Genotypic variation in carbon isotope discrimination and gas exchange of ponderosa pine seedlings under two levels of water stress

As part of a program to select ponderosa pine (Pinus ponderosa Dougl. ex Laws.) genotypes for improved drought tolerance, we examined physiological and morphological characteristics of 12 half-sib families of ponderosa pine from four seed sources; New Mexico, South Dakota, Nebraska, and Wyoming. We analyzed genetic variation in carbon isotope discrimination ( Δ), photosynthetic gas exchange, needle morphology, and growth of 2-year-old seedlings from the four seed sources grown under two levels of moisture availability. To gain a better understanding of within-provenance variation and identify opportunities to refine selection strategies, we also examined family within seed source variation in the traits. Water stress significantly (P < 0.05) reduced net photosynthesis (A), needle conductance to water vapor (g wv ), carbon isotope discrimination ( Δ), and growth of the seedlings as compared to well-watered seedlings. However, instantaneous water use efficiency (A/g wv ) did not differ between water treatments. Seedlings from New Mexico had significantly lower g wv and higher A/g wv than seedlings from the other sources. Carbon isotope discrimination was lowest for seedlings from New Mexico and Nebraska. Families within seed sources varied significantly in A, g wv , stomatal density, needle length, height increment, and Δ. Carbon isotope discrimination was significantly correlated with g wv but not with A, supporting results from mature trees suggesting that variation in Δ in ponderosa pine is more related to g wv than to A. Seed source × water treatment interactions were not observed for any of the traits analyzed. These results support our previous assertion that genotype × environment interaction in Δ of mature ponderosa pine trees from these sources grown in Nebraska and Oklahoma was related to factors other than moisture availability.