Isoprene Emission

The emission of isoprene was measured in the field from the same leaves of A. donax used for gas-exchange analysis, under the same environmental settings, but a LiCor Li6400 fitted with a 6 cm² cuvette and LED light unit was used. When monitoring isoprene emission, air from the cuvette with the enclosed leaf passed through a biphasic adsorbent trap containing 30 mg of Tenax and 20 mg of Carboxen (GERSTEL GmbH & Co.KG, Germany). A pump (Elite 5, A.P. Buck, Orlando, FL, United States) was used to pass 2 L of air through each trap at a rate of 200 ml min^-1. Measurements of the concentration of isoprene in the ambient air (blanks) were performed using an empty leaf cuvette before and after each measurement. The traps were then stored at 4°C prior to analysis in the laboratory. Isoprene was first desorbed from traps at high temperature and then measured using a gas chromatograph – mass spectrometer (GC-MS) with an Agilent HP-INNOWAX (30 m × 0.32 mm × 0.15 μm) GC column. A 5977A mass selective detector with electron ionization operating at 70eV was used for analysis. Isoprene was identified by matching the spectrum peak with a library spectral database (NIST 11.L) and through comparison of the retention time and mass spectrum with an isoprene analytical standard (Sigma Aldrich, St. Louis, MO, United States) injected into the GC-MS at different concentrations. The isoprene analytical standard was also used to construct a calibration curve by injecting known concentrations of isoprene into the GC-MS. The data was analyzed using Agilent MassHunter Workstation software (Agilent 7890A, Agilent Technologies, Santa Clara, CA, United States). The concentration of isoprene within the leaf was calculated using the approach of Singsaas et al. (1997).