Shifting global climate patterns will put selective pressure on many plant species to survive more severe and variable precipitation regimes. Plants can survive drought by either growing and reproducing quickly while resources are abundant (escape) or more slowly and conservatively (tolerance). However, fitness tradeoffs suggest that plants must invest in a single strategy or increase their plasticity to variable environments. Previous work has demonstrated these tradeoffs among species and across populations, but few, if any, studies have examined them within a population where selection occurs. Further, most studies use coarse, whole-organism traits to measure fitness and drought response, so we do not fully understand the physiological mechanisms underlying drought response. I hypothesized that measuring an array of physiological and whole-organism traits in a drought experiment would reveal intra-population diversity that predicts drought survival. I grew seeds from 8 maternal families in a population of Silenelatifolia, an invasive grassland species in North America from Eurasia, and identified variation in leaf conductance, photosynthesis, stomatal pore area, growth, biomass, leaf carbon:nitrogen, and flowering in well-watered and drought-stressed plants in a 10-week growth chamber experiment.
Results/Conclusions
I found variation between maternal families in the population and co-variation between physiological and whole-organism traits, demonstrating that leaf water use and photosynthetic rate influence growth and flowering rates under drought. Evidence also emerged for tradeoffs between drought coping strategy and plasticity, with stronger escape traits: maternal families with high stomatal conductance and early flowering were less plastic and more sensitive to drought stress. Similarly, slow-growing and late-flowering plants grew more consistently during drought. These results highlight the importance of examining genetic variation and plasticity at the population level to understand how environmental change will influence the direction of plant evolution.
