Defensive chemical production of mustard plant Thlaspi arvense increases in response to reduced water availability in a greenhouse; evidence for local adaptation in subalpine meadows

Background/Question/Methods

With the expected increase in severity and duration of early summer droughts in the Rocky Mountains, USA, understanding how plants respond to water stress is key to predicting broader community responses to climate change. This study investigates the effects of water stress on non-native mustard plant Thlaspi arvense morphology, reproduction, and defensive chemical production. Mustard plants produce glucosinolates, defensive chemicals that can alter plant and microbe functioning. T. arvense is a plant of Eurasian origin that is expanding its range into subalpine meadows in Colorado, where snowmelt time is a major factor in early season water availability. We conducted a greenhouse experiment with seeds from two populations of T. arvense plants at low and high elevation subalpine meadows. We predicted that if there is localized population adaptation, plants from the higher elevation, later snowmelt meadow would show lower tolerance for water stress. Greenhouse plants were either consistently watered or dried down for 10 days during early growth. We measured morphological traits of all plants at rosette, flowering, and harvest, and used high performance liquid chromatography to quantify foliar glucosinolate content of a subset of plants. We compared this data to field observations of T. arvense populations across 10 subalpine meadows.

Results/Conclusions

In the greenhouse, we observed a significant increase in foliar glucosinolate concentration of both populations in response to reduced water availability (p< 0.01), suggesting that glucosinolate production increases under water stress. Additionally, our analysis showed significant differences in other plant performance and reproductive capacity metrics between populations, between treatments, and in strength and direction of population responses to water stress. The results suggest that plants sourced from the lower elevation site were less affected by water stress than plants sourced from the higher elevation site, potentially showing local adaptation to the prolonged water stress that occurs at lower elevations. We conclude that retaining the potential to produce higher quantities of defensive chemicals in response to environmental stressors may aid this species’ competitive ability and likelihood of continued range expansion. Phenotypic selection analysis of field observations is ongoing, and will link these results to selection pressures specific to the subalpine meadow environment.