Comparing two methods for quantifying cold tolerance in Mimulus

An important question in evolutionary ecology is how biotic and abiotic stressors drive divergent natural selection leading to speciation. Temperature is a key abiotic driver of local adaptation, population differentiation, and natural selection. Elevation gradients can produce many microclimates that can allow species to adapt to different environments over small or large geographic areas. One particularly strong environmental factor that varies along such gradients is temperature, which plays a key role in determining range limits of many plant species. There are three techniques currently used to quantify cold tolerance in plants including photosynthetic activity, whole-plant tissue assessment, and electrolyte leakage assays. This study compares the freeze tolerance abilities of two species of yellow monkeyflower, Mimulus guttatus and M. tilingii. The goal of this work was to determine which of these assays was optimal for measurement of freeze tolerance in the Mimulus system. The electrolyte leakage assay was more reproducible and reliable than the whole-plant visual assessment of tissue damage. Variation between the species was observed at lower minimum temperatures with the alpines species, M. tilingii, showing greater freeze tolerance, which cold temperatures may play an important role in the species’ divergence and that physiological differences between the species contribute to overall freeze tolerance capabilities