Anatomy of seedling tap roots of loblolly pine (Pinus taeda L.)

The development of tap root anatomical features was investigated in seedlings of loblolly pine (Pinus taeda L.) under both pot and pouch growth regimes. The roots possessed the three anatomical zones previously observed in jack pine (Pinus banksiana Lamb) and Eucalyptus pilularis Sm. - white, condensed tannin (CT), and cork - suggesting that this developmental sequence is preserved over species and growth conditions. Xylem development was centripetal and similar to that found earlier in P. sylvestris. Tracheids with lignified, secondary walls were detected distal to the point of endodermal Casparian band deposition. However, tests for ability to conduct fluid indicated that the protoxylem was capable of transport only proximal to the Casparian bands. Detailed examination of suberin lamella deposition in the endodermis demonstrated that passage cells were present through the white and CT zones. Progressive; centripetal cortical death in the CT zone did not Include the endodermis, which remained alive until the cork layer formed, at which point the endodermis was crushed. Therefore, passage cells remain as functional portals for nutrient and water uptake in the CT zone even though the central cortex is dead. Tracer tests indicated that the endodermis provides an apoplastic barrier to tracer diffusion into the stele and that this function was taken over by the young cork layers. Results of this study point to a strong role for the endodermis in the regulation of nutrient and water uptake until the maturation of the first cork layer.