During photosynthesis, reactive oxygen species (ROS) are formed, including hydrogen peroxide (H2O2) and singlet oxygen (1O2), which have putative roles in signalling, but their involvement in photosynthetic acclimation is unclear. Due to extreme reactivity and a short lifetime, 1O2 signalling occurs via its reaction products, such as oxidised poly-unsaturated fatty acids in thylakoid membranes. The resulting lipid peroxides decay to various aldehydes and reactive electrophile species (RES). Here, we investigated the role of ROS in the signal transduction of high light (HL), focusing on GreenCut2 genes unique to photosynthetic organisms. Using RNA seq. data, the transcriptional responses of Chlamydomonas reinhardtii to 2 h HL were compared with responses under low light to exogenous RES (acrolein; 4-hydroxynonenal), β-cyclocitral, a β-carotene oxidation product, as well as Rose Bengal, a 1O2-producing photosensitiser, and H2O2. HL induced significant (p < 0.05) up- and down-regulation of 108 and 23 GreenCut2 genes, respectively. Of all HL up-regulated genes, over half were also up-regulated by RES, including RBCS1 (ribulose bisphosphate carboxylase small subunit), NPQ-related PSBS1 and LHCSR1. Furthermore, 96% of the genes down-regulated by HL were also down-regulated by 1O2 or RES, including CAO1 (chlorophyllide-a oxygnease), MDH2 (NADP-malate dehydrogenase) and PGM4 (phosphoglycerate mutase) for glycolysis. In comparison, only 0-4% of HL-affected GreenCut2 genes were similarly affected by H2O2 or β-cyclocitral. Overall, 1O2 plays a significant role in signalling during the initial acclimation of C. reinhardtii to HL by up-regulating photo-protection and carbon assimilation and down-regulating specific primary metabolic pathways. Our data support that this pathway involves RES.

In recent years, inorganic nanoparticles, including calcium hydroxide nanoparticles [Ca Ca(OH)2 NPs], have attracted significant interest for their ability to impact plant photosynthesis and boost agricultural productivity. In this study, the effects of 15 and 30 mg L-1 oleylamine-coated calcium hydroxide nanoparticles [Ca(OH)2@OAm NPs] on photosystem II (PSII) photochemistry were investigated on tomato plants at their growth irradiance (GI) (580 μmol photons m-2 s-1) and at high irradiance (HI) (1000 μmol photons m-2 s-1). Ca(OH)2@OAm NPs synthesized via a microwave-assisted method revealed a crystallite size of 25 nm with 34% w/w of oleylamine coater, a hydrodynamic size of 145 nm, and a ζ-potential of 4 mV. Compared with the control plants (sprayed with distilled water), PSII efficiency in tomato plants sprayed with Ca(OH)2@OAm NPs declined as soon as 90 min after the spray, accompanied by a higher excess excitation energy at PSII. Nevertheless, after 72 h, the effective quantum yield of PSII electron transport (ΦPSII) in tomato plants sprayed with Ca(OH)2@OAm NPs enhanced due to both an increase in the fraction of open PSII reaction centers (qp) and to the enhancement in the excitation capture efficiency (Fv\'/Fm\') of these centers. However, the decrease at the same time in non-photochemical quenching (NPQ) resulted in an increased generation of reactive oxygen species (ROS). It can be concluded that Ca(OH)2@OAm NPs, by effectively regulating the non-photochemical quenching (NPQ) mechanism, enhanced the electron transport rate (ETR) and decreased the excess excitation energy in tomato leaves. The delay in the enhancement of PSII photochemistry by the calcium hydroxide NPs was less at the GI than at the HI. The enhancement of PSII function by calcium hydroxide NPs is suggested to be triggered by the NPQ mechanism that intensifies ROS generation, which is considered to be beneficial. Calcium hydroxide nanoparticles, in less than 72 h, activated a ROS regulatory network of light energy partitioning signaling that enhanced PSII function. Therefore, synthesized Ca(OH)2@OAm NPs could potentially be used as photosynthetic biostimulants to enhance crop yields, pending further testing on other plant species.

OBJECTIVE: Long-term exposure over several days to Far-Red (FR) increases leaf expansion, while short-term exposure (minutes) may enhance the PSII operating efficiency (ϕPSII). The interaction between these responses at different time scales, and their impact on photosynthesis at whole-plant level is not well understood. Our study aimed to assess the effects of FR in an irradiance mimicking the spectrum of sunlight (referred to as artificial solar irradiance) both in the long and short-term, on whole-plant CO2 assimilation rates and in leaves at different positions in the plant.
METHODS: Tomato (Solanum lycopersicum) plants were grown under artificial solar irradiance conditions with either a severely reduced or normal fraction of FR(SUN(FR-) vs. SUN). To elucidate the interplay between the growth light treatment and the short-term reduction of FR, we investigated this interaction at both the whole-plant and leaf level. At whole-plant level, CO2 assimilation rates were assessed under artificial solar irradiance with a normal and a reduced fraction of FR. At the leaf level, the effects of removal and presence of FR (0FR and 60FR) during transition from high to low light on CO2 assimilation rates and chlorophyll fluorescence were evaluated in upper and lower leaves.
RESULTS: SUN(FR-) plants had lower leaf area, shorter stems, and darker leaves than SUN plants. While reducing FR during growth did not affect whole-plant photosynthesis under high light intensity, it had a negative impact at low light intensity. Short-term FR removal reduced both plant and leaf CO2 assimilation rates, but only at low light intensity and irrespective of the growth light treatment and leaf position. Interestingly, the kinetics of ϕPSII from high to low light were accelerated by 60FR, with a larger effect in lower leaves of SUN than in SUN(FR-) plants.
CONCLUSIONS: Growing plants with a reduced amount of FR light lowers whole-plant CO2 assimilation rates at low light intensity through reduced leaf area, despite maintaining similar leaf-level CO2 assimilation to leaves grown with a normal amount of FR. The short-term removal of FR brings about significant but marginal reductions in photosynthetic efficiency at the leaf level, regardless of the long-term growth light treatment.

Photosystem II (PSII) functions were investigated in basil (Ocimum basilicum L.) plants sprayed with 1 mM salicylic acid (SA) under non-stress (NS) or mild drought-stress (MiDS) conditions. Under MiDS, SA-sprayed leaves retained significantly higher (+36%) chlorophyll content compared to NS, SA-sprayed leaves. PSII efficiency in SA-sprayed leaves under NS conditions, evaluated at both low light (LL, 200 μmol photons m-2 s-1) and high light (HL, 900 μmol photons m-2 s-1), increased significantly with a parallel significant decrease in the excitation pressure at PSII (1-qL) and the excess excitation energy (EXC). This enhancement of PSII efficiency under NS conditions was induced by the mechanism of non-photochemical quenching (NPQ) that reduced singlet oxygen (1O2) production, as indicated by the reduced quantum yield of non-regulated energy loss in PSII (ΦNO). Under MiDS, the thylakoid structure of water-sprayed leaves appeared slightly dilated, and the efficiency of PSII declined, compared to NS conditions. In contrast, the thylakoid structure of SA-sprayed leaves did not change under MiDS, while PSII functionality was retained, similar to NS plants at HL. This was due to the photoprotective heat dissipation by NPQ, which was sufficient to retain the same percentage of open PSII reaction centers (qp), as in NS conditions and HL. We suggest that the redox status of the plastoquinone pool (qp) under MiDS and HL initiated the acclimation response to MiDS in SA-sprayed leaves, which retained the same electron transport rate (ETR) with control plants. Foliar spray of SA could be considered as a method to improve PSII efficiency in basil plants under NS conditions, at both LL and HL, while under MiDS and HL conditions, basil plants could retain PSII efficiency similar to control plants.

Non-photochemical quenching (NPQ) is a protective mechanism for dissipating excess energy generated during photosynthesis in the form of heat. The accelerated relaxation of the NPQ in fluctuating light can lead to an increase in the yield and dry matter productivity of crops. Since the measurement of NPQ is time-consuming and requires specific light conditions, theoretical NPQ (NPQ(T)) was introduced for rapid estimation, which could be suitable for High-throughput Phenotyping. We investigated the potential of NPQ(T) to be used for testing plant genetic resources of chickpea under drought stress with non-invasive High-throughput Phenotyping complemented with yield traits. Besides a high correlation between the hundred-seed-weight and the Estimated Biovolume, significant differences were observed between the two types of chickpea desi and kabuli for Estimated Biovolume and NPQ(T). Desi was able to maintain the Estimated Biovolume significantly better under drought stress. One reason could be the effective dissipation of excess excitation energy in photosystem II, which can be efficiently measured as NPQ(T). Screening of plant genetic resources for photosynthetic performance could take pre-breeding to a higher level and can be implemented in a variety of studies, such as here with drought stress or under fluctuating light in a High-throughput Phenotyping manner using NPQ(T).

The study evaluated the effects of treating irrigation water with a coaxial flow variator (CFV) on the morpho-physiology of pot-cultivated test species, including cucumber (Cucumis sativus, CU), lettuce (Lactuca sativa, LE), and sorghum (Sorghum vulgare, SO), in early stages of growth. CFV caused a lower oxidation reduction potential (ORP), increased pH and flow resistance and inductance. It induced changes in the absorbance characteristics of water in specific spectral regions, likely associated with greater stretching and reduced bending vibrations compared to untreated water. While assimilation rate and photosynthetic efficiency were not significantly affected at 60 days after sowing, treated water increased the stomatal conductance to water vapour gsw (+79%) and the electron transport rate ETR (+10%) in CU, as well as the non-photochemical quenching NPQ (+33%) in SO. Treated water also reduced leaf temperature in all species (-0.86 °C on average). This translated into improved plant biomass (leaves: +34%; roots: +140%) and reduced leaf-to-root biomass ratio (-42%) in SO, allowing both faster aerial growth and soil colonization, which can be exploited to improve plant tolerance against abiotic stresses. In the C3 species CU and LE, plant biomass was instead reduced, although significantly in LE only, while the leaf-to-root biomass ratio was generally enhanced, a result likely profitable in the cultivation of leafy vegetables. This is a preliminary trial on the effects of functionalized water and much remains to be investigated in other physiological processes, plant species, and growth stages for the full exploitation of this water treatment in agronomy.

Plants have a protective mechanism called non-photochemical quenching to prevent damage caused by excessive sunlight. A critical component of this mechanism is energy-dependent quenching (qE). In Chlamydomonas reinhardtii, the protein expression called light-harvesting complex stress-related protein 3 (LHCSR3) is crucial for the qE mechanism. LHCSR3 expression is observed in various conditions that result in photooxidation, such as exposure to high light or nutrient deprivation, where the amount of captured light surpasses the maximum photosynthetic capacity. Although the role of LHCSR3 has been extensively studied under high light (HL) conditions, its function during nutrient starvation remains unclear. In this study, we demonstrate that LHCSR3 expression can occur under light intensities below saturation without triggering qE, particularly when nutrients are limited. To investigate this, we cultivated C. reinhardtii cells under osmotic stress, which replicates conditions of nutrient scarcity. Furthermore, we examined the photosynthetic membrane complexes of wild-type (WT) and npq4 mutant strains grown under osmotic stress. Our analysis revealed that LHCSR3 expression might modify the interaction between the photosystem II core and its peripheral light-harvesting complex II antennae. This alteration could potentially impede the transfer of excitation energy from the antenna to the reaction center.

Orange carotenoid proteins (OCPs) are unique photoreceptors that are critical for cyanobacterial photoprotection. Upon exposure to blue-green light, OCPs are activated from a stable orange form, OCPO, to an active red form, OCPR, which binds to phycobilisomes (PBSs) and performs photoprotective non-photochemical quenching (NPQ). OCPs can be divided into three main families: the most abundant and best studied OCP1, and two others, OCP2 and OCP3, which have different activation and quenching properties and are yet underexplored. Crystal structures have been acquired for the three OCP clades, providing a glimpse into the conformational underpinnings of their light-absorption and energy dissipation attributes. Recently, the structure of the PBS-OCPR complex has been obtained allowing for an unprecedented insight into the photoprotective action of OCPs. Here, we review the latest findings in the field that have substantially improved our understanding of how cyanobacteria protect themselves from the toxic consequences of excess light absorption. Furthermore, current research is applying the structure of OCPs to bio-inspired optogenetic tools, to function as carotenoid delivery devices, as well as engineering the NPQ mechanism of cyanobacteria to enhance their photosynthetic biomass production.

Non-photochemical quenching (NPQ) protects plants from photodamage caused by excess light energy. Substantial variation in NPQ has been reported among different genotypes of the same species. However, comparatively little is known about how environmental perturbations, including nutrient deficits, impact natural variation in NPQ kinetics. Here, we analyzed a natural variation in NPQ kinetics of a diversity panel of 225 maize (Zea mays L.) genotypes under nitrogen replete and nitrogen deficient field conditions. Individual maize genotypes from a diversity panel exhibited a range of changes in NPQ in response to low nitrogen. Replicated genotypes exhibited consistent responses across two field experiments conducted in different years. At the seedling and pre-flowering stages, a similar portion of the genotypes (∼33%) showed decrease, no-change or increase in NPQ under low nitrogen relative to control. Genotypes with increased NPQ under low nitrogen also showed greater reductions in dry biomass and photosynthesis than genotypes with stable NPQ when exposed to low nitrogen conditions. Maize genotypes where an increase in NPQ was observed under low nitrogen also exhibited a reduction in the ratio of chlorophyll a to chlorophyll b. Our results underline that since thermal dissipation of excess excitation energy measured via NPQ helps to balance the energy absorbed with energy utilized, the NPQ changes are the reflection of broader molecular and biochemical changes which occur under the stresses such as low soil fertility. Here, we have demonstrated that variation in NPQ kinetics resulted from genetic and environmental factors, are not independent of each other. Natural genetic variation controlling plastic responses of NPQ kinetics to environmental perturbation increases the likelihood it will be possible to optimize NPQ kinetics in crop plants for different environments.

Zinc (Zn) is an essential trace element but can lead to water contamination and ecological deterioration when present in excessive amounts. Therefore, investigating the photosynthetic response of microalgae to Zn stress is of great significance. In this study, we assessed the photosynthetic responses of neutrophilic Chlamydomonas reinhardtii and acidophilic Chlamydomonas sp. 1710 to Zn exposure for 96 h. The specific growth rate (μ), chlorophyll-a (Chl-a) content, and chlorophyll fluorescence parameters were determined. The results demonstrated that Chlamydomonas sp. 1710 was much more tolerant to Zn than C. reinhardtii, with the half-maximal inhibitory concentration (IC50) values of 225.4 mg/L and 23.4  mg/L, respectively. The μ and Chl-a content of C. reinhardtii decreased in the presence of 15  mg/L Zn, whereas those of Chlamydomonas sp. 1710 were unaffected by as high as 100  mg/L Zn. Chlorophyll fluorescence parameters indicated that the regulation of energy dissipation, including non-photochemical quenching, played a crucial role in Zn stress resistance for both Chlamydomonas strains. However, in the case of C. reinhardtii, non-photochemical quenching was inhibited by 5  mg/L Zn in the first 48 h, whereas for Chlamydomonas sp. 1710, it remained unaffected under 100  mg/L Zn. Chlamydomonas sp. 1710 also exhibited a 20 times stronger capacity for regulating the electron transfer rate than C. reinhardtii under Zn stress. The light energy utilization efficiency (α) of Chlamydomonas sp. 1710 had the most highly non-linear correlation with μ, indicating the energy utilization and regulation process of Chlamydomonas sp. 1710 was well protected under Zn stress. Collectively, our findings demonstrate that the photosystem of Chlamydomonas sp. 1710 is much more resilient and tolerant than that of C. reinhardtii under Zn stress.