## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Protective Roles and Mechanisms of Volatile Terpenoids Under Abiotic Stress
Solid lines represent direct reactions, and broken lines represent indirect reactions. Environmental stress (high light, temperature, and ozone exposure) causes oxidative stress (Ox), which results in the production of ROS (for example, hydrogen peroxide, singlet oxygen, and superoxide) and reactive nitrogen species (RNS; for example, nitric oxide, peroxynitrite). These compounds initiate cell signaling directly and also through interactions with the hormonal response network, as well as causing further direct oxidative damage. Different stresses trigger different response pathways. For example, ozone exposure also triggers a response that overlaps with biotic stress responses through the plant hormone network; salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) trigger signal cascades that initiate programmed cell death (PCD), resulting in accelerated senescence via an inappropriate hypersensitive response (HR). Isoprene may act at several different levels to arrest oxidative stress-response processes. (1) Because it is lipophilic, isoprene may physically stabilize hydrophobic interactions in membranes, minimizing lipid peroxidation and reducing oxidative stress and downstream buildup of ROS/RNS. (2) Isoprene may react with ROS/RNS to produce reactive electrophile species (RES) such as metracolein and methylvinylketone (products of isoprene/ozone reaction), which are known to induce antioxidant and other defenses. If the stressor is itself an ROS (for example, ozone), isoprene may react directly with the stressor. (3) Direct antioxidant behavior (scavenging ROS/RNS) also prevents accumulation to damaging levels, thus preventing further oxidative damage. As a consequence, ROS/RNS-activated signal cascades and PCD pathways that normally result in tissue necrosis are prevented. alleviating oxidative injury. Structurally, monoterepenes are composed of two isoprene units and can form acyclic, monocyclic, or bicyclic skeletons, often bearing functional groups such as alcohols, ketones, and aldehydes. This chemical diversity underlies their broad reactivity and biological functionality [142]. Studies in _Q. ilex_ L. [143] and thyme [144] have shown that monoterepenes like linalool, geraniol, and \(\alpha\)-pinene are released in large quantities under drought or heat conditions [145]. In cork [146] oak (_Quercus suber_), fumigation with \(\alpha\)-pinene significantly reduced H\({}_{2}\)O\({}_{2}\) and malondialdehydedehyde (MDA) accumulation, enhanced APX activity, and delayed photosynthetic decline under heat stress. Beyond biochemical antioxidant roles, monoterepenes also participate in the integration of stress signaling. Under cold stress, expression of monoterepene biosynthetic genes in rice is upregulated and co-activated with the MAPK cascade and ethylene signaling pathways, indicating monoterepenes as signaling intermediates [147]. Under drought conditions, monoterepene accumulation strongly correlates with antioxidant enzyme activity, suggesting a dual role as non-enzymatic ROS scavengers and membrane protectants [148]. In wild rose petals, a negative correlation between geraniol levels and H\({}_{2}\)O\({}_{2}\) content has been observed, supporting the idea that monoterepene-rich organs maintain lower oxidative stress, potentially delaying senescence and extending floral longevity [149]. Compared to monoterepenes, sesquiterpenenes (C\({}_{15}\))--such as \(\beta\)-caryophyllene and farnesene--are more hydrophobic and thus more likely to integrate into lipid membranes. This property enhances their capacity to protect membranes from lipid peroxidation, particularly under high-temperature or salinity stress [150; 151; 152]. In addition to directly neutralizing ROS as lipid-soluble radical scavengers, sesquiterpenes have been reported to induce the expression of heat shock proteins (HSPs) and modulate osmoprotectant accumulation, thereby supporting plant tolerance under compound stress environments [153; 139].