AMINO-ACID UTILIZATION IN SEEDS OF LOBLOLLY-PINE DURING GERMINATION AND EARLY SEEDLING GROWTH .1. ARGININE AND ARGINASE ACTIVITY

The mobilization and utilization of the major storage proteins in lobl olly pine (Pinus taeda L.) seeds following imbibition were investigate d. Most of the seed protein reserves were contained within the megagam etophyte. Breakdown of these proteins occurred primarily following rad icle emergence and correlated with a substantial increase in the free amino acid pool in the seedling; the majority of this increase appeare d to be the result of export from the megagametophyte. The megagametop hyte was able to break down storage proteins and export free amino aci ds in the absence of the seedling. Arginine (Arg) was the most abundan t amino acid among the principal storage proteins of the megagametophy te and was a major component of the free amino acid pools in both the seedling and the megagametophyte. The increase in free Arg coincided w ith a marked increase in arginase activity, mainly localized within th e cotyledons and epicotyl of the seedling. Arginase activity was negli gible in isolated seedlings. Experiments with phenylphosphorodiamidate , a urease inhibitor, supported the hypothesis that arginase participa tes in Arg metabolism in the seedling. The results of this study indic ate that Arg could play an important role in the nutrition of loblolly pine during early seedling growth.