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Levels of hybridization between related species can vary in response to evolutionary history or local environmental conditions, such as pollinators and abiotic factors, that affect reproductive isolation. Contact sites between the herbs Ipomopsis aggregata and Ipomopsis tenuituba vary in habitat type, spatial distribution of the parental species, and level of hybridization, partly because of more species-specific pollinator foraging at the contact site with fewest hybrids. We hypothesized that such a contact site, with low hybridization and stronger genetic differentiation between the species, would also show greater divergence in ecophysiology, potentially contributing to reduced hybrid viability. We compared photosynthetic responses to temperature and water for the two species and their natural hybrids (where present) between the unusual site where hybrids are scarce and two contact sites with extensive hybridization. Measurements were made under field conditions and as a function of leaf temperature for potted plants in a common environment. Ipomopsis tenuituba had higher intrinsic water-use efficiency than I. aggregata, with plants of the two species showing a similar difference in water-use efficiency across all contact sites. The two species also showed similar photosynthetic responses to temperature. In a common environment, the greatest difference between species in photosynthetic rate was for a single contact site where hybridization is common. Differences in gas exchange, although consistent with survival and distribution within hybrid zones, do not help explain the geographical variation in hybridization.
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