Correlated evolution of chloroplast heat shock protein expression in closely related plant species.

Interspecific variation in chloroplast low molecular weight (cLMW) HSP (heat shock protein) expression was examined with respect to phylogeny, species specific leaf area, chlorophyll fluorescence, and mean environmental conditions within species ranges. Eight species of Ceanothus (Rhamnaceae) were heat shocked for 4 h at several different temperatures. Leaf samples were collected immediately after the heat shock, and cLMW HSP expression was quantified using Western blots. At 45°C species from the subgenus Cerastes had significantly greater cLMW HSP expression than species from the subgenus Ceanothus. Specific leaf area was negatively correlated with cLMW HSP expression after the 45°C heat treatment. In addition, chlorophyll fluorescence (F(v)/F(m)) 1 h after the heat shocks was positively correlated with cLMW HSP expression. Contrary to our prediction, there was no correlation between July maximum temperature within species ranges and cLMW HSP expression. These results suggest that evolutionary differentiation in cLMW HSP expression is associated with leaf physiological parameters and related aspects of life history, yet associations between climatic conditions within species ranges and cLMW HSP expression require further study.