GENETIC-VARIATION IN NITROGEN USE EFFICIENCY OF LOBLOLLY-PINE SEEDLINGS

Seedlings of 23 open-pollinated loblolly pine (Pinus taeda L.) families were grown in a greenhouse at two nitrogen (N) levels (5 and 50 ppm). Differences among families for components of N use efficiency (NUE) were evaluated from measurements of biomass and N accumulation in different parts of seedlings. NUE, defined as stem biomass produced per unit of N applied, was partitioned into two major components: uptake and utilization efficiencies. Family differences were detected for NUE and its two components at low N and for utilization efficiency at high N. Heritability estimates indicated that NUE traits were under a moderate to high degree of genetic control. NUE and its components were generally positively correlated with root and shoot growth of seedlings. Families with more nitrogen (high uptake efficiency) had greater root length and stem height at low N but not at high N. Strong correlation of seedling root and shoot traits with NUE components indicated that genetic differences in N use may reflect differences in seedling growth. The relative contribution of uptake or utilization efficiency to the variation in NUE was different at the two N levels. At the high N level, differences among families for NUE were due largely to variation in utilization of absorbed N, while uptake and utilization efficiencies contributed equally to the variation in NUE at the low N level. Furthermore, some families with similar levels of NUE showed large differences in the relative contribution of uptake and utilization efficiency.