Variable Signaling of Defense Status in Natural Populations of the Wild Radish Raphanus raphanistrum (Brassicaceae)

Glucosinolates (GS) are sulfur-containing compounds that serve as a deterrentto herbivores, though are tolerated by some specialized insects. The correlation of GS with floral traits such as colour and scent might represent a signaling of defense status, a pattern rarely recorded in natural populations. We used Raphanus raphanistrum to discover GS signaling patterns and their effects on plant fitness. We recorded fitness, floral colour, GS, and floral Volatile Organic Compounds (fVOCs) from five natural populations. We then tested correlations among traits at the species level and at the population level. Plant fitness was not correlated with any of the GS and fVOCs analysed. At the species level, flower colour was correlated with GS levels in leaves indicating that there might be visual signaling of defense status, while there was a weak correlation between minor GS and fVOCs. However, at the population level, we found multiple and discordant significant correlations, mainly negative, between GS and fVOCs. Our study indicates that signaling channels used by R. raphanistrum to advertise its chemical defense are complex and variable among populations suggesting that they can be shaped by biotic networks of pollinators and herbivores varying in space.