Heat stress disruption and contrasting avoidance-tolerance strategies from thermally contrasting climates in Arabidopsis thaliana

Background/Question/Methods

Temperature, especially high temperature, is one of the major ecological factors that limit plant distribution. Plants can avoid heat stress by reducing tissue temperature below ambient air temperature. In contrast, plants may exhibit tolerance, i.e. maintain key functions at high temperature. Here we ask: 1) Do plants from thermally contrasting environments show dramatically variable heat stress damage and 2) Do they adopt different avoidance-tolerance strategies? 8 Arabidopsis thaliana populations from thermally contrasting environments, 4 low- and 4 high-elevation populations, were planted in a growth chamber and then exposed to severe heat stress (45^oC) at flowering stage. Plants from low elevation tend to experience more frequent hot and dry conditions, while plants from high elevation experience greater frequency of cold and wet conditions but still experience occasional heat stress. We measure avoidance by recording rosette compared to ambient temperature. We measure tolerance by comparing photosynthetic rate under control (22 ^oC) and high (45 ^oC) temperatures.  Finally, we estimated fitness as the summed length of all fruits produced.  We also quantified heat stress disruption on reproduction characteristics (branch number, reproductive length) and resource allocation (root, rosette and inflorescence dry mass).

Results/Conclusions

We found that for all populations, heat stress equally induced more inflorescence branches, 25% longer inflorescence length and 12% lower root dry mass compared with control. However, the greater resource allocation to inflorescences with heat stress did not contribute increased fitness. All populations exhibited reduced (15%) summed fruit length with heat stress.  Importantly, we found that populations from thermally contrasting climates adopt significantly different avoidance and tolerance strategies. For avoidance, all populations showed reduced rosette temperature compared with ambient temperature (45 ^oC), with mean rosette temperature 38 ^oC for low elevation and 35 ^oC for high elevation populations. High elevation populations are more thermally homeostatic than low elevtion populations. For tolerance, low elevation populations reduced photosynthesis ~30% while high elevation populations reduced photosynthesis ~75% with heat stress compared with control. In summary, populations from low elevation exhibit less avoidance but more tolerance, while populations from high elevation show greater avoidance but lower tolerance. The different avoidance–tolerance strategies for populations from thermally contrasting climates indicates constraints on the evolution of both strategies and their adaptive divergence regulated by local climates.