UV-B causes both damage to the photosynthetic apparatus (PA) and the activation of specific mechanisms that protect the PA from excess energy and trigger a cascade of regulatory interactions with different photoreceptors, including phytochromes (PHYs) and cryptochromes (CRYs). However, the role of photoreceptors in plants’ responses to UV-B radiation remains undiscovered. This study explores some of these responses using tomato photoreceptor mutants (phya, phyb1, phyab2, cry1). The effects of UV-B exposure (12.3 µmol (photons) m⁻² s⁻¹) on photosynthetic rates and PSII photochemical activity, the contents of photosynthetic and UV-absorbing pigments and anthocyanins, and the nonenzymatic antioxidant capacity (TEAC) were studied. The expression of key light-signaling genes, including UV-B signaling and genes associated with the biosynthesis of chlorophylls, carotenoids, anthocyanins, and flavonoids, was also determined. Under UV-B, phyab2 and cry1 mutants demonstrated a reduction in the PSII effective quantum yield and photosynthetic rate, as well as a reduced value of TEAC. At the same time, UV-B irradiation led to a noticeable decrease in the expression of the ultraviolet-B receptor (UVR8), repressor of UV-B photomorphogenesis 2 (RUP2), cullin 4 (CUL4), anthocyanidin synthase (ANT), phenylalanine ammonia-lease (PAL), and phytochrome B2 (PHYB2) genes in phyab2 and RUP2, CUL4, ANT, PAL, and elongated hypocotyl 5 (HY5) genes in the cry1 mutant. The results indicate the mutual regulation of UVR8, PHYB2, and CRY1 photoreceptors, but not PHYB1 and PHYA, in the process of forming a response to UV-B irradiation in tomato.

The influence of short-term additional white (WL), red (RL) and far-red (FRL) light and combined RL+FRL on the physiological morphological and molecular characteristics of two-year-old Scots pine plants grown in a greenhouse under sunlight was studied. Additional RL and RL+FRL increased the number of xylem cells, transpiration and the expression of a group of genes responsible for the biosynthesis and signaling of auxins (AUX/IAA, ARF3/4, and ARF16) and brassinosteroids (BR-α-RED and BRZ2), while the expression of genes related to the signaling pathway related to jasmonic acid was reduced. Additionally, WL, RL and RL+FRL increased the content of proanthocyanidins and catechins in young needles; however, an increase in the expression of the chalcone synthase gene (CHS) was found under RL, especially under RL+FRL, which possibly indicates a greater influence of light intensity than observed in the spectrum. Additional WL increased photosynthetic activity, presumably by increasing the proportion and intensity of blue light; at the same time, the highest transpiration index was found under RL. The results obtained indicate that the combined effect of additional RL+FRL can accelerate the development of pine plants by increasing the number of xylem cells and increasing the number of aboveground parts but not the photosynthetic activity or the accumulation of secondary metabolites.

The effects of high-intensity blue light (HIBL, 500/1000 µmol m⁻²s⁻¹, 450 nm) on Solanum lycopersicum mutants with high pigment (hp) and low pigment (lp) levels and cryptochrome 1 (cry1) deficiency on photosynthesis, chlorophylls, phenols, anthocyanins, nonenzymatic antioxidant activity, carotenoid composition, and the expression of light-dependent genes were investigated. The plants, grown under white light for 42 days, were exposed to HIBL for 72 h. The hp mutant quickly adapted to 500 µmol m⁻²s⁻¹ HIBL, exhibiting enhanced photosynthesis, increased anthocyanin and carotenoids (beta-carotene, zeaxanthin), and increased expression of key genes involved in pigment biosynthesis (PSY1, PAL1, CHS, ANS) and PSII proteins along with an increase in nonenzymatic antioxidant activity. At 1000 µmol m⁻²s⁻¹ HIBL, the lp mutant showed the highest photosynthetic activity, enhanced expression of genes associated with PSII external proteins (psbO, psbP, psbQ), and increased in neoxanthin content. This mutant demonstrated greater resistance at the higher HIBL, demonstrating increased stomatal conductance and photosynthesis rate. The cry1 mutant exhibited the highest non-photochemical quenching (NPQ) but had the lowest pigment contents and decreased photosynthetic rate and PSII activity, highlighting the critical role of CRY1 in adaptation to HIBL. The hp and lp mutants use distinct adaptation strategies, which are significantly hindered by the cry1 mutation. The pigment content appears to be crucial for adaptation at moderate HIBL doses, while CRY1 content and stomatal activity become more critical at higher doses.

Plants respond to ultraviolet B radiation (280–320 nm) with an integrated reaction that includes the reception of the acting stress factor, followed by the generation of reactive oxygen species and damage to macromolecules and membrane structures, as well as changes in cellular metabolism and the formation of protective systems. However, the involvement of key UV-B–related signalling components such as HY5, SPA1 and BIC1 or BIC2 proteins in physiological, biochemical and molecular responses remains insufficiently understood. The effects of 8, 16 and 24 h of UV-B exposure (within an 8 h photoperiod over three days) on the net photosynthetic rate (Pn), chlorophyll fluorescence parameters Y(II) and F_v/F_m, reflecting the functional state of PSII, nonphotochemical quenching (NPQ), pigment contents (Chl(a+b), carotenoids, anthocyanins and UV-absorbing pigments (UAPs) and the expression of key light-induced genes in wild-type Arabidopsis thaliana and spa1, bic1,2 and hy5 mutants were studied. UV-B irradiation resulted in a gradual reduction in the Pn, Y(II), F_v/F_m values and Chl(a+b) but caused a marked increase in the anthocyanin and UAP contents and only minor changes in the carotenoid content. The hy5 mutant presented the lowest net photosynthetic rate (Pn), chlorophyll fluorescence parameters, and chlorophyll and carotenoid contents under all the UV-B exposures. In addition, the accumulation of anthocyanins and UAPs during UV-B treatment was consistently the lowest in hy5. After any UV-B exposure, the highest accumulation of UAPs and anthocyanins was observed in the spa1 mutant, whereas the highest Pn values were detected after 24 h in bic1,2. One of the reasons for the reduced photosynthetic activity and antioxidant capacity in hy5 may be the lower expression levels of CHS and PAL in this variety than in the other genotypes. Our results indicate that HY5 is required to maintain antioxidant responses and photosynthetic performance under repeated daytime UV-B exposure (16.8 kJ m⁻² per day). In contrast, BIC1, BIC2, and SPA1 also contribute to UV-B tolerance, but through distinct regulatory mechanisms and to a lesser extent.

The physiological, biochemical, and morphometric responses of two lettuce cultivars (Lactuca sativa L.), Gypsy and Pomegranate Lace, which differ in terms of leaf morphology and anthocyanin pigmentation, were examined under moderate light (290 µmol m⁻² s⁻¹) with the addition of blue light (BL, peak at 450 nm), UV-B (peak at 306 nm), and their combinations. Continuous low-intensity UV-B (30 mW m⁻²) was applied for 48 h—during the day with white (WL, Red: 51%, Green: 38%, Blue: 11%) or white + blue (WL + BL, Red: 30%, Green: 22%, Blue: 48%) light and at night alone—to assess the effects of sustained UVR8 activation in the absence of visible light. In the Pomegranate Lace cultivar, which has wrinkled leaves and localized anthocyanin pigmentation, the combination of WL + BL + UV-B enhanced the chlorophyll and carotenoid contents, photosynthetic rate, and stomatal conductance, whereas respiration did not change. These coordinated changes indicate efficient integration of cryptochrome and UVR8 signaling, which sustains photochemical efficiency and stimulates phenolic and carotenoid accumulation, reinforcing antioxidant capacity. In the Gypsy cultivar, which is characterized by smooth leaves and uniform pigmentation, UV-B + BL increased g_S along with the rates of respiration and photosynthesis and improved PSII efficiency. However, both cultivars showed a decrease in biomass and leaf area. Nevertheless, both cultivars exhibited increased antioxidant capacity, but in Gypsy, the addition of BL or UV-B affected the antioxidant capacity and PSII photochemical efficiency more effectively than in the Pomegranate Lace, likely due to deeper penetration in leaves and lower reflectance. Thus, long-term low-intensity UV-B radiation acts as a regulatory spectral cue that differentially modulates photosynthetic and antioxidant pathways. Its integration with blue light enables cultivar-specific optimization of photochemical resistance and metabolic resilience.

Cryptochrome 1a (CRY1a) mediates blue-light signalling in tomato, but its contribution to acclimation to combined blue and green light remains unresolved. We compared wild-type (WT) and cry1 plants under white fluorescent light (WFL), blue light (BL), green light (GL), and BL/GL using gas exchange, A/Ci curves, chlorophyll fluorescence, pigment composition, electron microscopy, transcript abundance, and phenolic metabolism. Under BL, cry1 had a lower photosynthesis rate, stomatal conductance, and Y(II), despite higher chlorophyll content and grana area. Under BL/GL, the photosynthesis rate and stomatal conductance were approximately 35% lower in cry1, and Vcmax, Jmax, and TPU decreased by 48–57%. Y(II) was lower, and Y(NO) was higher in cry1 than in WT. Under BL/GL, the grana area was lower in cry1, whereas the starch-grain area, stroma area, and plastoglobule number showed small differences that were not statistically significant. Transcript responses varied among spectra and functional groups. These results support a contribution of CRY1a to the coordination of structural and physiological acclimation to light quality. The BL/GL response is consistent with an imbalance between CO₂ supply and carbon utilization that may promote photoprotective feedback.