## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Mechanical Damage  
Mechanical damage, as a typical physical stressor, reprograms VT emission profiles through multi-layered signaling networks. This response is mediated by rapid electrochemical signal propagation, which prioritizes VT metabolic activation over photosynthetic adjustments, coupled with oxidative stress and transcriptional cascades that drive dynamic VT synthesis. Isoprene metabolism shows particularly swift responsiveness to mechanical injury. [PERSON] and [PERSON] [208] demonstrated that mechanical damage (cutting, abrasion) triggers immediate declines in isoprene emissions despite stable photosynthetic rates, indicating autonomous damage-sensing systems with heightened mechanosensitivity. Electrophysiological studies confirm that distal leaf tissues perceive damage via membrane potential shifts, suppressing _IspS_ activity through long-distance electrical signaling. Disruption of membrane integrity and the consequent oxidative burst constitute core drivers of VOC dynamics. In mechanically wounded mango (_Mangifera indica_) branches, plasma membrane rupture induces ROS surges, elevating MVK and MACR concentrations by 0.8-0.9 ppbv and 0.3-0.6 ppbv, respectively, through lipid peroxidation [209; 210]. This oxidative cascade is tightly coordinated with jasmonate signaling: mechanical stress activates LOX (lipoxygenase) and HPL (hydroperoxide lyase) genes, synchronously boosting precursor availability for stress-inducible VOC biosynthesis [211]. The spatiotemporal patterns of VT emissions manifest a dual-phase emission strategy encompassing immediate release and inducible biosynthesis. Constitutive (storage-type) VTs undergo rapid release upon mechanical disruption of secretory structures--exemplified by _Ageratina adenophora_ releasing preformed amorpha-4, 7(11)-diene from laticifers via _EuTPS1_-mediated regulation [212], and freshly excised _Eucalyptus sideravylon_ leaves emitting dominant terpenoids (1,8-cineole, \(\alpha\)-pinene, and limonene) from resin ducts [213]. Induced (synthesized de novo) VTs exhibit temporally phased biosynthesis, initiated by systemic signals like systemin that activate terpene synthase genes. Conifers subjected to wounding show progressive increases in xylem-emitted limonene and \(\alpha\)-pinene under both light/dark cycles [214], demonstrating light-independent induction. This mechanical priming effect has agrotechnological relevance: in oolong tea production, the Yaoshiqing process (mechanical rocking) enhances key aroma compounds--linaload and geraniol concentrations increase 1.8-2.3-fold and 1.5-1.7-fold (reaching 38.7 \(\pm\) 2.1 pmol g\({}^{-1}\) h\({}^{-1}\) and 25.3 \(\pm\) 1.8 \(\upmu\)g g\({}^{-1}\) FW, respectively), collectively contributing over 67% of the characteristic floral notes. Mechanistically, wounding triggers rapid activation of ERF-family transcription factors, elevating their binding affinity to terpene synthase promoters (LIS, GES) by 3.2-fold. Concurrently, chromatin remodeling increases enhancer region accessibility (2.5-fold nucleosome displacement) at distal regulatory elements, potentiating transcriptional initiation efficiency of terpenoid biosynthetic genes [215]. Furthermore, mechanically induced VTs facilitate cross-individual defense networks through airborne signaling. For instance, herbivory by _Spodoptera littoralis_ larvae induces a cascade effect in maize (_Zea mays_), where herbivore-induced plant volatiles (HIPVs) enhance terpenoid biosynthesis in both damaged and neighboring plants, strengthening systemic resistance against subsequent attacks [216]. In the field of agro-product processing, the epigenetic regulatory effects of mechanical damage warrant attention. During the shaking process of oolong tea, mechanical stress induces a decrease in DNA methylation levels within the promoter regions of terpene biosynthesis genes, which may sustain the upregulation of VOC production by alleviating gene silencing [215].