## 2 Materials and Methods
### X-ray Absorption Near-Edge Structure (XANES)  
XANES measurements have also previously been described in detail [15; 16]. Briefly, measurements were performed in the Soft X-ray Spectroscopy beamline (SXS) of the Brazilian Synchrotron Laboratory (LNLS Brazil) [21; 22]. The beam energy was tuned within a selected photon energy range, encompassing the sulfur K-edge. A Si (111) double crystal with an energy bandwidth of 0.5 eV was used in the monochromatization of the incident beam. In this beamline, XANES measurements were performed in total electron yield (TEY) mode using an electrometer (Keithley, Cleveland, OH, USA, Model 6514) and in fluorescence yield (FL) mode using a silicon drift detector (SuperFast SDD, AMPTEK, Bedford, MA, USA, Model XR-100 SDD). Energy calibration was carried out by measuring the XANES spectrum of an ultrapure molybdenum foil in the TEY mode and setting the zero of the second derivative of the spectra to 2520 eV [21].  
Spectra around the sulfur K-edge were obtained from 2400 to 2600 eV in the photon energy range, with the following: increments of 2 eV in the ranges between 2400-2440 eV and 2560-2600 eV; steps of 1 eV in the ranges between 2440-2465 eV and 2500-2560 eV; and steps of 0.2 eV in the 2465-2500 eV energy range. The latter interval covers the main absorption near edge for the sulfur K-edge (\(\approx\)2470 eV). The first normalization of the spectra was carried out with respect to the beam intensity by measuring it using a gold mesh placed before the sample. The second normalization, a step height normalization, was performed using the procedure available in the ATHENA software package (Demeter_0.9.24) [23]; the wide photon energy range of the pre-edge and post-edge regions allows for a precise step height normalization. Spectra of the plant samples were measured in the fluorescence mode, since due to the sulfur dilution in these samples, self-absorption issues were not expected.  
A collection of standards was previously measured in order to obtain a library of spectra of sulfur functional groups with the aim of carrying out sulfur chemical speciation in different plant tissues [15; 16]. The set of standards is composed of thiols (cysteine, reduced glutathione), thioether (methionine), disulfide (cystine, oxidized glutathione), sulfoxide (dimethyl sulfoxide (DMSO)), sulfone (L-methionine sulfone), sulfonic acid (cysteine acid), organic sulfate (chondroitin sulfate), and inorganic sulfate (zinc sulfate). Solid standards were purchased from Sigma Aldrich (Burlington, MA, USA), and they were milled and measured in the TEY mode in order to avoid the distortion of the spectra caused by self-absorption. For DMSO, a solution with a 0.4% mass of sulfur was prepared, and the spectrum was obtained in fluorescence mode. A special Teflon sample holder, designed for liquid samples and covered with a 4 \(\upmu\)m X-ray film (3525 Ultralene-SPEX(r)SamplePrep), was used.  
To check for reproducibility and stability, a cysteine spectrum was measured several times during the course of spectra acquisition for the plant samples and standards. Cysteine was chosen because it has been proven that its spectra remain unchanged following repeated irradiation [24]. The white line in the cysteine NEXAFS spectrum was observed at 2471.3 eV, whereas its second derivative determination was observed at 2470 eV.