Timing of floral volatiles emission in two sympatric Schiedea species (Caryophyllaceae) coincides with foraging activity of a shared crepuscular pollinator

Floral scent is a source of information for crepuscular and nocturnal pollinators that rely on volatile plumes for long-distance and/or local navigation to floral rewards. In addition, plants that signal to pollinators only during periods of pollinator activity should enjoy increased fitness due to reductions in metabolic costs or by avoiding attraction of herbivores. Studies of floral volatile emissions at fine time scales can determine which volatiles are upregulated during narrow periods of pollinator activity, versus those synthesized or released constantly. We asked how the floral volatile profiles of two sympatric, moth-pollinated species of Hawaiian Schiedea (Caryophyllaceae), S. kaalae and S. hookeri, vary throughout the day and night, and whether the emission of volatiles matches the period of foraging activity of Pseudoschrankia brevipalpis (Erebidae). This endemic microlepidopteran is the sole pollinator of S. kaalae and also pollinates S. hookeri. Inflorescence volatiles were collected from greenhouse-grown plants of each species by either dynamic headspace sampling during day or night followed by thermal desorption GC-MS, or by measuring real-time fluxes by proton transfer reaction mass spectrometry over 2-4 d in an environmental chamber. Differences in diurnal and species profiles were examined through linear discriminant analysis and PERMANOVA. The two Schiedea species produced qualitatively distinct floral volatiles profiles during both night and day (based on n = 32 individuals from 2-3 populations of each species), although they also shared one major compound, 1-octen-3-ol. Schiedea kaalae (n = 5 individuals) emitted pyranoid linalool oxide, a related ketone, and benzaldehyde starting 2 h before and peaking 2 h after dark, phenylacetaldehyde starting immediately after dark, cis-3-hexen-1-ol constantly, and alpha-phellandrene only during illumination. Schiedea hookeri (n = 3 individuals) displayed elevated emissions of benzaldehyde and an unidentified benzenoid compound 0-5 h after dark. Benzaldehyde and four compounds produced by S. kaalae in the evening are known to be detected by moths. Changes or constancy in the timing of emissions when photoperiod was altered suggest light-dependent or circadian regulation for some compounds. Overall, each species released a species-specific blend of volatiles that increased before dark and peaked during previously reported times of evening pollinator activity, consistent with temporal regulation of volatiles that has evolved to meet the constraints of efficiently securing pollination. Diurnal emission patterns may suggest functional roles for individual volatiles or for suites of volatiles released in tandem, a useful screen for potential attractants before conducting behavioral assays with pollinators.