Experimental Studies of Ponderosa Pine. Iii. Differences in Photosynthesis, Stomatal Conductance, and Water‐use Efficiency Between Two Genetic Lines

As part of an intensive study of heritable differences among the progeny of Pinus ponderosa parents from two contrasting habitats (coastal vs. interior, continental), we examined the potential for differences in photosynthesis rate, stomatal conductance, and photosynthetic water‐use efficiency. Plants from a cross between two coastal parents (ponderosa × ponderosa) exhibited lower photosynthetic water‐use efficiencies, relative to plants from a coastal × interior cross (ponderosa × scopulorum). The lower water‐use efficiencies in the ponderosa × ponderosa plants were evident as a lower ratio of external to intercellular CO2 concentrations and higher stomatal conductances at any given rate of photosynthesis. The ponderosa × scopulorum plants exhibited lower stomatal conductances over a range of leaf‐to‐air water vapor concentration differences, which was partially explained by lower stomatal densities. The ponderosa × scopulorum plants also exhibited lower maximum photosynthesis rates and lower needle nitrogen concentrations. Taken together, the results suggest that in adapting to drier habitats, P. ponderosa has acquired improved water‐use efficiencies and lower transpiration rates, but at the expense of reduced maximum photosynthesis rates.