The evolutionary and ecological physiology of plant thermal tolerance

I studied the interaction between plants and their environment and how evolutionary differentiation between species affects the outcome of these interactions. My approach was integrative, from leaf level physiology and biochemistry to large-scale analyses of species distributions across environmental gradients. Part of my research dealt with the heat shock protein (HSP) response. For some time it has been speculated that evolutionary change in the HSP response might be correlated with the frequency of temperature stress experienced by a species. To test this hypothesis, I performed experiments involving closely related Ceanothus and Encelia species from contrasting thermal environments. Results indicated that while for Encelia, thermotolerant species had higher induction temperatures and higher thermal maxima for HSP expression, in Ceanothus they did not. However, there were significant functional relationships. Ceanothus species that expressed HSPs after similar temperature treatments were better able to maintain electron transport. Variation in HSP expression was also negatively correlated with the mass to area ratio of leaves, with is itself correlated with a suite of traits associated with carbon gain and stress tolerance.; Using several congeneric species pairs (Encelia, Salvia, Eriogonum, Atriplex) native to contrasting thermal environments I also tested the hypotheses that desert species have greater photosynthetic thermotolerance and a greater investment in leaf mass per unit leaf area (LMA). Measurements were made both in a common environment and in the field to examine plastic and genetic components of phenotypic variation. In the field, all of the desert species within a congeneric pair had greater LMA and photosynthetic thermotolerance. In the common environment photosynthetic thermotolerance was not significantly different, while LMA was significantly different for desert and coastal congeneric species. These results suggest that phenotypic plasticity and genetic divergence both contribute to observe patterns of differentiation in the field.; I also performed a large-scale analysis of genome size variation across environmental gradients. I found a decreased representation of species with large genomes at both temperature extremes and along a gradient of increasing aridity. These patterns do not simply represent a change in mean genome size but rather a decreased variance in genome size across these gradients.