The Mexican Salvia blepharophylla and S. greggii in an ecological context : scents, biotic interactions and exudates

Strategies adopted by plants to interact with other organisms are very sophisticated and the study of these relationships are full of charm for researchers and extremely important to understand evolutionary perspectives. Our work arises in this framework. We focused our attention on two Mexican Salvia species (Lamiaceae) preserved at the Ghirardi Botanic Garden of the University of Milan: Salvia blepharophylla Brandegee ex Epling and Salvia greggii A. Gray. Both species show nice red flowers, of the kind usually pollinated by hummingbirds. We explored the phytochemical characterization related to the productivity in volatile organic compounds (VOCs) from both flowers and leaves, and the epicuticular depositions, in concert with the biotic interactions established by the target species. We evaluated the biodiversity of insect pollinators visiting the flowers and of the microorganisms inhabiting the phyllosphere. Moreover, we performed a micro-morphological investigation on the glandular trichomes. Our aim is giving a first attempt to sketch a link of the plant productivity in secondary metabolites to observed biotic relationships concurrently occurring at the botanic garden. Phytochemical survey revealed that sesquiterpene hydrocarbons were the most abundant chemical class of volatiles in the leaf of both species (42.34% in S. greggii and 54.97% in S. blepharophylla), while monoterpenes dominated the flower emission profiles. As a whole, different emission profiles characterized the two species thanks to exclusive compounds: the floral bouquets are dominated by limonene and β-pinene in S. greggii, and by 1,8-cineole in S. blepharophylla. On the other hand, common principal compounds occurring in high relative percentages in flowers are β-caryophyllene and germacrene D, both also detected in the leaf emission profile. Slight differences were found for the composition of the secondary metabolites present in the leaf exudates, with isopimaric acid and a compound structurally related to the cloredane diterpenes blepharolide A and B prevailing in S. greggii and S. blepharophylla, respectively. Biological surveys were equally interesting. Although the relatively high biodiversity of pollinators recorded at the botanic garden, the target species attracted only two functional pollinator groups: small (Lasioglossum morphotypes) and large bees (Xylocopa violacea L.). These insects applied different but interesting handling strategies to reach flower resources: the described strategies are significant from an evolutionary point of view, considering the probable link of the Mexican Salvia with bird pollinators. The microbiological analysis revealed that S. greggii harbours a more abundant and diverse phyllospheric bacterial community than S. blepharophylla did, although a considerable variability between the replicates was observed within each species. Our data can be linked to existent literature concerning chemical compounds and biotic responses. With regards to VOCs, an attractive role towards bees is documented only for limonene, β-pinene and 1,8-cineole [1-3]. Conversely, β-caryophyllene and germacrene D are common deterrents [4]. Our data, as existent literature [5], sustain the hypothesis that local bees do adapt to novel food sources. The chemical compounds we found may moreover allow to infer that bees are able to recognise the chemical signals emitted by the target-species and possibly use them to locate the plants. In this view, also chemicals produced by leaves may be used by bees: more data are needed to investigate this potential role of leaf chemistry. Compounds in leaves are so far known to exert defensive functions [6]. Finally, our is the first report on the microflora inhabiting the phyllosphere of the two Salvia species and represents an initial step towards the understanding of the adaptive mechanisms between microorganisms and leaf exudates.