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We examined whether increased high temperature photosynthetic thermal tolerance (PT), reduced specific leaf area (SLA) and reduced leaf size represent correlated and convergent adaptations for recently diverged Encelia, Salvia, Atriplex and Eriogonum congeneric species pairs from contrasting thermal and water environments (the Mojave Desert and coastal California). We also studied whether variation in PT is associated with inducible small heat shock protein expression (sHsp).

Traits were measured in a common environment (CE) and in the field to partition effects of phenotypic plasticity and genetic divergence.

We found little evidence for convergent adaptation of PT (CE measurements). Field measurements revealed significant plasticity for PT, which was also associated with increased sHsp expression. Compared to coastal congeners desert species had lower SLA in the CE. These differences were magnified in the field. There was a negative correlation between SLA and PT. Desert species also tended to have smaller leaves both in the CE and in the field.

SLA and leaf size reductions represent repeated evolutionary divergences and are perhaps convergent adaptations for species radiating into the desert, while PT is highly plastic and shows little evidence for convergent adaptation in the congeneric species pairs we studied.