Effects of root and shoot genotypes on productivity and physiology of loblolly pine (Pinus taeda) seedlings were evaluated in contrasting nutrient regimes. Twelve-week-old seedlings from contrasting provenances were grafted reciprocally to facilitate distinction of rootstock and scion effects. Five open-pollinated families each from a mesic region (Atlantic Coastal Plain) and from a xeric region (Lost Pines Texas) were planted in a split-plot design on a nutrient-poor site in the Sandhills region of central North Carolina. A total of 1800 seedlings were used, including non-grafted trees as controls. Half of the plots were fertilized annually, and after one and two growing seasons, seedlings were harvested for component biomass determinations. Total biomass production among families was positively related to proportional biomass allocation to roots. Generally, mesic sources produced more total biomass and allocated proportionally more biomass to roots. When fertilized, mesic rootstocks were always associated with greater mass in aboveground components, regardless of scion genotype. Rootstock affected stem growth efficiency, in that xeric rootstocks were associated with increased proportional allocation to stem, regardless of scion type. Effects of root system genotype on leaf gas exchange of selected families were evaluated and related to tree growth. In two families from each provenance, light-saturated net photosynthesis (An) and stomatal conductance (gs) were measured monthly during the second growing season. Leaf carbon isotope discrimination (C13) was analyzed to estimate water use efficiency (WUE) of genotypes. Provenances differed in stomatal conductance (gs) but not in photosynthesis (An), averaged over all months. The mesic sources had lower gs and higher intrinsic WUE (An/gs). Rootstocks affected gs and leaf C13 but not An nor WUEi of scions. Leaf tissue C13 was inversely associated with stem growth efficiency and with root biomass among families, but was u.