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Chlorophyll a fluorescence (ChF) signal analysis has become a widely used and rapid, non-invasive technique to study the photosynthetic process under stress conditions. It monitors plant responses to various environmental factors affecting plants under experimental and field conditions. Thus, it enables extensive research in ecology and benefits forestry, agriculture, horticulture, and arboriculture. Woody plants, especially trees, as organisms with a considerable life span, have a different life strategy than herbaceous plants and show more complex responses to stress. The range of changes in photosynthetic efficiency of trees depends on their age, ontogeny, species-specific characteristics, and acclimation ability. This review compiles the results of the most commonly used ChF techniques at the foliar scale. We describe the results of experimental studies to identify stress factors that affect photosynthetic efficiency and analyse the experience of assessing tree vigour in natural and human-modified environments. We discuss both the circumstances under which ChF can be successfully used to assess woody plant health and the ChF parameters that can be useful in field research. Finally, we summarise the advantages and limitations of the ChF method in research on trees, shrubs, and woody vines.

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0–200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

The variability of chlorophyll a fluorescence (ChlF) parameters of forest tree species was investigated in 209 stands belonging to six European forests, from Mediterranean to boreal regions. The modifying role of environmental factors, forest structure and tree diversity (species richness and composition) on ChlF signature was analysed. At the European level, conifers showed higher potential performance than broadleaf species. Forests in central Europe performed better than those in Mediterranean and boreal regions. At the site level, homogeneous clusters of tree species were identified by means of a principal component analysis (PCA) of ChlF parameters. The discrimination of the clusters of species was influenced by their taxonomic position and ecological characteristics. The species richness influenced the tree ChlF properties in different ways depending on tree species and site. Tree species and site also affected the relationships between ChlF parameters and other plant functional traits (specific leaf area, leaf nitrogen content, light-saturated photosynthesis, wood density, leaf carbon isotope composition). The assessment of the photosynthetic properties of tree species, by means of ChlF parameters, in relation to their functional traits, is a relevant issue for studies in forest ecology. The connections of data from field surveys with remotely assessed parameters must be carefully explored.

Despite a range of initiatives to reduce global carbon emission, the mean global temperature is increasing due to climate change. Since rising temperatures pose a serious threat of food insecurity, it is important to further explore important biological molecules that can confer thermotolerance to plants. Recently, melatonin has emerged as a universal abiotic stress regulator that can enhance plant tolerance to high temperature. Nonetheless, such regulatory roles of melatonin were unraveled mainly by assessing the effect of exogenous melatonin on plant tolerance to abiotic stress. Here, we generated melatonin deficient tomato plants by silencing of a melatonin biosynthetic gene, ( ), to unveil the role of endogenous melatonin in photosynthesis under heat stress. We examined photosynthetic pigment content, leaf gas exchange, and a range of chlorophyll fluorescence parameters. The results showed that silencing of aggravated heat stress by inhibiting both the light reactions and the carbon fixation reactions of photosynthesis. The photosynthetic pigment content, light absorption flux, trapped energy flux, energy dissipation, density of active reaction center per photosystem II (PSII) cross-section, the photosynthetic electron transport rate, the maximum photochemical efficiency of PSII photochemistry, and the rate of CO assimilation all decreased in -silenced plants compared with that of non-silenced plants particularly under heat stress. However, exogenous melatonin alleviated heat-induced photosynthetic inhibition in both genotypes, indicating that melatonin is essential for maintaining photosynthetic capacity under stressful conditions. These findings provide genetic evidence on the vital role of melatonin in photosynthesis and thus may have useful implication in horticultural crop management in the face of climate change.

ABSTRACT Popularly known as araçá-una, Psidium myrtoides is an endemic tree species in Brazil, with fruits much appreciated by the fauna. It is indicated for the composition of reforestation and for enriching the vegetation of degraded areas. This work aimed to evaluate the effects of drought, heat, and the interaction of both in the physiological attributes of Psidium myrtoides plants. Monitored fluorescence, gas exchange, and chlorophyll index in araçá-una plants induced by drought, heat, and the combination of both during 1, 3, and 7 days after treatment induction (DAT). After, the plants were returned to their initial condition, and their recovery was evaluated at 15 DAT. The results indicate that Psidium myrtoides plants reduce photosynthetic activity in the absence of water, contrary to what has been shown in other studies; in addition, they are not potentiated by the combination with heat. Therefore, we conclude that this species has a potential tolerance to heat (Tmean 30.3 °C). However, if water is available in the soil, it maintains photosynthetic activity at normal levels.

In the present study, the effect of heat stress (45 ºC for 20 min) on some physiological changes was investigated through a chlorophyll afluorescence technique, and some endogenous resistance mechanisms (activities of some antioxidant enzymes, free proline, and reduced ascorbate contents) in two maize cultivars (Zea mays L. cvs. P3167 and DKC7221). Chlorophyll fluorescence measurements demonstrated that heat stress led to the reduction in the efficiency of the Hill reaction, accumulation of inactive reaction centers, inhibition of electron flow from reaction centers to the plastoquinone pool, and induction of non-photochemical dissipation of absorbed light energy. Changes in Φo/(1 – Φo), SFIABS and PIABS indicated that electron transport reactions in P3167 were almost completely inhibited by heat stress. In DKC7221, however, photosynthetic electron transport reactions were maintained under heat stress conditions. As a result of impairment in the photosynthetic efficiency in P3167 under heat stress, oxidative stress appeared as shown by lower antioxidant activity, accumulation of H2O2, malondialdehyde, and formazon and photooxidative injuries in chlorophyll pigments in the leaf tissue. DKC7221, on the other hand, had a higher antioxidant efficiency and lower oxidative damage under heat stress. FeSOD activity was found to be responsible for the dismutation of superoxide radicals in both maize genotypes under heat stress. As a result, it may be concluded that the genotype DKC7221 is more tolerant to heat stress than P3167.

Photosynthesis can be probed through Chlorophyll a fluorescence induction (FI), which provides detailed insight into the electron transfer process in Photosystem II, and beyond. Here, we have systematically studied the natural variation of the fast phase of the FI, i.e. the OJIP phase, in rice. The OJIP phase of the Chl a fluorescence induction curve is referred to as “fast transient” lasting for less than a second; it is obtained after a dark-adapted sample is exposed to saturating light. In the OJIP curve, “O” stands for “origin” (minimal fluorescence), “P” for “peak” (maximum fluorescence), and J and I for inflection points between the O and P levels. Further, Fo is the fluorescence intensity at the “O” level, whereas Fm is the intensity at the P level, and Fv (= Fm − Fo) is the variable fluorescence. We surveyed a set of quantitative parameters derived from the FI curves of 199 rice accessions, grown under both field condition (FC) and growth room condition (GC). Our results show a significant variation between Japonica (JAP) and Indica (IND) subgroups, under both the growth conditions, in almost all the parameters derived from the OJIP curves. The ratio of the variable to the maximum (Fv/Fm) and of the variable to the minimum (Fv/Fo) fluorescence, the performance index (PIabs), as well as the amplitude of the I–P phase (AI–P) show higher values in JAP compared to that in the IND subpopulation. In contrast, the amplitude of the O–J phase (AO–J) and the normalized area above the OJIP curve (Sm) show an opposite trend. The performed genetic analysis shows that plants grown under GC appear much more affected by environmental factors than those grown in the field. We further conducted a genome-wide association study (GWAS) using 11 parameters derived from plants grown in the field. In total, 596 non-unique significant loci based on these parameters were identified by GWAS. Several photosynthesis-related proteins were identified to be associated with different OJIP parameters. We found that traits with high correlation are usually associated with similar genomic regions. Specifically, the thermal phase of FI, which includes the amplitudes of the J–I and I–P subphases (AJ–I and AI–P) of the OJIP curve, is, in turn, associated with certain common genomic regions. Our study is the first one dealing with the natural variations in rice, with the aim to characterize potential candidate genes controlling the magnitude and half-time of each of the phases in the OJIP FI curve.

Despite the plethora of published reports on ameliorative effects of exogenously applied salicylic acid (SA) to plants under salt stress, a critical role of SA in redox balance, photosynthetic and electron transport in mediating salt tolerance in plants is still ignored. The present study was aimed to assess the beneficial effects of SA on photosynthetic electron transport in five radish cultivars during salt stress which may translate into protection from salt-induced oxidative damage. Seeds of five radish cultivars (Red Neck Purple, Early Long White, Minnu Radish, Radish 40-day and Gong Swang Radish) were allowed to germinate for five days after which seedlings were sprayed with different concentrations of SA (0, 2 and 5 mM). After 48 h of SA treatment, seedlings were subjected to salt stress (0, 100, and 200 mM NaCl). Salt stress cause reduction in biomass, chlorophyll contents and PSII photochemistry in all five radish cultivars. However, salt stress causes an increase in proline content, lipid peroxidation, ROS content and antioxidant enzymatic activity. Foliar spray of SA downregulated the absorption, trapping and photosynthetic electron transport fluxes while it increased dissipation flux consequently improving photochemistry of most of the radish cultivars under salt stress. This Improved photochemistry and enhanced antioxidant enzymes activity of radish cultivars by SA application resulted in lower ROS generation and membrane damage. Greater accumulation of compatible solutes due to foliar spray of SA might have additional protective effects on photosynthetic machinery by lowering oxidative stress. Moreover, ameliorative effects of SA were cultivar specific. The study suggested that SA could mitigate the detrimental effects of salt stress by regulating physiological and biochemical mechanisms in radish plant.

In this study, the differences in chlorophyll fluorescence transient (OJIP) and modulated 820 nm reflection (MR 820 ) of cucumber leaves were probed to demonstrate an insight into the precise influence of melatonin (MT) on cucumber photosystems under low temperature stress. We pre-treated cucumber seedlings with different levels of MT (0, 25, 50, 100, 200, and 400 μmol · L -1 ) before imposing low temperature stress (10 °C/6 °C). The results indicated that moderate concentrations of MT had a positive effect on the growth of low temperature-stressed cucumber seedlings. Under low temperature stress conditions, 100 μmol · L -1 (MT 100) improved the performance of the active photosystem II (PSII) reaction centers (PIabs), the oxygen evolving complex activity (OEC centers) and electron transport between PSII and PSI, mainly by decreasing the L-band, K-band, and G-band, but showed differences with different duration of low temperature stress. In addition, these indicators related to quantum yield and energy flux of PSII regulated by MT indicated that MT (MT 100) effectively protected the electron transport and energy distribution in the photosystem. According to the results of W O-I ≥ 1 and MR 820 signals, MT also affected PSI activity. MT 100 decreased the minimal value of MR/MR O and the oxidation rate of plastocyanin (PC) and PSI reaction center (P700) ( V ox ), while increased △MR slow /MR O and deoxidation rates of PC + and P 700 + ( V red ). The loss of the slow phase of MT 200 and MT 400-treated plants in the MR 820 kinetics was due to the complete prevention of electron movement from PSII to re-reduce the PC + and P700 + . These results suggest that appropriate MT concentration (100 μmol · L -1 ) can improve the photosynthetic performance of PS II and electron transport from primary quinone electron acceptor (Q A ) to secondary quinone electron acceptor (Q B ), promote the balance of energy distribution, strengthen the connectivity of PSI and PSII, improve the electron flow of PSII via Q A to PC + and P 700 + from reaching PSI by regulating multiple sites of electron transport chain in photosynthesis, and increase the pool size and reduction rates of PSI in low temperature-stressed cucumber plants, All these modifications by MT 100 treatment promoted the photosynthetic electron transfer smoothly, and further restored the cucumber plant growth under low temperature stress. Therefore, we conclude that spraying MT at an appropriate concentration is beneficial for protecting the photosynthetic electron transport chain, while spraying high concentrations of MT has a negative effect on regulating the low temperature tolerance in cucumber.

Root cutting caused by underground coal mining subsidence is among the leading causes of plant damage in western China. Detection of root cutting stress is of great importance in evaluating the degree of plant damage and changes in physiological conditions in underground coal mining disturbance conditions. The present study assessed the use of chlorophyll fluorescence OJIP transient data to evaluate the disturbance characteristics of root cutting stress on leaf photosynthetic mechanisms in the typical shrub Krasch. Different root cutting ratios (10%, 20%, 30%, 50%, 75%, and 100%) were established on the roots of in the field, and the OJIP transient and JIP parameters of the leaves were measured. The overall OJIP curves and each OJIP step in leaves decreased as the root cutting ratio increased, but the impact was relatively small for root cutting ratios of less than 30%. Through the analysis of JIP parameters and the established energy pipeline model, it was found that the energy capture efficiency and electron transfer efficiency of photosystem II decreased as the root cutting ratio increased. Therefore, we also inferred that the threshold for the plant root cutting ratio at which leaf photosynthetic mechanisms begin to change is 30-50%. These results indicate that OJIP transient analysis can serve as a non-destructive, rapid technique for detecting plant root cutting stress in coal mining subsidence areas, which is of great value for non-destructive monitoring of plant root damage.