Wheat (C3) and maize (C4) adaptive responses to soil thallium toxicity under elevated CO<sub>2</sub> conditions

This study investigated how wheat (C3) and maize (C4) respond to soil thallium (Tl) contamination and elevated CO2 (eCO2), aiming to understand strategies for mitigating oxidative stress. Under eCO2, both crops showed higher biomass production. However, high Tl concentration (120 mg/kg) significantly decreased fresh and dry weights by 31–59%, which translated directly to compromised yield. This growth decline is linked to impaired photosynthesis, evidenced by a 54–57% drop in net photosynthetic rate under elevated Tl. Such photosynthetic inhibition intensifies oxidative stress, marked by increased membrane damage and hydrogen peroxide (H2O2). Furthermore, photorespiration contributed to oxidative stress by generating H2O2, with increased activities of glycolate oxidase and hydroxypyruvate reductase rising by 122% and 201%, in wheat and by 179% and 39% in maize, respectively, in response to 120 mg/kg TI under eCO2 conditions. Simultaneously, to mitigate oxidative damage, antioxidant defences were significantly enhanced, resulting in increased activity of the ascorbate (ASC)/glutathione (GSH) cycle, along with elevated levels of metallothionein and phytochelatin for Tl sequestration, as well as augmented glutathioneS-transferase activity. Overall, findings reveal complex interactions between CO2 and Tl, highlighting species-specific adaptive responses of C3 and C4 plants. C3 plants use photorespiration to combat oxidative stress, while C3 and C4 plants have strong antioxidant systems to reduce the effects of oxidative stress, promoting crop resilience and growth despite Tl toxicity.