Related Experiment Video
Updated: May 28, 2026

High Throughput Image-Based Phenotyping for Determining Morphological and Physiological Responses to Single and Combined Stresses in Potato
Published on: June 7, 2024
Humic Acid Modulates Photosynthetic Responses to PEG-Induced Drought in Ocimum basilicum L
Martin A Stefanov1, Georgi D Rashkov1, Preslava B Borisova1
1Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, 1113 Sofia, Bulgaria.
None:
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 QA- reoxidation, reduced open PSII reaction centers (qP), diminished electron transport (ETo/RC, REo/RC), and substantial declines in performance indices (PIABS, PItotal). Energy dissipation increased (DI0/RC), with regulated energy losses (ΦNPQ) rising more strongly than non-regulated losses (ΦNO). 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 QA- 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.
More Related Videos
Related Concept Videos
Adaptations that Reduce Water Loss
Responses to Salt Stress
Responses to Drought and Flooding
Regulation of Transpiration by Stomata
C4 Pathway and CAM
C4 Pathway
The C4 pathway is used by plants such as...
Responses to Heat and Cold Stress

