Co-variation of chemical and mechanical defenses in lima bean (Phaseolus lunatus L.)

Plants usually express multiple chemical and mechanical defenses simultaneously. The interplay of these defenses is still poorly understood, as predictions range from negative associations such as allocation tradeoffs to positive correlations forming synergistic defense syndromes. Surprisingly, little empirical evidence exists on the co-variation of multiple plant defenses. In the present study, we analyzed different genotypes of lima bean (Phaseolus lunatus L.) for the expression of two direct chemical defenses [cyanogenic potential (constitutive), polyphenol oxidase activity (inducible)], two indirect chemical defenses [volatiles (VOCs) and extrafloral nectar (EFN; both inducible)] and a constitutive mechanical defense (hook-shaped trichomes). While the occurrence of trichomes was positively correlated with cyanogenesis, these traits showed a tradeoff with polyphenol oxidase activity, release of VOCs, and secretion of EFN. Hook-shaped trichomes were abundantly present in four of 14 genotypes investigated, and were found only in one monophyletic group of an AFLP-based tree, thus indicating a single evolutionary origin within the species. Our findings show that different lima bean genotypes express either one of two defense systems: 1) high constitutive defense via cyanogenesis and trichomes or 2) high inducible defense via VOCs, EFN, and PPO activity.