Contrasting latitudinal patterns in plant defence in Oenothera biennis (Onagraceae)
The observation that herbivory and plant defenses increase towards the equator has long been viewed as a well-supported pattern called the Latitudinal Herbivory Defense Hypothesis (LHDH). Recent studies call this hypothesis into question, which suggests alternative hypotheses might explain biogeographic patterns of herbivory and plant defense in nature. Here we test LHDH by growing Oenothera biennis plants from 137 populations in a common garden to examine if genetically based latitudinal gradients are present in life history traits (flowering time and growth rate), physiological traits (SLA, water content), physical resistance traits (trichomes, leaf toughness), and plant susceptibility to four herbivores. We also examined latitudinal patterns in total phenolics, oxidative capacity, and concentrations of the three most common ellagitannins in O. biennis, Oenothein A, Oenothein B and oxidized Oenothein A. The variety of compounds measured, and number of population replicates make this one of the largest and more rigours tests of latitudinal gradients in plant defences within a single species.
Key plant defence traits, which are related to decreased herbivory, show genetically based latitudinal clines. Specifically, fruit total phenolics and oxidative capacity are higher at lower latitudes, providing support for LHDH. However, other tissues show no latitudinal patterns or even higher herbivory at higher latitudes. The conflicting patterns may be explained by a trade-off between Oenothein A and B. Across all leaf, fruit, and flower tissues, Oenothein A is higher at lower latitudes and correlates negatively with herbivory, while Oenothein B is higher at higher latitudes and positively correlates with herbivory. Flowering date was also an important predictor for herbivory and showed a latitudinal cline. Physical and physiological traits showed only weak patterns that were not significant after multiple comparison corrections. Oenothera biennis genotypes from higher latitudes were also more susceptible to all herbivores. Together these pattern suggest that higher herbivory observed in colder climates may be due to lower levels of plant defences in these regions. Furthermore, a lack of consistent latitudinal patterns in chemical defences may be due to the utilization of broad measures of plant defence, such as “total phenolics”. These broad measures might often not capture the patterns of compounds that are actually involved in defence.