Differences in germination response to temperature, salinity, and water potential among Prosopis laevigata populations are guided by the tolerance-exploitation trade-off.

Abstract

Exploring the mechanisms that promote population divergence represents a central point in evolutionary ecology. Along their distribution, species commonly experience contrasting environmental conditions, which impose local selection pressures to which populations tend to adapt resulting, for example, in ways to deal with resource shortage or abundance. In this study, we explored if germination performance varies under different levels of temperature, water potential and salinity among populations of Prosopis laevigata, a widespread mesquite species from the arid and semi-arid zones of Mexico. Given the wide distribution of P. laevigata and the sensitivity of the germination phase to environmental conditions, we hypothesized the existence of differential responses during this crucial stage of plant development. Thus, we expected germination in populations of P. laevigata from more arid zones to be less sensitive to high temperatures, salinity, and water deficit. Also, we explored if the exploitation–tolerance trade-off guides the differentiation of P. laevigata populations in germination performance under varying conditions. Germination of all populations was very sensitive to water scarcity but not to variation in temperature or salinity. We identified two major axes of trait covariation in germination physiology: one defined by tolerance of germination to water and saline stress versus germination velocity, and the other defined by the tolerance to extreme temperatures. We found evidence that variation in tolerance among the populations was related with their distribution along environmental gradients. Overall, populations with a higher capacity to germinate under water deficit stress, salinity, and extreme temperatures inhabit warmer and more saline areas. Our study provides strong evidence of existing local adaptations during germination of P. laevigata from across its distribution in the arid lands of Mexico.