## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Co\({}_{2}\) Concentration  
At the leaf level, elevated CO\({}_{2}\) concentrations typically suppress isoprene emissions in both controlled environments [177; 178] and open ecosystems [179]. This suppression is primarily attributed to CO\({}_{2}\)-enhanced photosynthesis, which redirects key photosynthetic precursors, such as phosphoenolpyruvate (PEP), toward CO\({}_{2}\) fixation, thereby reducing the availability of DMAPP--the direct precursor of isoprene--for _IspS_-mediated biosynthesis [177]. Notably, [PERSON] et al. [178] reported that isoprene emissions remained suppressed even after plants were re-exposed to ambient CO\({}_{2}\) levels, indicating potential long-term metabolic reprogramming. Further mechanistic studies by [PERSON] et al. [180] on _Populus tremuloides_ clones with varying isoprene sensitivity demonstrated a linear decline in emissions with increasing intracellular CO\({}_{2}\), linking clonal differences to variations in _IspS_ gene expression and enzymatic activity rather than DMAPP substrate availability.  
In _Quercus rubra_ L, isoprene emissions increased with photon flux density (PFD) under low CO\({}_{2}\) conditions (\(\leq\)200 \(\upmu\)mol\(\cdot\)mol\({}^{-1}\)) but sharply declined beyond this threshold, suggesting that CO\({}_{2}\) modulates ATP allocation and carbon metabolism within photosynthetic partitioning [181]. Untargeted metabolomics analyses conducted by [PERSON] et al. [182] on wild-type and RNAi-suppressed _IspS_ poplars under elevated CO\({}_{2}\) revealed metabolic compensation mechanisms. Specifically, the suppression of isoprene biosynthesis was associated with increased antioxidant levels, alterations in lipid composition, and enhanced accumulation of photosynthetic pigments. Notably, this CO\({}_{2}\)-driven metabolic shift contrasts with the heat-induced upregulation of isoprene, underscoring the context-dependent protective functions of isoprene, which likely arise from the integrated regulation of multiple stress-response pathways through combinatorial defense signaling networks.  
Elevated CO\({}_{2}\) concentrations elicit divergent responses in the emission of BVOCs, particularly monoterepenes, across different plant species. [PERSON] et al. [183] reported a 1.8-fold increase in monoterepne emissions from _Quercus ilex_ under elevated CO\({}_{2}\), potentially due to a transient carbon surplus resulting from CO\({}_{2}\)-enhanced biomass productivity, which redirected metabolic flux toward the synthesis of secondary metabolites. Conversely, [PERSON] et al. [184] observed significant reductions in monoterepne emissions from _Q. ilex_ grown at 700 ppm CO\({}_{2}\), which were attributed to suppressed monoterepne synthase enzymatic activity. In contrast, studies on conifers (_Pinus_ spp. and _Picen_ spp.) revealed no significant CO\({}_{2}\)-driven changes in foliar monoterepne concentrations or emissions [185; 186], likely due to the metabolic costs associated with the development and maintenance of resin ducts--specialized secretory structures for monoterepne storage--rather than the biosynthesis of monoterepnes themselves [187]. This variability has been attributed to species-specific carbon allocation strategies between growth (primary metabolism) and defense (secondary metabolism). Despite significant progress, the regulatory mechanisms underlying CO\({}_{2}\)-monoterpene interactions remain incompletely understood. The current literature reveals significant inconsistencies regarding the effects of CO\({}_{2}\) on BVOC emissions in woody plants and crops, with no consensus on the direction of these responses. Critical knowledge gaps remain in understanding the long-term acclimation dynamics and ecological consequences of CO\({}_{2}\)-modulated BVOC fluxes, highlighting the need for integrated molecular-physiological studies to unravel this complexity.