Islands provide unique opportunities to study the evolutionary ecology of plant-animal interactions. Because many herbivorous taxa are absent on islands (e.g. mammalian browsers and grazers), it is widely predicted that island plants have lower levels of defense than continental plants, although this has rarely been tested. The endemic Hawaiian prickly poppy (Argemone glauca, Papaveraceae) is an example of a species that appears to contradict the prediction, displaying obvious defense traits, including both chemical (latex) and mechanical (prickles) defenses. However, the evolutionary and functional ecology of latex and prickles in A. glauca has not been previously investigated. Using a combination of pot experiments, expression patterns of latex exudation and prickle density were characterized in order to identify important sources of variation, including both genetic factors (plant ontogeny and population differences among islands) and abiotic factors (light, water availability, and mechanical damage). To investigate whether prickles and latex may be maintained in A. glauca because of eco-physiological roles instead of or in addition to defense, the relationships between prickle density, latex exudation and key physiological parameters (photosynthesis measured using pulse amplitude modulated fluorometry, leaf morphology and growth) were tested.
Results/Conclusions
Prickle density was highly variable among plants, ranging from 0.53 - 13.15 prickles/cm2. Prickle density did not change during ontogeny, but differed significantly among populations. In contrast, latex exudation increased significantly during ontogeny and also varied significantly among populations. Neither latex nor prickles were induced by 50% mechanical defoliation at either the juvenile or mature plant stage. In contrast, expression of these traits were plastic in response to light and water availability. High light was associated with high prickle density and high rates of photosynthesis, but had no effect on latex. Low water availability was associated with high levels of latex, smaller plants, and low rates of photosynthesis. These results suggest significant variation in prickle and latex expression for A. glauca, indicating considerable potential for these traits to respond to natural selection by herbivores or other important environmental conditions. The link between prickles, high light and photosynthesis reveals that prickles may play an important role in the eco-physiology of A. glauca, in addition to or instead of its role in defense against herbivores. Future studies examining these traits under field conditions and exposed to herbivores and resource stresses will shed further light into the evolution of putative defense traits on islands.