Ceyda Ozfidan-Konakci

Current knowledge about what the protective roles of elevated CO ₂ concentration (eCO ₂ ) on chlorophyll fluorescence, PSII photochemistry and antioxidant capacity of Arabidopsis with heat stress remains insufficient. Besides, there is a lack of information on the tolerance mechanism of plants against stress in the presence or absence of sirtuin genes. This study aimed to investigate how eCO ₂ (800 ppm) mitigated heat stress (36 °C)-dependent oxidative damages through analyzing growth, water content, gas exchange, chlorophyll fluorescence, electron flux efficiency in PSII, antioxidant capacity and lipid peroxidation in Arabidopsis thaliana cv. Columbia-0 (Col-0) and its sirtuin-deficient mutant ( srt2 ) plants. The focus was on investigating the function of the SRT2 gene, particularly in comparison with wild-type plants. Therefore, the sirtuin inhibitor (sirtinol, 1 µM S) was applied to Col-0 plants and the effects of ambient (400 ppm) and eCO ₂ heat (H) under stress were also examined. The reductions in growth and water content of heat stress-treated Col-0 and srt2 mutants were eliminated by eCO ₂ exposure. The elevated CO ₂ was removed the reductions on chlorophyll fluorescence, carbon assimilation rate and carboxylation efficiency dependent by H stress in Col-0 plants, but not in srt2 mutants. S supplementation to Col-0 plants produced effects similar to those observed under stress across all measured parameters. While the structural indicators and electron fluxes of PSII and performance indices were disrupted by H stress, eCO ₂ provided the positive effects on the photochemistry of PSII in the wild type of Arabidopsis . Interestingly, the eCO ₂ -triggered alleviation against stress in srt2 mutants was not maintain. The induced (SOD and GR) and reduced activities (POX, APX, AsA/DHA and GSH redox state) of antioxidant systems point to increased H ₂ O ₂ accumulation in both Col-0 (123%) and srt2 mutant (40%). Both of the Arabidopsis plants experienced oxidative stress as proved by the high levels of lipid peroxidation. eCO ₂ under H stress resulted in remarkable decline in H ₂ O ₂ accumulation, which contributed to POX and AsA regeneration in Col-0 and POX, AsA and GSH redox status in srt2 . Sirtinol application to Col-0 plants also eliminated eCO ₂ -mediated protection on TBARS content. This study provides the first evidence of SRT2 involvement in CO₂-mediated heat stress tolerance.

Heavy metal pollution, especially arsenic toxicity, significantly impairs plant growth and development. Phenolic acids, known for their antioxidant properties and involvement in stress signaling, are gaining increased attention as plant secondary metabolites with the potential to enhance plant resistance to these stressors. This study aimed to investigate the effects of different concentrations of syringic acid (SA1, 10 μM; SA2, 250 μM; SA3, 500 μM) on growth, photosynthetic parameters, and antioxidant activity in lettuce seedlings subjected to arsenic stress (As, 100 μM). Arsenic stress reduced growth by 56.7%, water content by 7.39%, and osmotic potential by 26.2% in lettuce leaves compared to control. Conversely, SA1 and SA2 treatments mitigated the adverse effects of arsenic on growth and preserved the water balance in plants. However, the SA3 treatment led to a decrease in growth by 18.9% and 39.5% in the SA3 and As+SA3 groups, respectively, indicating that high‐dose SA treatment adversely affected lettuce leaves under both control and stress conditions. Exogenous SA1 treatment significantly improved photosynthesis, whereas SA2 provided milder benefits and SA3 did not reduce the adverse effects of arsenic exposure. Arsenic stress increased H ₂ O ₂ content by 47.3% and lipid peroxidation by 33.4% in lettuce seedlings. SA1 treatment effectively reduced oxidative stress by enhancing the activities of key antioxidant enzymes, such as superoxide dismutase (SOD) and peroxidase (POX). Moreover, SA1 was successful in maintaining the glutathione (GSH) pool, whereas SA2 primarily promoted ascorbate (AsA) regeneration. In conclusion, 10 μM of syringic acid (SA1) was identified as the optimal dose for reducing arsenic stress in lettuce by enhancing antioxidant activity and supporting growth. Overall, the findings underscore the potential of SA1 treatment in enhancing the resilience of lettuce to heavy metal toxicity.

Lead (Pb) and arsenic (As) pollution seriously affect the physiological and biochemical processes of plants, reduce agricultural productivity, and pose a serious threat to human health. Recently, there has been an increasing interest in studies aimed at mitigating these negative effects of heavy metals. Curcumin is an important secondary metabolite belonging to the polyphenol class that provides stress tolerance to plants through its metal‐chelating activity. Therefore, in our study, curcumin (Cur1; 50 μM and Cur2; 100 μM) was applied to lettuce (Lactuca sativa L.) under lead (Pb; 100 μM) and/or arsenic (As; 100 μM) stress. It was observed that the relative growth rate of the plant decreased by 12% at Pb stress and 27% under As stress. Pb and combined stress conditions (Pb + As) reduced the relative water content by 23%. Notably, significant increases in these parameters were observed following curcumin treatment. Furthermore, exogenous curcumin application effectively mitigated the detrimental effects induced by heavy‐metal stress. On gas exchange and photosynthetic parameters, Cur1 treatment resulted in more than a twofold increase in carbon assimilation rate under Pb and Pb + As conditions. Due to Pb + As exposure, H₂O₂ content increased by 25% and TBARS content increased by twofold. These elevations were attenuated by curcumin with enhanced antioxidant enzyme activity. Compared to the combined stress exposure, Pb + As + Cur1 and Pb + As + Cur2 treatments resulted in 40% and 24% increased SOD activity, respectively. Curcumin treatments contributed to the maintenance of redox balance in the cell by increasing the activity of the ascorbate–glutathione (AsA‐GSH) cycle. In conclusion, curcumin applications to lettuce under Pb and/or As stress contributed to the reduction of ROS accumulation by increasing the water uptake, photosynthesis efficiency, and antioxidant enzyme capacity. These findings suggest that phenolic compounds like curcumin could be a promising strategy in agricultural practices to enhance plant resistance to heavy‐metal stress, thereby improving productivity.