficit (WD) conditions. The relative water content (RWC), stomatal conductance, chlorophyll (Chl) index, and Chl a fluorescence were evaluated. FMS CR 1326 showed greater reductions in RWC under WD. The FMS CR 1307 showed greater reductions in the connectivity of energy (L-band) and electron transfer from the oxygen-evolving complex (K-band), followed by a lower ability to maintain the reduction of plastoquinone. However, the increased size of the pool of the final electron acceptors in PSI for this lineage was observed. The FMS CR 1326 plant lineage showed an advantage over the FMS CR 1307 in terms of stability and efficiency of PSII under low water availability conditions.

Due to the overall low thermodynamic efficiency (1–4%) of photosynthesis (Ort et al. 2011, Subramanian et al. 2013) and its impact on crop productivity, substantial efforts are being made to engineer photosynthesis to improve its light use and carbon capture efficiency to increase crop yields.

This Special issue of Photosynthetica has been compiled to honour Professor Reto Jörg Strasser’s 75th birthday which was on 28 December, 2018. Reto has been a famous scientist in the field of photosynthesis and chlorophyll a fluorescence since the beginning of his PhD studies in 1968 at University of Bern. In 1976, he got a habilitation degree in photobiology at the same university and within the next year (1977) he moved to the USA to work as Principal Acting Investigator (PAI) at University of California, San Diego (UCSD).

The effects of 20-hydroxyecdysone on the primary photosynthetic processes (PPP) were examined across twelve plant species using the OJIP chlorophyll fluorescence analysis. Our experiments brought evidence that the ecdysteroids can function not only as protective compounds in plants but participate also in the regulation of photosynthesis. Exogenously applied 20-hydroxyecdysone positively affected the PPP particularly in French bean and canola. Contrary to this, the PPP were negatively influenced by 20-hydroxyecdysone in pea, sunflower, and wheat. The performace of the oxygen-evolving complex, the excitonic connectivity among PSII units, the electron transport between PSII and PSI, the size of the pool of the end electron acceptors, and the absorption and trapping of the excitation energy in the light-harvesting antennae were differently affected in individual plant species. Additionally, the older and younger leaves did not respond to 20-hydroxyecdysone in the same way and this was also a species-specific phenomenon.

Replicate common gardens of Italian and Greek provenances of Quercus ilex, Q. pubescens, and Q. frainetto seedlings were established in 2017 in Italy, Greece, and Germany (here: also Q. robur) to assess their potential for plantations for future climate scenarios. The JIP-test was applied on saplings (2016) and in the summers of 2017-2019 to assess the efficiency of the photosynthetic apparatus. Seedlings from the nurseries differed in the tested JIP parameters from older plants in the field. Further, considerable inter-annual and between-sites variability of the parameters occurred. Q. pubescens revealed the most stable behavior across time and site, with the Greek provenance usually showing higher values of performance index and the probability of electron transport beyond Q_A^- than the Italian provenance. In the German sites, Greek Q. pubescens revealed higher values of most of the tested JIP parameters than the local species, Q. robur.

The recovery of photosynthetic activity during rehydration of Haberlea rhodopensis from drought- and freezing-induced desiccation were investigated. The water uptake during the initial 15 h was slow thus preventing cellular damages. The results showed faster recovery of quantum efficiency of PSII in plants rehydrated after freezing stress (RAF) compared to plants rehydrated after drought stress (RAD) and the most significant differences between them were evident after 9-15 h of rehydration. Following rehydration, PSI activity recovered faster compared to PSII and in contrast to PSII, its activity was higher in RAD compared to RAF plants. During the first hours of rehydration, prominent alterations in energy transfer between photosynthetic complexes occurred as revealed by 77 K fluorescence of isolated thylakoids. High proportion of thermal energy dissipation in dry plants and during the first hours of rehydration protects them from photooxidation; the role of PSII reaction center quenching during the recovery was suggested.

Silver nanoparticles (AgNPs) are used in several industries and their continuous release into environment can damage the ecosystem. Our study observed the impact of two concentrations of AgNPs (1 and 5 mM) on wheat seedlings. In this study, the treatment of AgNPs was found to have a significant impact on growth parameters, such as root and shoot length of wheat seedlings, in addition to that biochemical parameters, such as activity of catalase, glutathione, flavonoid, and chlorophyll concentrations, were also affected. AgNPs apparently suppressed the photosynthetic activity of the seedlings, with a clear destruction of photosystems at 5 mM concentration. The result also indicated an improper regulation of PSI electron transport, resulting in a damage to chloroplast structure, including photosystem itself. Our study presents a mechanism of the AgNPs action of on photosynthetic processes in plants.

Predicting responses to stressful conditions is very important. Chlorophyll a fluorescence (ChlF) can be used to assess effects of various stresses on photosynthetic performance. We tested the responses of five 10-d old maize hybrids to salinity stress by measuring ChlF parameters, fresh (FM) and dry mass (DM). ChlF data were incorporated into a penalized regression model to predict biomass traits. The values of FM and DM significantly decreased under salt stress by 42 and 25%, respectively. Strong responses in ChlF parameters assessing the absorption dissipation and trapping fluxes to NaCl treatment were detected. In penalized regression models, 118 transients showed greater (R² = 0.663 for FM and R² = 0.678 for DM), although comparable, predictive abilities as 18 selected JIP-test parameters (R² = 0.597 for FM and R² = 0.636 for DM). Genetic assessment of developed models is needed, as they efficiently predict biomass traits and provide physiological context to the obtained predictions.

The aim of the present study was to investigate the photosynthetic properties and transcriptomic profiles of wildtype and chlorophyll (Chl) b-lacking rice (Oryza sativa L.). The plastid ultrastructure of the Chl b-lacking rice (i.e., loss of starch granules, abundant vesicles, and abundant plastoglobuli) indicated abnormal plastid development, whereas the analysis of transcriptome profiles and differentially expressed genes revealed that gene encoding PsbR (PSII core protein) was downregulated in the mutant, thereby reducing the Chl accumulation of the mutant. Meanwhile, in regards to Chl biosynthesis and degradation pathways, GluTR gene was downregulated, whereas UROD, CPOX, and MgCH genes were upregulated. The qPCR results were generally consistent with those of the transcription analysis, except for the finding that NOL genes, which regulate Chl b degradation, were upregulated. These results suggest that both the reduction in Chl accumulation and increase in conversion rate of Chl b to Chl a caused Chl a/b ratio amplification in mutant. The present study also provides evidence for Chl b degradation via pheophorbide b.

The study was carried out to examine the photosynthetic efficiency of Scot pine populations, originated from various European countries and Anatolia in Turkey, exposed to UV-B radiation. Seeds were germinated and grown in the nursery; three-year-old seedlings were exposed to UV-B for two days, eight hours per day. The photosynthetic efficiency was probed by chlorophyll a fluorescence at 24 h after UV-B treatment and analysed by JIP-test. UV-B caused reduction in quantum yields and electron transport at both donor and acceptor sides of photosystems. The performance indexes of seedlings were also negatively affected by the treatment. Populations from France, England, and three provenances of Turkey showed sensitive responses, while the populations from Spain, Romania, and Ilgaz performed their photosynthesis better. We found that UV-B affected at different levels the photosynthetic functionality of populations. Chlorophyll a fluorescence technique is very useful to identify the UV-B tolerance of different Scots pine populations, however, further studies based on molecular analyses should be applied to explain their evolutionary history.

iH026a is a formulation containing a biochemical class of plant growth regulator that modulates glycoconjugation through the plant lectin cycle. While lectins are common to vascular plants, we observed, consistent with reversible binding of sugars from lectins, enhancements of quantities and qualities of various features, including significant enrichment of Brix soluble sugars compared to controls in cherry, grape, and melon in trials conducted in Arizona and California, USA.

We tested the seasonal variability of frost resistance and photosynthesis characteristics, namely net photosynthetic rate at saturating irradiance and ambient atmospheric CO₂ concentrations, maximum carboxylation rate, and measurements based on the JIP-test, namely maximum quantum yield of PSII and the performance index. Additionally, we measured leaf functional traits, namely specific leaf area, leaf dry matter content, stomatal pore area index, and chlorophyll content. We assessed all traits weekly on two evergreen species Berberis vulgaris and Quercus × hispanica as well as the two summer green species Betula pendula and Quercus pubescens. We found that there was a high seasonal variability in all traits except stomatal pore area index. In a multivariate analysis, we could show trade-offs between growth rates and leaf thickness on the first axis, as well as resistance and performance on the second axis. The findings help understand ecophysiological processes and fine-tune biosphere models.

In this investigation, we used the growth, photosynthetic physiological parameters, and targeted metabolite analysis to evaluate the responses of Eleutherococcus senticosus in different shading treatments. The results showed that the moderate shading treatment (Z₁) promoted the growth and inhibited photosynthesis of plants. The severe shading treatment (Z₂)inhibited both the growth and photosynthesis of the plants. Besides, Z₁ had no significant effect on the PSII, while Z₂ inhibited the PSII. Most of the eight medicinal metabolites accumulated in the Z₁. The C₆C₁- and C₆C₃-type phenolics accumulated in the Z₁, and the C₆C₃C₆-type in the Z₂. In conclusion, the moderate shading treatment accumulated more defensive phenolics; this might be the reason for this shading condition promoting the growth and the accumulation of medicinal metabolites of the plant. The result of this study laid a theoretical foundation for the further study of shading treatments on the secondary metabolism of Eleutherococcus senticosus.

Different light filters affect leaf photosynthetic features and fruit quality. Consequently, selecting the appropriate covering filter for rain-shelter cultivation of peaches is a key part of successful production. We used a late-maturing peach variety 'Xiahui 8' to study differences in leaf photosynthetic features, chlorophyll fluorescence characteristics, and fruit quality under neutral, red, yellow, green, and blue filter, with natural light as control. The results showed that the leaf photosynthetic ability and internal quality under the neutral filter treatment were elevated compared with the control, and the appearance color was the same as the control. Leaves under neutral filter could maintain higher photosynthetic ability than other filter treatments. In addition, the fruits could also keep higher quality when treated with neutral filter. Therefore, the application of neutral filter in rain-shelter cultivation of 'Xinhui 8' peaches is recommended for maintaining high photosynthetic capacity and for improving fruit quality.

Accumulation and distribution of zinc within Miscanthus × giganteus plants grown on elevated Zn concentrations and their photosynthetic performance were investigated. High concentrations of Zn in soils caused an increase of its concentrations in all plant organs. The bioconcentration factor, bioaccumulation factor, and translocation factor were lower than one indicating that M. × giganteus is an excluder plant species. Excessive Zn induced visible leaf damage, i.e. chlorosis and necrosis, only in the oldest leaves, pointing to Zn accumulation. Elevated amounts of Zn in leaves significantly lowered the photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO₂ concentrations, parameters of chlorophyll a fluorescence, and chlorophyll b content. Despite Zn excess in leaves, there was no severe reduction in the maximal quantum yield of PSII photochemistry, indicating a high photosynthetic capacity, high tolerance to elevated Zn concetrations, and ability of M. × giganteus to grow on Zn-contaminated soils.