Real-time AA and MeOH emission measurements
Experimental details of the leaf, branch, and ecosystem gas exchange methods to determine AA and MeOH emissions, as well as the detached stem, detached leaf, and hydrated AIR temperature response curves can be found in the supplementary methods. Briefly, emission rates of MeOH and AA were quantified in real-time (roughly 2.5 measurements per minute) using a high sensitivity quadrupole proton transfer reaction mass spectrometer (PTR-MS, Ionicon, Innsbruck Austria, with a QMZ 422 quadrupole, Balzers, Switzerland). The PTR-MS was regularly calibrated to a primary standard by dynamic dilution (Supplementary Figure S1 ). AA and MeOH emissions were determined using PTR-MS at the leaf level using an environmentally controlled leaf photosynthesis system (Model 6800, Licor Biosciences, USA), branch level using a custom 5.0 L transparent Tedlar gas exchange enclosure with artificial lighting, and from a temperature-controlled chamber used for detached leaf, stem, and hydrated AIR AA and MeOH emission studies (Model 150 Dynacalibrator, +/- 0.01 C temperature accuracy, Vici Metronics, USA). Together with air temperature, continuous above canopy ambient AA and MeOH concentrations during the growing season were made at a poplar plantation in Belgium (Portillo-Estrada et al., 2018), a mixed hardwood forest in Alabama (Suet al., 2016), and above a citrus grove in California (Parket al., 2013). Vertical ecosystem fluxes of MeOH and AA were estimated at the Belgium field site using the technique of eddy covariance employing high frequency vertical wind and MeOH and AA concentration measurements (Portillo-Estradaet al., 2018). While ecosystem concentration and flux measurements MeOH were collected at all three sites using eddy covariance with PTR-TOF-MS, only the Belgium poplar plantation reported ecosystem scale AA flux data. At the Alabama mixed forest site, AA fluxes were not reported (Suet al., 2016) and at the citrus grove in California, AA fluxes were reported to suffer from inlet dampening of high frequency concentration variations (Parket al., 2013). This loss of AA flux signal was explained by dampening of fast fluctuations in the sample tube due to stickiness of AA with respect to the inert tubing walls. Therefore, at the Alabama and California sites, diurnal ambient concentrations of MeOH and AA were analyzed instead of fluxes as a function of air temperature.
Long-distance13C2-acetatetransport in the transpiration stream and leaf cell wall O-acetylation interactions
In order to evaluate the possibility of long-distance metabolic interactions between plant tissues mediated by acetate in the transpiration stream, including influencing O -acetylation dynamics of cell walls, 13C2-acetate labeling studies were carried out on individual P. trichocarpatrees transferred from the greenhouse to the laboratory.13C2-acetate delivery to leaves was accomplished using detached branches (N = 3 branches, 1 branch/individual) placed in a 10 mM solution of sodium13C2-acetate (Sigma-Aldrich, USA) for 2 days inside an environmentally controlled growth chamber (Percival Intellus Control System, USA) maintained at 27.5 °C daytime temperature (6:00–20:00; 30% light) and 23 °C nighttime temperature (20:00-6:00). After 2 days, the branches took up roughly 30-40 ml of the13C2-acetate -acetate solution. In addition, a single individual of 2.1 m height was placed in the laboratory under automated daytime lighting with continuous daytime (150 µl min-1) and nighttime (70 µl min-1) xylem injection at the base of the stem with a 10 mM sodium 13C2-acetate acetate solution (1,176 ml injected over 7 days using a flow controlled M6 Pump, Valco Instruments Co. Inc., USA). Following the13C2-acetate -acetate labeling period (branch: 2 day, tree: 7 day), a mature leaf was removed and flash frozen under liquid nitrogen and stored at -80 °C before isolating whole leaf cell walls through the generation of AIR. Leaf AIR samples were also prepared from detached branches fed with water and 10 mM acetate with natural 13C/12C abundance as controls. Experimental details of the AIR saponification followed by13C-labeling analysis of the released acetate can be found in the supplementary methods.