Drought induces moderate, diverse changes in the odour of grassland species.

Plants react to drought stress with numerous changes including altered emissions of volatile organic compounds (VOC) from leaves, which provide protection against oxidative tissue damage and mediate numerous biotic interactions. Despite the share of grasslands in the terrestrial biosphere, their importance as carbon sinks and their contribution to global biodiversity, little is known about the influence of drought on VOC profiles of grassland species. Using coupled gas chromatography-mass spectrometry, we analysed the odorants emitted by 22 European grassland species exposed to an eight-week-lasting drought treatment (DT; 30% water holding capacity, WHC). We focused on the odorants emitted during the light phase from whole plant shoots in their vegetative stage. Emission rates were standardised to the dry weight of each shoot. Well-watered (WW) plants (70% WHC) served as control. Drought-induced significant changes included an increase in total emission rates of plant VOC in six and a decrease in three species. Diverging effects on the number of emitted VOC (chemical richness) or on the Shannon diversity of the VOC profiles were detected in 13 species. Biosynthetic pathways-targeted analyses revealed 13 species showing drought-induced higher emission rates of VOC from one, two, three, or four major biosynthetic pathways (lipoxygenase, shikimate, mevalonate and methylerythritol phosphate pathway), while six species exhibited reduced emission rates from one or two of these pathways. Similarity trees of odorant profiles and their drought-induced changes based on a biosynthetically informed distance metric did not match species phylogeny. However, a phylogenetic signal was detected for the amount of terpenoids released by the studied species under WW and DT conditions. A comparative analysis of emission rates of single compounds released by WW and DT plants revealed significant VOC profile dissimilarities in four species only. The moderate drought-induced changes in the odorant emissions of grassland species are discussed with respect to their impact on trophic interactions across the food web. (294 words).

An unusual recruitment strategy in a mass-recruiting stingless bee, Partamona orizabaensis.

Foragers of several stingless bee species deposit attractive scent marks on solid substrates to precisely recruit nestmates to food. Interestingly, Partamona workers quickly recruit large numbers of nest mates to resources, likely even without the deposition of attractive scent marks. However, systematic studies of the recruitment system of these bees are lacking. We now studied the recruitment behavior of P. orizabaensis. Our findings show that foragers of this species can recruit large numbers of nestmates to food sources at a particular location. The precise nestmate recruitment does not rely on attractive scent marks deposited on substrates. We never observed any scent marking behavior and feeders baited with labial or mandibular gland extracts were not attractive for the bees. Chemical analyses showed that the foragers' labial gland secretions exclusively contain long chain hydrocarbons, which render their role in recruitment communication unlikely. Whether mandibular gland secretions, which contain esters and alcohols that are known as attractive pheromones in other bee species, are used to guide recruits toward food during flight, remains elusive. We conclude that Partamona's quick recruitment system that does not rely on conspicuous scent marks has evolved as a strategy against competition with sympatrically occurring and more aggressive bee species.