Testing predictions of the optimal defense theory in vegetative and reproductive structures of lima bean (Phaseolus lunatus L.)

Background/Question/Methods  

Plants show a broad range of chemical defenses against herbivores. Cyanogenesis, the wound-induced release of hydrogen cyanide from preformed precursors, belongs to the most common chemical defense in the plant kingdom. About 10% of all vascular plants are cyanogenic. According to the optimal defense theory (ODT), within a given plant young leaves and in particular reproductive structures should be the best defended plant parts. These parts are the most relevant for plant fitness and at the same time are under the highest risk of being attacked by herbivores due to their relatively high nutritive value compared to older tissues. Although widely accepted, there is surprisingly little empirical support for the ODT. The quantitative distribution of defensive and nutritive traits among vegetative and reproductive tissues and the variation of these traits with respect to organ age remain elusive in most cases. The goal of this study was to determine variation in cyanogenesis in leaves and reproductive structures, i.e., flowers and fruits on a quantitative basis, and to test whether the assumptions of the ODT are met. Using cyanogenic lima bean (Phaseolus lunatus) as experimental plant we spectrophotometrically measured concentrations of cyanide in leaves, flowers, and fruits of three different age groups.

Results/Conclusions

In both leaves and reproductive structures, we found a strong quantitative variation in cyanide concentration depending on developmental stage. For each type of organ the youngest tissues showed the highest concentrations of cyanide whereas the oldest developmental stages were least cyanogenic. Young leaves (66.40±3.15 µmol HCN g^-1fwt) showed significantly higher amounts of cyanide than intermediate (32.70±0.25) and mature leaves (6.51±3.03 µmol HCN g^-1fwt), while also the difference in cyanide concentration between intermediate and mature leaves was significant. In developing flowers and fruits we found the same ontogenetic patterns of cyanide accumulation as observed in leaves. Small buds showed concentrations of 26.32±3.67; large buds of 9.8±1.25, and fully developed flowers of 6.57±1.68 µmol HCN g^-1fwt. Similarly, in fruits cyanide values decreased from 32.95±7.61 in young fruits to 2.02±0.91 in intermediate fruits, and 0.08±0.10µmol HCN g^-1fwt in mature fruits. Overall, leaves had highest values of cyanide, followed by fruit, while flowers showed the lowest values.

While the assumptions of the ODT are met when referring to the developmental stage of a given organ (leaf, flower, or fruit), among organs predictions of ODT do not hold up as reproductive structures—which are most important for plant fitness—are significantly less defended than leaves.