Species-specific responses of wheat and maize to thallium stress under elevated CO<sub>2</sub>: effects on yield, photosynthesis, and metabolism

Heavy metal stress inhibits plant growth, but this impact is less studied and pronounced under climate change conditions. The present study investigates the physiological, biochemical, and agronomic responses of wheat (C3) and maize (C4) exposed to varying thallium (Tl) stress (60 and 120 mg/kg) under ambient (aCO2) and elevated (eCO2,710 µmol/mol) CO2 levels. High Tl exposure markedly reduced grain yield by 58% in wheat and 68% in maize at 120 mg/kgunder aCO2. However, eCO2 partially offset the negative effects, increasing yield by ~20% in wheat and 36% in maize at 60 mg/kg Tl. eCO2 enhanced photosynthetic activity under eCO2, which increased the accumulation of soluble sugars under TI stress. These provide carbon skeletons for the synthesis of primary metabolites such as amino acids, organic acids and fatty acids. Although total fatty acid content declined under stress, the metabolic crosstalk initiated by improved photosynthesis and sugar availability enables plants to maintain key fatty acids (such as palmitic, linolenic, and oleic acids) essential for membrane stability and function. Amino acids, especially proline and cysteine, accumulated significantly under Tl stress. These primary metabolites, in turn, feed into secondary metabolic pathways, promoting the formation of phenolic acids and flavonoids that enhance antioxidant defence and stress tolerance. This metabolic cascade explains eCO2’s capacity to alleviate TI stress and improve crop performance, and underscores the value of leveraging eCO2 environments to support agricultural productivity and food security under challenging conditions.