Relationships between gas exchange and carbon isotope discrimination of Sitka x interior spruce introgressive genotypes and ribosomal DNA markers

Effects of hybridization on physiological performance were investigated in seven natural introgressive populations and one F-1 population of Sitka (Pi cea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss, x P. engelmannii Parry ex Engelm.). Each population was represented by a Sit ka-interior spruce ribosomal DNA (Si rDNA) index that was calculated from t he relative abundance of species-specific DNA polymorphisms. Gas exchange p arameters were measured under well-watered conditions on current-year needl e tissues, which were also analyzed for carbon isotope discrimination. Sitk a spruce populations had higher gas exchange rates (net photosynthesis, sto matal conductance to water vapor), but lower carbon isotope discrimination values than interior spruce populations. The natural hybrid populations had intermediate values of these parameters consistent with their Si rDNA inde x. The F1 population had gas exchange parameters resembling those of Sitka spruce populations, but its carbon isotope discrimination was skewed toward that of interior spruce populations. These results confirmed previous find ings that physiological performance of introgressive hybrid spruce populati ons varied as their DNA constitution changed.