# Below versus above Ground Plant Sources of
## 1 Introduction
### Abscisic Acid (ABA) and Reactive Oxygen Species (ROS) Signaling during Warming and Water Deficit  
Studies in the Amazon have found high mid-day leaf temperatures up to 42 \({}^{\circ}\)C resulting in large leaf-to-atmosphere water vapor pressure deficits, which drive high leaf transpiration rates and reductions in leaf water potentials [28]. To avoid excessive water loss and potential hydraulic failure, an afternoon reduction in stomatal conductance is often observed, resulting in an afternoon depression of leaf net photosynthesis rates [29; 30] and ecosystem NPP [31; 32].  
One of the earliest processes in plant response to HT and drought stress is the rapid accumulation of the isoprenoid hormone abscisic acid (ABA) stimulating stomatal closure [33] and reactive oxygen species (ROS) that initially function as warning signals that activate defense responses before triggering programmed cell death under excessive ROS accumulation [34]. ABA signaling stimulates stomatal closure leading to reductions in transpiration and net photosynthesis [35], increases in hydraulic conductivities, in part through aquaporin activity [36; 37], and activation of defense gene expression including the antioxidant enzymes catalase, ascorbate peroxidase, glutathione reductase, and superoxide dismutase [38] as well as other ABA-induced abiotic stress resistance genes [39]. A recent study using next generation sequencing technology found that exogenous application of ABA to tomato fruit revealed the crucial role of ABA in flavonoids synthesis and regulation of antioxidant systems [40]. The three major components of the ABA signaling network have been described including an ABA receptor, a negative regulator (type 2C protein phosphatase, PP2C), and a positive regulator (SNF1-related protein kinase 2, SnRK2). Together they constitute a double negative regulatory system [41] which has been shown to modify the expression of 10,388 genes in tomato [40]. ABA signaling is intimately linked to ROS signaling. For example, stomatal closure by ABA is mediated by ROS signaling within guard cells [42] and increasing biochemical, genetic, and cell biological evidence points to the emerging view that ROS function as second messengers in ABA signaling [43].  
Thus, through stomatal regulation and ABA-ROS stress signaling which alters water and carbon fluxes [44; 45], it can be hypothesized that ABA lies at the heart of the Carbon-Water Nexus of plant response to HT and drought stress. A growing body of literature suggests that isoprenoids, including ABA, carotenoids, isoprene, and monoterenes, play important roles in minimizing ROS accumulation in plants through antioxidant mechanisms including the consumption of excess photosynthetic energyduring isoprenoid biosynthesis [46], direct ROS-isoprenoid antioxidant reactions [47; 48], and signaling properties of oxidation products [49]. Therefore, ABA plays a central role in plant thermotolerance by increasing hydraulic conductivities and decreasing stomatal conductance to help replenish plant water reserves and mitigate oxidative stress resulting in enhanced cell membrane integrity and continued function carbon assimilation via photosynthesis. Because stomatal closure under HT and drought stress reduces plant uptake flux of atmospheric CO\({}_{2}\), continued efficient operation of carbon assimilation mechanisms is mediated by enhanced re-assimilation of plant internally produced CO\({}_{2}\) generated by photorespiration, respiration, and various biosynthetic pathways including the isoprenoid and fatty acid pathways [50].  
In contrast, if ROS production overwhelms the scavenging action of the antioxidant system, extensive cellular damage including membrane peroxidation and the reduction of ecosystem net primary productivity (NPP) with a shift from terrestrial sinks to sources of atmospheric CO\({}_{2}\)[21]. Such a shift in tropical forest carbon balance would eliminate a critical ecosystem service and accelerate global warming.