Chemical analyses
We performed gas chromatography to quantify the sampled VOCs. We sampled
the volatile compounds of the two plant species by passive trapping with
polydimethylsiloxane (PDMS) tube cuttings following a modified protocol
of that used by (Kallenbach et al. 2015) and (Klimm et al.2020). Using the PDMS tubes, we sampled volatile compounds from ca. half
of the saplings. First, we attached the tubes by copper wire to the
experimental sapling. We then tightly enclosed as many leaves as
possible and the PDMS tube in polyamide oven bags. We sampled VOCs from
the headspace for 24 hours. We removed the tubes, placed them to sterile
glassy vials and brought them to lab for further analyses. The PDMS
cuttings were analysed by a thermal desorption-gas chromatograph-mass
spectrometer (TD-GC-MS) consisting of a thermodesorption unit (MARKES,
Unity 2, Llantrisant, United Kingdom) equipped with an autosampler
(MARKES, Ultra 50/50). PDMS cuttings were transferred to empty stainless
steel tubes (MARKES) and desorbed with helium as carrier gas and a flow
path temperature of 160°C using the following conditions: dry purge 5
min at 20 ml/min, pre purge 1 min at 10 ml/min, desorption 8 min at
200°C with 60 ml/min, trap temperature 0°C, pre trap fire purge 1 min at
60 ml/min, split flow 20ml/min, trap heated to 230°C and hold for 4 min.
The VOCs were separated on a gas chromatograph (Bruker, GC-456, Bremen,
Germany) connected to a triple-quad mass spectrometer (Bruker, SCION)
equipped with DB-WAX column: (30m x 0.25mm inner diameter x 0.25um film
thickness, Restek). The temperature program was the following: 60°C
(hold 1 min), 30°C/min to 150 °C, 10°C/min to 200°C and 30°C/min to
230°C (hold 1 min). Helium was used as carrier gas at a constant flow
rate of 1 ml/mi. MS conditions were set at a 40°C manifold, 240°C
transfer line and 220°C for the ion source. The scan-range was 33 –500
m/z for a full scan and scan-time was 250 ms. We selected the most
prominent peaks in the chromatograms (signal to noise ratio
> 10). Peaks that were also present in air blanks were
regarded as systemic contamination and were excluded from further
analysis. VOCs were tentatively identified by comparison to the NIST
database and comparison to retention indices from the literature. The
peak areas of these compounds were calculated using the Bruker
Workstation software (v8.0.1).