Prickles, latex, and tolerance in the endemic Hawaiian prickly poppy (Argemone glauca): variation between populations, across ontogeny, and in response to abiotic factors.

Covariance among plant defense traits is predicted to occur both within and among plant species, potentially leading to characteristic defense syndromes. I examined patterns of variation in prickle density, latex exudation, and tolerance in order to assess whether traits varied between populations, across plant ontogeny, and as phenotypic plasticity in response to water and light limitation and physical damage using the endemic Hawaiian prickly poppy, Argemone glauca, as a model system. Plants produced copious latex, had extremely variable prickle densities, and were generally tolerant of 50% defoliation. However, expression patterns differed among defense traits. Prickle density was consistent across ontogeny and was not induced by either water limitation or mechanical damage, but was significantly induced under high light conditions. In contrast, latex exudation increased significantly across ontogeny and was reduced by water limitation, but had no response to mechanical damage or light. Prickles, latex, and tolerance differed considerably between populations, suggesting different evolutionary histories for these populations. These disparate patterns indicate that latex and prickles are unlinked within A. glauca, potentially as a result of differences in their function, and providing little evidence that they jointly function as a defense syndrome. Moreover, this study provides the first description patterns of variation for multiple defense traits in an island endemic, and high levels of prickles, latex, and tolerance suggest that A. glauca is well defended against herbivores. Future research in the field will provide additional insights into the functional ecology of these traits in A. glauca.