## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Light  
Light intensity, a critical environmental factor influencing plant growth and metabolism, exerts significant regulatory effects on VT emissions, especially the light-dependent release of isoprene and monoterepenes. Studies demonstrate that emission rates of isoprene and monoterepenes exhibit a light-dependent increase with rising photosynthetically active radiation (PAR) until reaching species-specific saturation thresholds, where the light saturation points (LSPs) for isoprene and monoterepenes serve as key determinants of characterizing this photoresponse [188]. Specifically, the emission rate of isoprene increases gradually between 0 and the saturation point of PAR, as exemplified by _Loropetalum chinese_ and _Nandina domestica_, which reach maximum isoprene emission rates of 3279.21 and 7355.17 pmol m\({}^{-2}\) s\({}^{-1}\) at 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) PAR [189]. Similarly, _Mangifera indica_ shows an increase in isoprene emissions from 6 to 16 nmol m\({}^{-2}\) s\({}^{-1}\) as PAR rises from 500 to 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)[190], highlighting isoprene high photosensitivity. Monoterpene emissions demonstrate stronger PAR responsiveness than isoprene, with greater variability in light saturation thresholds and unstable post-LSP emission patterns [188]. For example, European beech (_Fagus sylatica_) shows minimal monoterepne emission fluctuations under varying PAR, while early- and late-flowering Norway spruce (_Picea abies_) achieve monoterepne emission saturation at 500 and 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) PAR, respectively [191], highlight ing interspecies divergence in photoresponsiveness. [PERSON] et al. [192] classified BVOC emissions into light-dependent (LD) and non-light-dependent (NLD) types based on enzymatic versus storage-pool dynamics in Aleppo pine (_Pinus halepensis_). LD emissions (e.g., (E)-\(\beta\)-ocimene and linalool) follow enzymatic kinetics, while NLD emissions (e.g., \(\alpha\)-pinene and myrcene) originate from storage pools, showing temperature-dependent exponential increases without light regulation. Conversely, all rosmary (_Rosmarinus officinalis_) VOCs, including oxygenated derivatives, originate from storage pools, suggesting anatomical and physiological constraints on emission mechanisms.  
Light perception through photoreceptors (phytochromes, cryptochromes, and UVR8) enables spectral quality-dependent regulation of terpenoid biosynthesis [193]. Illumination intensifies photosynthetic activity in plants and concurrently influences the pool size of DMADP, a precursor for isoprene biosynthesis in leaves, thereby increasing the rate of isoprene production [194; 195]. Certainly, Ca\({}^{2+}\) signaling contributes to the biosynthesis and emission of monoterpenes regulated by light intensity. The increase in light intensity initially triggers Ca\({}^{2+}\) influx into the cytoplasm, followed by the activation of downstream monoterpene synthase gene expression, which in turn governs the biosynthesis and emission of monoterpenes [196]. In strawberry, red light upregulates _FaNES1_ expression, enhancing trans-nerolidol synthesis, while blue light reduces terpenoid accumulation, and green/violet light suppresses _FaNES1_ activity and emission rates [197]. High light/heat stress shifts BVOC profiles: oxygenated monoterpenes and sesquiterpenenes increase, while monoterpene hydrocarbon emissions decline, concomitant with photosynthetic inhibition (reduced electron transport rates, ETRs) and precursor limitation [198]. [PERSON] et al. [199] demonstrated that monoterpenes and other BVOCs played a dominant role in stress resistance enhancement in _Hybrid poplars_ under severe photoinhibitory conditions, whereas isoprene exhibited diminished protective efficacy under such metabolic bottleneck.