Preslava B. Borisova

Drought is a major environmental constraint that disrupts photosynthetic processes. This study investigated the effects of foliar-applied commercial humic acid (HA) at different concentrations (1, 3 and 5 mg/mL) on the photosynthetic apparatus of sweet basil (Ocimum basilicum L. Italiano classico) under PEG-induced stress. The responses of the photosynthetic machinery were evaluated using chlorophyll a fluorescence analyses (JIP-test and PAM), leaf pigment composition, and assessments of membrane integrity. Drought stress caused pronounced alterations on both the donor and acceptor sides of photosystem II (PSII), including impaired Q_A⁻ reoxidation, reduced open PSII reaction centers (qP), diminished electron transport (ETo/RC, REo/RC), and substantial declines in performance indices (PI_ABS, PItotal). Energy dissipation increased (DI₀/RC), with regulated energy losses (Φ_NPQ) rising more strongly than non-regulated losses (Φ_NO). Drought also elevated oxidative stress markers (MDA and H₂O₂), leading to enhanced membrane injury. Among the tested concentrations, 5 mg/mL HA provided the most effective protection against drought stress. This treatment mitigated PEG-induced damage on both PSII donor and acceptor sides and increased the proportion of open reaction centers (qP). Improved PSII photochemistry corresponded with more efficient Q_A⁻ reoxidation, facilitated its interaction with plastoquinone, and caused the overall stabilization of photosynthetic functions under drought. The protective effects of HA were also evident for both PSI subpopulations. The enhanced tolerance was associated with the activation of antioxidant enzymes (CAT, SOD, APX) and the increased levels of anthocyanins and total phenolic content (TPC). In contrast, lower HA concentrations (1 and 3 mg/mL) provided insufficient protection. This study clearly demonstrates that HA enhances drought tolerance in basil in a concentration-dependent manner by protecting the structural and functional integrity of the photosynthetic apparatus, supporting its potential use as a foliar treatment to improve crop resilience under water-limited conditions.

Drought is a major environmental threat to agriculture. This study examined the role of anthocyanins in plant drought tolerance by comparing two basil varieties differing in leaf anthocyanin content: green basil (Ocimum basilicum L. Italiano Classico) and purple basil (Ocimum basilicum L. Dark Opal). The impact of the PEG-induced drought stress was assessed by monitoring changes in chlorophyll a fluorescence parameters (JIP and PAM), leaf pigment content, anthocyanin and total phenolic levels, oxidative stress markers (malondialdehyde, hydrogen peroxide and membrane integrity), as well as radical-scavenging capacity (DPPH assay). Drought stress led to a modification on both the donor (Wk) and acceptor (Vj) sides of PSII, which influences Q_A reoxidation and amounts of the closed reaction centers (1-qP). These changes inhibited photosystem II photochemistry, the rate of the electron transport (ETR), and the rate of the photosynthesis (R_Fd) and decreased performance indices (PI_ABS, PItotal), as well as the photosystem I photochemistry. The drought-induced changes were associated with an increase in the dissipated energy per reaction center (DI₀/RC). The results show that photosynthetic functions in purple basil were less affected under drought stress compared to green basil. The reason for better tolerance of purple basil is associated with elevated anthocyanin levels, which correlate with enhanced antioxidant capacity, reduced hydrogen peroxide accumulation, lower membrane lipid peroxidation, improved relative water content and membrane stability. In addition, rapid cyclic electron flow around photosystem I and a higher carotenoid to chlorophyll ratio contribute to drought tolerance in purple basil. After re-watering, purple basil recovers its photosynthetic function almost completely, unlike green basil, which shows further suppression. The increase in the anthocyanin content and radical-scavenging capacity, as well as the smaller oxidative damage under drought stress, are the main reasons for the better recovery in purple basil. Overall, the findings highlight that higher anthocyanin accumulation in purple basil confers greater drought tolerance and recovery capacity by stabilizing photosynthetic processes and reducing oxidative stress.

Drought stress is a major abiotic factor limiting plant growth and productivity. This study investigates the role of oligomerization of the light-harvesting complex of photosystem II (LHCII) in modulating plant responses to drought stress. Using pea plants (Pisum sativum L.): Borec (wild type) and its mutants Costata 2/133 and Coeruleovireus 2/16, with different degrees of LHCII oligomerization, we examined the impact of water deficit on the functions of the photosynthetic apparatus. This study demonstrated that plants with a higher degree of LHCII oligomerization (wild type and Coeruleovireus 2/16) have enhanced drought tolerance, expressed by reduced lipid peroxidation and membrane damage, protection of the photosynthetic pigment content, which corresponds with better photosynthetic performance. Data revealed only minor drought-induced inhibition of photosystem II (PSII) photochemistry (Fv/Fm, Φ_PSII), electron transport rate (ETR), and rate of photosynthesis (R_Fd)), along with sustained performance indices (PI_ABS and PItotal) in plants with higher LHCII oligomerization compared to those with lower levels (Costata 2/133). Additionally, the current study indicates that under drought stress and low actinic light, the interaction with plastoquinone and controlled dissipation of excess energy are promoted in thylakoid membranes with increased LHCII oligomerization. In contrast, drought-stressed plants with lower oligomerization (Costata 2/133) showed a significant increase in non-regulated energy losses under high actinic light. These results highlight the protective function of LHCII oligomerization in preserving photosynthetic integrity and functioning under drought stress and suggest that it could be a promising target for enhancing crop resilience in a changing climate.

The growing issue of salinity is a significant threat to global agriculture, affecting diverse regions worldwide. Nitric oxide (NO) serves as an essential signal molecule in regulating photosynthetic performance under physiological and stress conditions. The present study reveals the protective effects of different concentrations (0–300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the main complexes within the photosynthetic apparatus of maize (Zea mays L. Kerala) under salt stress (150 mM NaCl). The data showed that SNP alleviates salt-induced oxidative stress and prevents changes in the fluidity of thylakoid membranes (Laurdan GP) and energy redistribution between the two photosystems (77K chlorophyll fluorescence ratio F₇₃₅/F₆₈₅). Chlorophyll fluorescence measurements demonstrated that the foliar spray with SNP under salt stress prevents the decline of photosystem II (PSII) open reaction centers (qP) and improves their efficiency (Φexc), thereby influencing Q_A⁻ reoxidation. The data also revealed that SNP protects the rate constants for two pathways of Q_A⁻ reoxidation (k₁ and k₂) from the changes caused by NaCl treatment alone. Additionally, there is a predominance of Q_A⁻ interaction with plastoquinone in comparison to the recombination of electrons in Q_A Q_B⁻ with the oxygen-evolving complex (OEC). The analysis of flash oxygen evolution showed that SNP treatment prevents a salt-induced 10% increase in PSII centers in the S₀ state, i.e., protects the initial S₀–S₁ state distribution, and the modification of the Mn cluster in the OEC. Moreover, this study demonstrates that SNP-induced defense occurs on both the donor and acceptor sides of the PSII, leading to the protection of overall photosystems performance (PI_ABS) and efficient electron transfer from the PSII donor side to the reduction of PSI end electron acceptors (PItotal). This study clearly shows that the optimal protection under salt stress occurs at approximately 50–63 nmoles NO/g FW in leaves, corresponding to foliar spray with 50–150 µM SNP.