Temperature-stress resistance and tolerance along a latitudinal cline in North American Arabidopsis lyrata

Abstract

The study of latitudinal gradients can yield important insights into adaptation to temperature stress. Two strategies are available: resistance by limiting damage, or tolerance by reducing the fitness consequences of damage. Here we studied latitudinal variation in resistance and tolerance to frost and heat and tested the prediction of a trade-off between the two strategies and their costliness. We raised plants of replicate maternal seed families from eight populations of North American Arabidopsis lyrata collected along a latitudinal gradient in climate chambers and exposed them repeatedly to either frost or heat stress, while a set of control plants grew under standard conditions. When control plants reached maximum rosette size, leaf samples were exposed to frost and heat stress, and electrolyte leakage (PEL) was measured and treated as an estimate of resistance. Difference in maximum rosette size between stressed and control plants was used as an estimate of tolerance. Northern populations were more frost resistant, and less heat resistant and less heat tolerant, but-unexpectedly-they were also less frost tolerant. Negative genetic correlations between resistance and tolerance to the same and different thermal stress were generally not significant, indicating only weak trade-offs. However, tolerance to frost was consistently accompanied by small size under control conditions, which may explain the non-adaptive latitudinal pattern for frost tolerance. Our results suggest that adaptation to frost and heat is not constrained by trade-offs between them. But the cost of frost tolerance in terms of plant size reduction may be important for the limits of species distributions and climate niches.

Fig 1. Locations of the nine North American Arabidopsis lyrata populations included in this study.

The grey shading indicates the approximate distribution of the species based on herbarium records, regional botanical lists, personal communication with local botanists, and our own field experience. The actual distribution is highly fragmented. The eastern and western regions represent distinct ancestral genetic clusters [23].

Fig 2. Latitude of origin of Arabidopsis lyrata plants differing in electrolyte leakage (a), asymptotic size (b), resistance to frost and heat based on electrolyte leakage (c), and tolerance to frost and heat based on asymptotic size (d).

Symbols depict population means based on family means and one-/two-sided bars indicate standard errors. Regression lines on panels a and b represent the significant or close to significant latitude-by-treatment interaction, regression lines on panels c and d represent significant latitude effect. For statistics see Tables 1 and 2. Data for heat tolerance and frost resistance had been corrected for ancestral cluster.