Seasonal cambial activity and xylem development in <i>Pinus</i><i>contorta</i> in relation to endogenous indol-3-yl-acetic and (<i>S</i>)-abscisic acid levels

Seasonal cambial activity and xylem development were monitored anatomically at several main-stem positions in 20-year-old Pinuscontorta Dougl. Gas chromatography – mass spectrometry and electron-capture gas-chromatography quantifications of endogenous indol-3-yl-acetic acid (IAA) and (S)-abscisic acid (ABA), respectively, were made in these same tissues. In addition, IAA and ABA were measured in shoot apices, growing needles, and mature needles. Cambial reactivation occurred several weeks before budbreak at the base of the living crown, and it occurred only slightly before budbreak in younger cambia. Neither an acropetal nor a basipetal progression of reactivation were found. In contrast, early-wood development commenced in foliated stem locations well before commencing in nonfoliated regions. IAA and ABA levels in the cambial region increased, but little, immediately before cambial reactivation. IAA levels in extending apices, extending needle bases, and cambia reached maxima in midsummer. Cambial IAA levels remained high and ABA levels declined after both shoot and needle extension were complete. IAA levels in cambia were highest in the main stem near the crown base at all times; ABA levels varied little throughout the season. Following autumn inactivation of the cambium, IAA levels decreased and tracheids with narrow radial diameters differentiated at this time. ABA levels showed no significant increase as shoots, needles, and cambium ceased growing.