Optimal defence theory and flower petal colour predict variation in the secondary chemistry of wild radish

1 The presence, concentration and composition of plant secondary compounds, which confer plant resistance to herbivores and pathogens, vary greatly both within and among individuals. Optimal defence theory predicts that plant tissues most closely tied to plant fitness should be most defended at the constitutive level, and that more expendable tissues should be inducible with damage. 2 We examined variation in glucosinolate content between leaves and petals, as well as among four petal colour morphs of wild radish, Raphanus sativus. We predicted greater levels of constitutive defences in petals, and greater inducibility of glucosinolates in leaves, based on previous studies that could relate leaves and petals to plant fitness. 3 While, overall, optimal defence predictions were supported, individual glucosinolates differed in both their degree of inducibility as well as in their distribution between tissue types. 4 Petal colour variants differed in their induced responses to damage, but not in their constitutive levels of compounds. Yellow and white morphs, which are preferred by the dominant bee pollinators as well as by herbivores, were generally less inducible than anthocynanin-containing pink and bronze petal morphs. 5 Pleiotropic effects between petal colour and defence loci, or tight linkage between these loci, may allow pollinators to maintain variation in secondary chemistry, as well as allow herbivores to influence colour morph fitness and prevalence.