The radial distribution and metabolism of IAA-14C in Pinus echinata stems in relation to wood formation

Abstract

Radial distribution of auxin in differentiating xylem tissues was studied in decapitated 8- to 14-year-old Pinus echinata stems to which indoleacetic acid (IAA-14C) in lanolin paste was applied during early and late growing season. Small segments of the treated stems were sectioned tangentially with a sliding microtome, the serial sections were collected, and the radioactivity of each section was measured. The results indicate that IAA is radially distributed in the differentiating tissues along a steep gradient from the cambial region to the differentiating xylem and that this distribution varies during the growing season in relation to the seasonal transition to latewood formation in intact trees.Forty-five to 60% of the radioactivity recovered in methanolic extracts of the differentiating xylem after a 2-day labeling period was in the form of IAA. After 7 days 10–15% of the radioactivity recovered was in the form of IAA. The radioactivity recovered separated into ether and aqueous fractions when the methanol extract was reduced, acidified, and partitioned with ether. Chromatography of aliquots of both fractions in several solvent systems revealed no individually significant metabolite of IAA. The aqueous fraction derivatives accumulated in the tissue, and later in the season the metabolism of IAA and the accumulation of the aqueous fraction apparently decreased. In experiments where excised differentiating xylem was incubated with a liquid medium containing IAA-14C, the conversion of radioactivity to the aqueous fraction was twofold greater in early season than in late season. The significance of the results is discussed with possible relation to the control of the seasonal transition to latewood formation in conifers.