Seed Thermal Response, and Morphological and Biochemical Plasticity in Wigandia urens as Indicators of Its Utility for Elevational and Latitudinal Assisted Migration

Abstract

Thermal time models are useful for predicting the effects of global climate change on plants and enable us to know the phenotypic plasticity of germination responses to temperature. In five populations of Wigandia urens collected in Mexico at elevations of 1260, 1660, 2040, 2345, and 2500 m above sea level (a.s.l.), we determined soil temperatures, seed traits, cardinal temperatures, thermal time (T _t), and fatty acid content. In seeds collected at 2345 m, we also assessed the effect of osmotic potential on germination. Temperature in the elevational gradient differed by 10.7°C ± 1.37°C, variation higher than the increase in (6°C) predicted by CC models for the next century. Temperature was negatively related to elevation, positively related to germination capacity, and negatively related to lag time, seed mass, and lipid content. All elevational seed populations exhibited high germination capacity and a wide thermal germination window. Germination was thermophilic. T _base for germination was 9.81°C-12.48°C and T _ceiling ≈ 34.74°C. Variation in T _t was broader than that in T _base and T _ceiling. Elevation and/or percentile subpopulations significantly affected T _base and T _t, while elevation affected T _ceiling. For suboptimal and supraoptimal T _t, the lowest values were at 1660 m and the highest at 2345 and 2500 m. Suboptimal T _t exhibited greater variation than the other parameters, probably determining the W. urens distribution. This species germinated from 0 to -0.5 MPa in 2.4 days, the time required to reach a cumulative soil temperature equivalent to T _t for the 50% percentile. Even 32% germinated at -1 MPa. Unsaturated fatty acids content was highest at 2500 m. The phenotypic plasticity of W. urens, expressed in its germination and ecological distribution, suggests that it is a model pioneer tree useful as a facilitator species capable of facing temperature and precipitation predicted by climatic change and beneficial for ecological restoration and/or assisted migration across elevational or latitudinal gradients.

Keywords: base and ceiling temperatures; cumulative soil temperature; elevational gradient; maternal effects; stress temperature memory; thermal time model.