Inter-population and temporal variation of VOC emission from Mediterranean Helichrysum Mill. (Asteraceae)

Mediterranean Helichrysum species are highly diverse with respect to both phenotype and metabolite profile [1]. Volatile organic compounds (VOCs) were recently recognized as valuable tools for studying Helichrysum systematics at population level [2]. In seeking carry on the studies on the phytochemical typification for taxonomic purposes, we examined the diversity and temporal variation of volatile profiles in eight selected Mediterranean populations, included within the H. italicum complex, H. litoreum and H. stoechas. The goals of this study are: (i) to evaluate and compare the differences of volatile profiles across populations and collection time, and (ii) to characterize the intra-population variation over time. After collection in the wild, plants were cultivated in standard growing conditions at the Florence Botanical Garden, to remove the effects due to the different ecological conditions of the collection sites. The emissions of VOCs from annual leafy shoots were analysed in May, July and September 2012, by means of headspace solid phase microextraction coupled with gas-chromatography and mass spectrometry (HS-SPME-GC/MS). The VOC analysis revealed the production of overall 417 compounds, with terpenes being the dominant compound class, followed by esters and alcohols. Statistical data processing, based on a Permutational Multivariate Analysis of Variance and on a subsequent Variance Partitioning Analysis on the relative quantitative data of VOCs, showed a great variability in the volatile profiles across populations (69%, p<0.001), with a weak contribution due to time (5%, p<0.001), whereas the additional variance explained by the interaction term between these two factors is 19% (p<0.001). The volatile profiles of each population were consistent over time, as indicated by the high correlation coefficients (ca 65%), with the exception of only one provenance. In conclusion, VOC profiles as a whole varied considerably from one population to another, and over time. Conversely, the low diversity of time-to-time emission pattern within each population suggests that the volatile profiles exhibit phytochemical consistency, presumably due to the standardization of the plant growing conditions. These data further enhance the potentiality of VOCs as diacritical characters in discriminating among populations and in solving taxonomic problems.