Cyanogenesis, herbivory and plant defense in Turnera ulmifolia on Jamaica

Abstract

Field surveys of eight populations of Turnera ulmifolia L., a Jamaican weed exhibiting quantitative genetic variation for cyanogenesis, were undertaken to assess the effectiveness of cyanogenesis as a plant defense. Populations known to be characteristically acyanogenic, cyanogenic or to exhibit within-population variation were surveyed for cyanogenesis, plant size, and the presence and identity of invertebrate plant visitors. A developmental series of 10 leaves from a shoot of each surveyed plant was analyzed, using image analysis techniques, for the type and extent of damage present. We also surveyed two additional plant populations for the presence of plants with eggs or larvae of Euptoieta hegesia Cramer, a Nymphalid butterfly that is potentially the most damaging herbivore of T. ulmifolia, and a paired comparison analysis of cyanogenesis in plants with the herbivore versus plants without the herbivore was conducted. We found that T. ulmifolia are attacked by a reasonably diverse insect fauna, but a relatively small suite of specialist herbivores that are seemingly undeterred by cyanogenesis inflicts most of the leaf damage. Cyanogenesis does appear to play a role in determining the types, numbers, and presence of generalist herbivores found within and between populations. Tissue loss sustained by plants varies both within and between populations; however, the proportion of leaf tissue lost due to herbivory is low, ranging from only 1-9% on average, and does not appear to be correlated with cyanogenesis in a consistent way. An analysis of covariance did, however, reveal that cyanogenesis has a significant effect on both plant height and number of shoots per plant. These results suggest that cyanogenesis might afford protection against herbivory from generalizing herbivores.