## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Protective Roles and Mechanisms of Volatile Terpenoids Under Abiotic Stress
In addition to enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and non-enzymatic antioxidants like glutathione (GSH) and Ascorbic acid (AsA) [123; 124; 125], increasing attention has been given to volatile terpenoids as small-molecule antioxidants that confer protection at both the cellular and systemic levels [126; 127]. Antioxidants function by directly neutralizing ROS or interrupting free radical chain reactions, thereby preventing oxidative damage [128]. Isoprene, a volatile hemiterpenoid, has emerged as a multifunctional antioxidant [129; 130; 131]. Its conjugated double bonds enable direct scavenging of -OH, forming 2-methyltetrols in aqueous solution [132]. Given the high reactivity and cellular toxicity of -OH, isoprene's neutralization capacity significantly reduces membrane lipid peroxidation and limits oxidative injury. Under high light stress, isoprene also quenches singlet oxygen ('O\({}_{2}\)) generated at the thylakoid membrane, thereby maintaining photosynthetic efficiency and preserving membrane integrity [133]. Additionally, isoprene reacts with ozone in the leaf boundary layer, scavenging intercellular ozone, reducing O\({}_{3}\) concentration in mesophyll tissues, and preventing the formation of reactive nitrogen species [134], thereby protecting plant tissues from oxidative damage. Under O\({}_{3}\) stress, the inhibition of isoprene biosynthesis by fosmidomycin leads to marked accumulation of H\({}_{2}\)O\({}_{2}\), elevated lipid peroxidation, and enhanced activity of antioxidant enzymes in plant leaves. Exogenous fumigation with isoprene partially reverses these effects, restoring H\({}_{2}\)O\({}_{2}\) levels and lipid peroxidation to values closer to those observed in non-inhibited control leaves [134]. Furthermore, isoprene-emitting plants show higher concentrations of reduced AsA compared to non-emitting counterparts, indicating a reduced reliance on the enzymatic antioxidant system [135]. Collectively, these findings support the role of isoprene as a key volatile antioxidant that mitigates oxidative burden and contributes to cellular homeostasis under thermal stress. Notably, isoprene oxidation products such as methyl vinyl ketone (MVK) and methacrolein (MACR) [136], known as reactive electrophilic species (RES) [137; 138], can induce antioxidant gene expression and activate systemic signaling pathways mediated by SA, JA, and ET [136; 138]. Thus, isoprene performs a dual function: directly scavenging ROS and indirectly amplifying stress defense via signal transduction. Based on these findings, [PERSON] et al. proposed a 'common biochemical mechanism' hypothesis [139] (Figure 2), suggesting that oxidative damage represents a shared core across diverse abiotic stresses and that ROS-targeted mitigation is a unifying protective strategy. This concept provides a comprehensive framework for understanding oxidative regulation in plant stress biology. In the complex defense system that plants deploy against abiotic stressors, volatile terpenoids beyond isoprene--particularly monoterenes (C\({}_{10}\))--have demonstrated significant antioxidant capacity. These compounds contribute to multi-dimensional protection by scavenging ROS, stabilizing membrane structures, modulating signal transduction pathways, and inducing the expression of antioxidant-related genes. For instance, \(\alpha\)-pinene and \(\beta\)-pinene have been shown to enhance the thermal stability of the photosynthetic electron transport chain under heat stress, exhibiting functions comparable to isoprene in thermotolerance [140; 141]. Additionally, monoterenes can directly react with ozone in the leaf boundary layer [137], thereby reducing O\({}_{3}\) penetration into mesophyll tissues and  
Figure 2: The ‘single biochemical mechanism for multiple physiological stressors’ model. The model shows how oxidative damage resulting from environmental stress occurs (in purple) and how volatile isoprenoids may exert protective effects through antioxidant activity (in green).