The hypothesis that species inhabiting warmer regions have greater photosynthetic tolerance of high temperatures was tested using the temperature-dependent increase in fluorescence (T-F o). Congeneric species pairs of Atriplex, Salvia, Encelia, and Eriogonum with desert versus coastal distributions were studied in a common environment and in the field. In addition, 21 species with contrasting microclimate distributions were studied at a field site in a northern California chaparral community. The average July maximum temperature within the current distributions of species was quantified using a geographic information system. Four parameters (T crit, T S20, T 50, and T max) of the T-F o response were used to quantify photosynthetic thermotolerance. In the common environment, only the desert Atriplex species was significantly greater for all T-F o parameters when compared to its coastal congener. In the field, desert species had significantly greater T crit, T S20, T 50, and T max when compared to coastal species. The magnitude of variation between species and between genera was similar in the common environment and the field. However, T crit, T S20, T 50 and T max were all significantly greater when measured in the field. There was no relationship between T-F o parameters and the microclimate distribution of the 21 species at the chaparral field site. In addition, T-F o parameters for all 35 species were not correlated with the average July maximum temperature within the species ranges. However, there was a significant negative correlation between the average annual amount of precipitation inside species' ranges and T S20. Our results show that photosynthetic thermotolerance is (1) significantly different between genera and species, (2) highly plastic, (3) not necessarily greater for species with warm climate distributions when measured in a common environment, but (4) significantly greater overall for desert species compared to coastal species when measured in the field.
Keywords: Acclimation; Fluorescence; Photosystem II; Phylogenetic contrasts; Plasticity.