Data from: Herbivores disrupt clinal variation in plant responses to water limitation

Plasticity in plant traits, including secondary metabolites, is critical
to plant survival and competitiveness under stressful conditions. The
ability of a plant to respond effectively to combined stressors can be
impacted by crosstalk in biochemical pathways, resource availability, and
evolutionary history, but such responses remain underexplored. In
particular, we know little about intraspecific variation in response to
combined stressors or whether such variation is associated with the stress
history of a given population. Here, we investigated the consequences of
combined water and herbivory stress for plant traits, including relative
growth rate, leaf morphology, and various measures of phytochemistry,
using a common garden of Asclepias fascicularis milkweeds. To
examine how plant trait means and plasticities depend on the history of
environmental stress, seeds for the experiment were collected from across
a gradient of aridity in the Great Basin, USA. We then conducted a
factorial experiment crossing water limitation with herbivory. Plants
responded to water limitation alone by increasing the evenness of
UV-absorbent secondary metabolites, and to herbivory alone by increasing
the richness of metabolites. However, plants that experienced combined
water and herbivory stress exhibited similar phytochemical diversity to
well-watered control plants. This lack of plasticity in phytochemical
diversity in plants experiencing combined stressors was associated with a
reduction in relative growth rates. Leaf chemistry means and plasticities
exhibited clinal variation corresponding to seed-source water deficits.
The total concentration of UV-absorbent metabolites decreased with
increasing water availability among seed sources, driven by
higher concentrations of flavonol glycosides, which are hypothesized to
act as antioxidants, among plants from drier sites. Plants sourced from
drier sites exhibited higher plasticity in flavonol glycoside
concentrations in response to water limitation, which increased
phytochemical evenness, but simultaneous herbivory dampened plant
responses to water limitation irrespective of seed source. Synthesis:
These results suggest that climatic history can affect intraspecific
phytochemical plasticity, which may confer tolerance to water limitation,
but that co-occurring herbivory disrupts such patterns. Global change is
increasing the frequency and intensity of stress combinations, such that
understanding intraspecific responses to combined stressors is critical
for predicting the persistence of plant populations.