Temporal and spatial variation in chemical composition of susceptible and resistant alternate hosts of Cronartium pini, Melampyrum sylvaticum and M. pratense

Cronartium pini infects ca. 60 alternate host species among which Melampyrum sylvaticum is a susceptible and M. pratense a resistant species. Variation in rust resistance among these species may be connected to natural or induced leaf chemicals. In this study, we investigated the chemical variation of 11 compounds between two C. pini-resistant and -susceptible Melampyrum species in leaves of different age, temporally within growing season and spatially among different geographic locations. LC–MS (liquid chromatography mass spectrometry) was used to quantify the compounds. Concentrations of chlorogenic acid, syringic acid, hyperoside and quercitrin were significantly higher in M. pratense, while those of p-coumaric acid, rutin, ferulic acid and luteolin were significantly higher in M. sylvaticum. Temporal variation occurred in concentrations of the compounds. Old leaves of both Melampyrum species contained mostly higher concentrations than young leaves. Spatial variation occurred for individual compounds in both Melampyrum species. In conclusion, age of leaves, time of collection during growing season and geographic location affect concentrations of chemical compounds in leaves of both Melampyrum species, which should be taken into consideration when exploring the potential of plant chemicals in rust resistance.