# Below versus above Ground Plant Sources of
## 2 Metabolism of Abscisic Acid (ABA)
### Two Scenarios of ABA Biosynthesis with Distinct Environmental Controls
Using real-time observations, we present new theoretical estimates of transport times of ABA between roots and the upper canopy from 4 trees in an undisturbed mature tropical forest in the central Amazon during a 12-day dry season period (5-21 May 2015, Figure 2). The results show that for tree heights of 19.8 to 31.0 m, the mean daily sap velocity ranged between to is 0.4 to 1.4 m/day with transport from roots to canopy between 22 and 49 days. These extremely long transport times make the scenario of fast stomatal regulation through root to canopy transport of root-derived ABA unlikely, but support instead the scenario of a direct leaf source of ABA (Figure 3). Therefore, these two scenarios (not mutually exclusive) describe plant sources of ABA and suggest distinct environmental controls (Figure 3). In the first scenario (Scenario 1), ABA biosynthesis is carried out in the roots and transported to the leaves via the transpiration stream with ABA acting as a whole-plant messenger of low soil water potential and a leaf signal that the plant needs to save water by reducing g\({}_{8}\). In this classic scenario, root ABA biosynthesis is stimulated by a decrease in soil water potential. In the second scenario (Scenario 2), ABA is produced directly in the leaves in response to a number of physiological and environmental variables including leaf water potential, and  
Figure 2: Real-time sap velocities of 4 trees in an undisturbed mature tropical forest in the central Amazon during a 12-day dry season period (5–21 May 2015). Also shown in the table below are calculated transport times required for root-derived ABA to reach the top of the canopy via the transpiration stream. VPD. In addition, it can be hypothesized that, due to the tight connection between the Calvin cycle and the MEP pathway (Figure 1), variables influencing photosynthesis including leaf temperature, photosynthetically active radiation (PAR) and leaf internal concentrations of CO\({}_{2}\) may also affect leaf ABA production. In addition, variables influencing leaf water status including leaf to atmosphere vapor pressure deficit (VPD) and leaf water potential are also expected to influence leaf ABA production. Consistent with the second scenario, in response to high temperature stress, a large number of tropical plants synthesizes a number of secondary defense metabolites via the isoprenoid pathway in chloroplasts (e.g., isoprene and monoterpenes), some of which have sufficient vapor pressures to be directly emitted into the atmosphere at high rates as volatile organic compounds (VOCs). isoprene and monoterpene emissions generally account for 1-2% of net photosynthesis at leaf temperatures below the optimum for photosynthesis, but 10% or higher at temperatures above this optimum. A large fraction of tropical tree species emit isoprene and/or monoterpene to the atmosphere at high rates. Figure 3: Schematic diagram of two scenarios where ABA mediated plant responses to warming and drought are driven primarily by subsurface processes with environmental controls primarily in soil water potential (\(\Psi_{s}\)) (Scenario 1) and above ground processes linked with photosynthesis with potential environmental controls including leaf water potential (\(\Psi_{i}\)), Vapor Pressure Deficit (VPD), leaf temperature (Temp leaf), photosynthetically active radiation (PAR), and leaf internal CO\({}_{2}\) (Ci) (Scenario 2). In Scenario 1, the blue arrow with a star represents root-derived ABA transported to the leaves. In Scenario 2, the green arrow with a star represents leaf-derived ABA transported to the roots. A list of environmental controls influencing ABA biosynthesis are represented underneath the underlined text Root Sourced ABA and Leaf Sourced ABA. \({}^{13}\)CO\({}_{2}\) labeling experiments have shown that these volatile isoprenoid emissions derive from recently assimilated carbon rather than from stored reserves [14; 45; 83].