Phosphorus Uptake Capacity of 14-Year-Old Loblolly Pine as Indicated by a 32P Root Bioassay

Excised loblolly pine roots were exposed to a 32p-labelled solution for 20 minutes to measure their capacity for P uptake. On five dates from March 1985 to March 1986, root samples were collected from 14-year-old loblolly pine which had received 101 kg P · ha-1 and 0 kg P · ha-1 when they were planted. Phosphorus uptake by roots of nonfertilized loblolly pine (1.10 μmol P · g-1 · hr-1) was significantly greater than that by roots of fertilized loblolly pine (0.72 μmol P · g-1 · hr-1) when sampled between June and October, but no difference was detected when sampled in March. Phosphorus uptake was decreased by approximately 50% at 7°C compared to 25°C, and in the presence of metabolic inhibitors. Phosphorus concentrations, measured after the bioassay, of roots from fertilized trees (0.93 g P · kg-1) were significantly greater than those of roots from nonfertilized trees (0.45 g P · kg-1) on all five sampling dates. Capacity for root P uptake did not have an advantage over root or foliar P concentrations as an indicator of P stress, and does not appear to be a practical diagnostic tool for semimature loblolly pine. For. Sci. 36(2):358-366.