Preillumination of lettuce seedlings with red light enhances the resistance of photosynthetic apparatus to UV-A

Adaptation processes in Halomicronema hongdechloris, an example of the light-induced optimization of the photosynthetic apparatus on hierarchical time scales

Oxygenic photosynthesis in Halomicronema hongdechloris, one of a series of cyanobacteria producing red-shifted Chl f, is adapted to varying light conditions by a range of diverse processes acting over largely different time scales. Acclimation to far-red light (FRL) above 700 nm over several days is mirrored by reversible changes in the Chl f content. In several cyanobacteria that undergo FRL photoacclimation, Chl d and Chl f are directly involved in excitation energy transfer in the antenna system, form the primary donor in photosystem I (PSI), and are also involved in electron transfer within photosystem II (PSII), most probably at the ChlD1 position, with efficient charge transfer happening with comparable kinetics to reaction centers containing Chl a. In H. hongdechloris, the formation of Chl f under FRL comes along with slow adaptive proteomic shifts like the rebuilding of the D1 complex on the time scale of days. On shorter time scales, much faster adaptation mechanisms exist involving the phycobilisomes (PBSs), which mainly contain allophycocyanin upon adaptation to FRL. Short illumination with white, blue, or red light leads to reactive oxygen species-driven mobilization of the PBSs on the time scale of seconds, in effect recoupling the PBSs with Chl f-containing PSII to re-establish efficient excitation energy transfer within minutes. In summary, H. hongdechloris reorganizes PSII to act as a molecular heat pump lifting excited states from Chl f to Chl a on the picosecond time scale in combination with a light-driven PBS reorganization acting on the time scale of seconds to minutes depending on the actual light conditions. Thus, structure-function relationships in photosynthetic energy and electron transport in H. hongdechloris including long-term adaptation processes cover 10-12 to 106 seconds, i.e., 18 orders of magnitude in time.

The crosstalk of far-red energy and signaling defines the regulation of photosynthesis, growth, and flowering in tomatoes

This study investigated the effect of light intensity and signaling on the regulation of far-red (FR)-induced alteration in photosynthesis. The low (LL: 440 μmol m-2 s-1) and high (HL: 1135 μmol m-2 s-1) intensity of white light with or without FR (LLFR: 545 μmol m-2 s-1 including 115 μmol m-2 s-1; HLFR: 1254 μmol m-2 s-1 + 140 μmol m-2 s-1) was applied on the tomato cultivar (Solanum Lycopersicon cv. Moneymaker) and mutants of phytochrome A (phyA) and phytochrome B (phyB1, and phyB2). Both light intensity and FR affected plant morphological traits, leaf biomass, and flowering time. Irrespective of genotype, flowering was delayed by LLFR and accelerated by HLFR compared to the corresponding light intensity without FR. In LLFR, a reduced energy flux through the electron transfer chain along with a reduced energy dissipation per reaction center improved the maximum quantum yield of PSII, irrespective of genotype. HLFR increased net photosynthesis and gas exchange properties in a genotype-dependent manner. FR-dependent regulation of hormones was affected by light signaling. It appeared that PHYB affected the levels of abscisic acid and salicylic acid while PHYA took part in the regulation of CK in FR-exposed plants. Overall, light intensity and signaling of FR influenced plants' photosynthesis and growth by altering electron transport, gas exchange, and changes in the level of endogenous hormones.

High-Proportion Blue Light Irradiation at the End-of-Production Stage Promotes the Biosynthesis and Recycling of Ascorbate in Lettuce

Ascorbate (AsA), an essential antioxidant for both plants and the human body, plays a vital role in maintaining proper functionality. Light plays an important role in metabolism of AsA in horticultural plants. Our previous research has revealed that subjecting lettuce to high light irradiation (HLI) (500 μmol·m-2·s-1) at the end-of-production (EOP) stage effectively enhances AsA levels, while the optimal light quality for AsA accumulation is still unknown. In this study, four combinations of red (R) and blue (B) light spectra with the ratio of 1:1 (1R1B), 2:1 (2R1B), 3:1 (3R1B), and 4:1 (4R1B) were applied to investigate the biosynthesis and recycling of AsA in lettuce. The results demonstrated that the AsA/total-AsA content in lettuce leaves was notably augmented upon exposure to 1R1B and 2R1B. Interestingly, AsA levels across all treatments increased rapidly at the early stage (2-8 h) of irradiation, while they increased slowly at the late stage (8-16 h). The activity of L-galactono-1,4-lactone dehydrogenase was augmented under 1R1B treatment, which is pivotal to AsA production. Additionally, the activities of enzymes key to AsA cycling were enhanced by 1R1B and 2R1B treatments, including ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase. Notably, hydrogen peroxide and malondialdehyde accumulation increased dramatically following 16 h of 1R1B and 2R1B treatments. In addition, although soluble sugar and starch contents were enhanced by EOP-HLI, this effect was comparatively subdued under the 1R1B treatment. Overall, these results indicated that AsA accumulation was improved by irradiation with a blue light proportion of over 50% in lettuce, aligning with the heightened activities of key enzymes responsible for AsA synthesis, as well as the accrual of hydrogen peroxide. The effective strategy holds the potential to enhance the nutritional quality of lettuce while bolstering its antioxidant defenses.

Effect of Light of Different Spectral Compositions on Pro/Antioxidant Status, Content of Some Pigments and Secondary Metabolites and Expression of Related Genes in Scots Pine

The aim of this study was to investigate the effect of light quality (white fluorescent light, WFL, containing UV components), red light (RL, 660 nm), blue light (BL, 450 nm), and white LED light (WL, 450 + 580 nm) on the components of the cellular antioxidant system in Pinus sylvestris L. in needles, roots, and hypocotyls, focusing on the accumulation of key secondary metabolites and the expression of related genes. The qualitative and quantitative composition of carotenoids; the content of the main photosynthetic pigments, phenolic compounds, flavonoids (catechins, proanthocyanidins, anthocyanins), ascorbate, and glutathione; the activity of the main antioxidant enzymes; the content of hydrogen peroxide; and the intensity of lipid peroxidation (MDA and 4-HNE contents) were determined. RL resulted in an increase in the content of hydrogen peroxide and 4-HNE, as well as the total fraction of flavonoids in the needles. It also enhanced the expression of several PR (pathogen-related) genes compared to BL and WL. WFL increased the content of phenols, including flavonoids, and enhanced the overall activity of low-molecular antioxidants in needles and hypocotyls. BL increased the content of ascorbate and glutathione, including reduced glutathione, in the needles and simultaneously decreased the activity of peroxidases. Thus, by modifying the light quality, it is possible to regulate the accumulation of secondary metabolites in pine roots and needles, thereby influencing their resistance to various biotic and abiotic stressors.

A comparison of consistent UV treatment versus inconsistent UV treatment in horticultural production of lettuce

UV radiation is an underrated radiation currently missing in many horticultural production systems of vegetables in protected cultivation. It can be added e.g., in LED light sources. Using lettuce as a model plant, this study determined whether the use of UVB LEDs is suitable (1) for use in consistent systems (indoor farming) or (2) inconsistent systems (greenhouse). Blue and red LEDs were selected as additional artificial lighting to UVB LEDs. Both approaches led to a reproducible increase of desired flavonol glycosides, such as quercetin-3-O-(6''-O-malonyl)-glucoside or quercetin-3-O-glucuronide and the anthocyanin cyanidin-3-O-(6''-O-malonyl)-glucoside in lettuce. The impact of the consistent UVB treatment is higher with up to tenfold changes than that of the inconsistent UVB treatment in the greenhouse. Varying natural light and temperature conditions in greenhouses might affect the efficiency of the artificial UVB treatment. Here, UVB LEDs have been tested and can be recommended for further development of lighting systems in indoor farming and greenhouse approaches.

Molecular Processes of Dodder Haustorium Formation on Host Plant under Low Red/Far Red (R/FR) Irradiation

Low R/FR irradiation can promote dodder haustorium formation on the host plant; however, the mechanisms underlying the process are still unknown. In this study, we compared the transcriptomic data during the formation of haustorium of Cuscuta chinensis on host plant Arabidopsisthaliana under low (R/FR = 0.1) versus high (R/FR = 0.2) R/FR irradiation at 12 h, 24 h and 72 h time points. The results show that low R/FR radiation significantly promoted the entanglement and haustorium formation. Transcriptome analysis showed that during the early stage of haustorium formation, low R/FR radiation significantly up-regulated ARR-A related genes and down-regulated peroxidase related genes compared with high R/FR radiation. Meanwhile, during the middle stage of haustorium formation, low R/FR treatment significantly increased the expression of genes related to pectinesterase (PE), polygalacturonase (PG) and pectin lyase (Pel) production, while, during the late stage of haustorium formation, peroxidase (Prx)-related genes were differentially expressed under different R/FR treatments. Overall, our findings show that a low R/FR ratio promotes the parasitism of C. chinensis through plant hormone signal transduction and cell wall degradation pathways. This study provides a basis for the control of parasitic plants.

Supplemental light application can improve the growth and development of strawberry plants under salinity and alkalinity stress conditions

The use of complementary light spectra is a potential new approach to studying the increase in plant resilience under stress conditions. The purpose of this experiment was to investigate the effect of different spectra of complementary light on the growth and development of strawberry plants under salinity and alkalinity stress conditions. Plants were grown in the greenhouse under ambient light and irradiated with blue (460 nm), red (660 nm), blue/red (1:3), and white/yellow (400–700 nm) light during the developmental stages. The stress treatments were as follows: control (non-stress), alkalinity (40 mM NaHCO₃), and salinity (80 mM NaCl). Our results showed that salinity and alkalinity stress decreased fresh and dry weights and the number of green leaves, and increased chlorotic, tip burn, and dry leaves. The blue and red spectra had a greater effect on reducing the effects of stress compared to other spectra. Stress conditions decreased SPAD and RWC, although blue light increased SPAD, and blue/red light increased RWC under stress conditions. Blue/red and white/yellow light had the greatest effect on reproductive traits. Stress conditions affected fruit color indicators, and red and blue light had the most significant effect on these traits. Under stress conditions, sodium uptake increased, while K, Ca, Mg, and Fe uptake decreased, markedly. Blue and red light and their combination alleviated this reducing effect of stress. It can be concluded that the effects of salinity and alkalinity stresses can be reduced by manipulating the supplemental light spectrum. The use of artificial light can be extended to stresses.

Effects of light spectrum on morpho-physiological traits of grafted tomato seedlings

It is already known that there are many factors responsible for the successful grafting process in plants, including light intensity. However, the influence of the spectrum of light-emitting diodes (LEDs) on this process has almost never been tested. During the pre-grafting process tomato seedlings grew for 30 days under 100 μmol m-2 s-1 of mixed LEDs (red 70%+ blue 30%). During the post-grafting period, seedlings grew for 20 days under the same light intensity but the lightening source was either red LED, mixed LEDs (red 70% + blue 30%), blue LED or white fluorescent lamps. This was done to determine which light source(s) could better improve seedling quality and increase grafting success. Our results showed that application of red and blue light mixture (R7:B3) caused significant increase in total leaf area, dry weight (total, shoot and root), total chlorophyll/carotenoid ratio, soluble protein and sugar content. Moreover, this light treatment maintained better photosynthetic performance i.e. more effective quantum yield of PSII photochemistry Y(II), better photochemical quenching (qP), and higher electron transport rate (ETR). This can be partially explained by the observed upregulation of gene expression levels of PsaA and PsbA and the parallel protein expression levels. This in turn could lead to better functioning of the photosynthetic apparatus of tomato seedlings and then to faster production of photoassimilate ready to be translocated to various tissues and organs, including those most in need, i.e., involved in the formation of the graft union.

Effect of different shading materials on grain yield and quality of rice

Light is a basic environmental factor required for plant growth and development; however, these are not only affected by light quantity, but also by light quality - light and radiation of different wavelengths and different compositions. In four different rice varieties (Oryza sativa L.), two kinds of shading materials, white cotton yarn (Shading (W)) and black nylon net (Shading (B)) were used to simulate cloudy days. Yield decreased under Shading (W) (15.3-17.7%) and Shading (B) (16.6-20.0%) compared to under sunny day (CK), and different effects on rice quality, which is mainly affected by changes in light quality, were observed. The change in light quality (Blue, Green, Red and R/FR proportions) represented under Shading (W) was significantly different from that under CK and Shading (B) conditions. Red light composition under Shading (W) was closer to that of the cloudy day condition. The proportion of blue light under Shading (W) was significantly lower than that under CK conditions; under Shading (B), it was higher than that under all conditions. The differences in light quality may affect photosynthesis in leaves and final starch synthesis, resulting in increased chalky grain rate, chalkiness, and poor rice quality. White cotton yarn as the shading material for further research used to simulate the influence of the light environment on rice growth under cloudy conditions will be better than black net.

The effect of light quality on the pro-/antioxidant balance, activity of photosystem II, and expression of light-dependent genes in Eutrema salsugineum callus cells

The antioxidant balance, photochemical activity of photosystem II (PSII), and photosynthetic pigment content, as well as the expression of genes involved in the light signalling of callus lines of Eutrema salsugineum plants (earlier Thellungiella salsuginea) under different spectral light compositions were studied. Growth of callus in red light (RL, maximum 660 nm), in contrast to blue light (BL, maximum 450 nm), resulted in a lower H₂O₂ content and thiobarbituric acid reactive substances (TBARS). The BL increased the activities of key antioxidant enzymes in comparison with the white light (WL) and RL and demonstrated the minimum level of PSII photochemical activity. The activities of catalase (CAT) and peroxidase (POD) had the highest values in BL, which, along with the increased H₂O₂ and TBARS content, indicate a higher level of oxidative stress in the cells. The expression levels of the main chloroplast protein genes of PSII (PSBA and PSBD), the NADPH-dependent oxidase gene of the plasma membrane (RbohD), the protochlorophyllide oxidoreductase genes (POR B, C) involved in the biosynthesis of chlorophyll, and the key photoreceptor signalling genes (CIB1, CRY2, PhyB, PhyA, and PIF3) were determined. Possible mechanisms of light quality effects on the physiological parameters of callus cells are discussed.

Photochemical activity and the structure of chloroplasts in Arabidopsis thaliana L. mutants deficient in phytochrome A and B

The reduced content of photoreceptors, such as phytochromes, can decrease the efficiency of photosynthesis and activity of the photosystem II (PSII). For the confirmation of this hypothesis, the effect of deficiency in both phytochromes (Phy) A and B (double mutant, DM) in 7-27-day-old Arabidopsis thaliana plants on the photosynthetic activity was studied in absence and presence of UV-A radiation as a stress factor. The DM with reduced content of apoproteins of PhyA and PhyB and wild type (WT) plants with were grown in white and red light (WL and RL, respectively) of high (130 μmol quanta m^-2 s^-1) and low (40 μmol quanta m^-2 s^-1) intensity. For DM and WT grown in WL, no notable difference in the photochemical activity of PSII was observed. However, the resistance of the photosynthetic apparatus (PA) to UV-A and the rate of photosynthesis under light saturation were lower in the DM compared to those in the WT. Growth in RL, when the photoreceptors of blue light-cryptochromes-are inactive, resulted in the significant decrease of the photochemical activity of PSII in DM compared to that in WT including amounts of Q_B-non-reducing complexes of PSII and noticeable enhancement of thermal dissipation of absorbed light energy. In addition, marked distortion of the thylakoid membrane structure was observed for DM grown in RL. It is suggested that not only PhyA and PhyB but also cryptochromes are necessary for normal functioning of the PA and formation of the mechanisms of its resistance to UV-radiation.

UV-A radiation effects on higher plants: Exploring the known unknown

Ultraviolet-A radiation (UV-A: 315-400nm) is a component of solar radiation that exerts a wide range of physiological responses in plants. Currently, field attenuation experiments are the most reliable source of information on the effects of UV-A. Common plant responses to UV-A include both inhibitory and stimulatory effects on biomass accumulation and morphology. UV-A effects on biomass accumulation can differ from those on root: shoot ratio, and distinct responses are described for different leaf tissues. Inhibitory and enhancing effects of UV-A on photosynthesis are also analysed, as well as activation of photoprotective responses, including UV-absorbing pigments. UV-A-induced leaf flavonoids are highly compound-specific and species-dependent. Many of the effects on growth and development exerted by UV-A are distinct to those triggered by UV-B and vary considerably in terms of the direction the response takes. Such differences may reflect diverse UV-perception mechanisms with multiple photoreceptors operating in the UV-A range and/or variations in the experimental approaches used. This review highlights a role that various photoreceptors (UVR8, phototropins, phytochromes and cryptochromes) may play in plant responses to UV-A when dose, wavelength and other conditions are taken into account.

Protective effect of UV-A radiation during acclimation of the photosynthetic apparatus to UV-B treatment

We examined the acclimation response of the photosynthetic apparatus of barley (Hordeum vulgare L.) to a combination of UV-A and UV-B radiation (UVAB) and to UV-B radiation alone. Our aim was to evaluate whether UV-A radiation prevents UV-B-induced damage to the photosynthetic apparatus and whether UV-A pre-acclimation is required to mitigate the negative influence of UV-B radiation. Barley plants were grown from seeds under low photosynthetically active radiation (50 μmol m(-2) s(-1)) either in the absence or presence of UV-A radiation (UVA- and UVA+ plants, respectively). After 8 days of development, plants were exposed simultaneously to UV-A and UV-B radiation for the next 6 days. Additionally, UVA- plants were exposed to UV-B radiation alone. The UVA+ plants had a higher CO2 assimilation rate near the light-saturation region (A(N)) and a higher content of both total chlorophylls (Chls) and total carotenoids than the UVA- plants. Chls content, A(N), the potential quantum yield of photosystem II (PSII) photochemistry (F(V)/F(M)), the capacity of light-induced thermal energy dissipation and the efficiency of excitation energy transfer within PSII remained the same or even increased in both UVA+ and UVA- plants after UVAB treatment. On the contrary, exposure of UVA- plants to UV-B radiation itself led to a reduction in all these characteristics. We revealed that the presence of UV-A radiation during UVAB treatment not only mitigated but completely eliminated the negative effect of UV-B radiation on the functioning of the photosynthetic apparatus and that UV-A pre-acclimation was not crucial for development of this UV-A-induced resistance against UV-B irradiation.

Effect of preillumination with red light on photosynthetic parameters and oxidant-/antioxidant balance in Arabidopsis thaliana in response to UV-A

The effect of preillumination with low intensity (10μmol quanta m(-2)s(-1), 10min) light of different wavelengths in the spectral range of 550-730nm on photosynthesis and activity of PSII, the content of photosynthetic pigments and H2O2, as well as the peroxidase activity in the leaves of 26-d-old Arabidopsis thaliana wild-type (WT) plants in response to UV-A radiation was studied. UV-A decreased the activity of the PSII, the content of Chl a, Chl b and carotenoids, as well as increased the peroxidase activity and H2O2 level in the WT leaves. Preillumination of the leaves with red light (RL, λmax=664nm) reduced the inhibitory effect of UV radiation on photosynthesis and activity of the PSII, indicated by delayed light emission as well as the H2O2 level, but increased the peroxidase activity in the leaves compared to illumination by UV radiation only. Illumination with RL alone and the subsequent exposure of plants to darkness increased the peroxidase activity and the transcription activity of genes of the transcription factors APX1 and HYH. Preillumination of leaves with RL, then far red light (FRL, λmax=727nm) partially compensated the effect of the RL for all studied parameters, suggesting that the active form of phytochrome (PFR) is involved in these processes. Preillumination with the wavelengths of 550, 594 and 727nm only did not have a marked effect on photosynthesis. The hy2 mutant of Arabidopsis with reduced synthesis of the phytochrome B chromophore showed decreased resistance of PSII to UV-A compared with the WT of Arabidopsis. UV radiation reduced Chl a fluorescence much faster in the hy2 mutant compared to the WT. Preillumination of the hy2 mutant with RL did not affect the PSII activity and H2O2 level in UV-irradiated leaves. It is assumed that the formation of the increased resistance of the photosynthetic apparatus of Arabidopsis to UV-A radiation involves PFR and the antioxidant system of plants, partly by inducing transcriptional activity of some antioxidant and transcription factors genes.