Cross-correlations of Biogenic Volatile Organic Compounds (BVOC) emissions typify different phenological stages and stressful events in a Mediterranean Sorghum plantation

Climate change will affect the growing season and increase the occurrence of extreme stressful events, thus altering crop phenological phases and the associated emission of biogenic volatile organic compounds (BVOC). BVOC exchange has been poorly investigated in field crops, especially in the Mediterranean area. In this study we report continuous measurements of BVOC fluxes and CO₂ net ecosystem exchange (NEE), together with environmental variables, green area index (GAI) and aboveground biomass (AGB) during a whole growing season in a grain sorghum (Sorghum bicolor x Sorghum sudangrass., cv. Nicol, Pioneer) plantation located in Southern Europe. Results of this intensive field campaign showed that, while the bare soil of our site was a sink of BVOC, the sorghum plantation became a source of oxygenated BVOC, mainly methanol and acetaldehyde, which were emitted over the season at an average rate of 0.137 ± 0.013 and 0.070 ± 0.004 nmol m⁻² s⁻¹, respectively. In addition, the application of the advanced data mining method of Self-Organizing Maps (SOM) revealed distinctive patterns of BVOC fluxes correlating with sorghum growth stages (GS): in the first stage (GS1), developing plantlets emitted a mixture of BVOC uniquely characterized by monoterpenes; in GS2, adult plants forming an homogeneous dense canopy emitted the most abundant fluxes of a mixture of oxygenated BVOC comprising methanol, acetaldehyde, formic acid, acetone, acetic acid and n-pentenol; once plants entered the flowering stage (in GS3), only a few BVOC continued to be emitted at the highest rates (i.e. formic acid, acetone, acetic acid, n-pentenol). Moreover, the application of SOM to a sub-set of BVOC fluxes highlighted the possibility to qualitatively differentiate stressful events of plant lodging and harvest cutting. In fact, enhanced emission of acetaldehyde distinguished the BVOC mixture emitted from lodged rather than from cut and harvested sorghum plants in the field.