# Below versus above Ground Plant Sources of
## 5 Conclusions  
Under drought and HT stress, the phytohormone ABA has been well documented to induce stomatal closure leading to a reduction in transpiration and net photosynthesis, increase plant hydraulic conductivities, and activates defense gene expression including antioxidant systems. Therefore, ABA lies at the heart of the Carbon-Water-ROS Nexus of plant response to environment extremes and may be a critical plant endogenous factor that integrates hydraulics, carbon and energy metabolism, and defense mechanisms with environmental variables including moisture availability and temperature. Until recently, nearly all plant ABA experimental observations and models of ABA production, stomatal conductance, and gas exchange assumed a root source as the principal source of ABA in plants (Figure 1: Scenario 1). Here we show that the theoretical ABA transport time between the roots and main canopy leaves in the central Amazon is too long (>3 weeks) to account for rapid changes in g\({}_{\text{s}}\) throughout the day (e.g., mid-day suppression of g\({}_{\text{s}}\) associated with high VPD). This is consistent with recent experimental evidence that suggests a leaf source as the principal source of ABA in plants (Figure 1: Scenario 2). As tropical leaf emissions of isoprene and monoterenes derive from recent photosynthesis via the same biochemical pathway as ABA (MEP), the possibility of a direct energetic and carbon link between leaf ABA biosynthesis and photosynthesis exists. This possibility suggests the potential for a positive feedback between leaf warming and enhanced ABA production together with reduced stomatal conductance and transpiration. Moreover, variations in stomatal sensitivities to increases in VPD and decreases in \(\Psi_{1}\) across diverse hydraulic functional traits maybe partially attributed to variations in ABA biosynthesis sensitivities to VPD and \(\Psi_{1}\). Thus, species-specific variations in ABA biosynthesis sensitivities to VPD and \(\Psi_{1}\), may help explain isohydric stomatal behavior in Amazon forest plateaus and anisohydric stomatal behavior in valleys and secondary forests. Given the predictions of increasing mean surface temperatures and frequency and duration of widespread droughts in the tropics, an accurate representation of stomatal conductance behavior in ESMs is critical for predicting future carbon and water fluxes between terrestrial ecosystems and the atmosphere. For example, a reduced stomatal sensitivity to VPD in valley ecosystems relative to plateau ecosystems may potentially buffer overall decreases in ecosystem NPP during HT stress. This knowledge is also essential for predicting which species succumb to future climate warming and drought and which species are resistant and thrive. If a photosynthetic source of ABA is verified, an integrated gas exchange model could be developed linking photosynthesis and ABA production to stomatal conductance. Such an integrated model could be incorporated into modern ESMs to potentially improve predictions of the interactions and feedbacks between terrestrial ecosystems and the atmosphere under a changing climate. **Author Contributions:** [PERSON], [PERSON] and [PERSON] conceived of the study, analyzed the data, generated the graphics, and wrote the manuscript. [PERSON] and [PERSON] collected the field data. [PERSON] and [PERSON] edited the manuscript, [PERSON], [PERSON], and [PERSON] acquired the research funding, assisted with logistics, and edited the paper. **Acknowledgments:** This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research through contract No. DE-AC02-05 CH11231 to LBNL, as part of DOE's Terrestrial Ecosystem Science Program. Additional funding for this research was provided by the Coordination for the Improvement of Higher Education Personal (CAPES) in Brazil.