Effect of laterally applied gibberellin A4/7 on cambial growth and the level of indole‐3‐acetic acid in Pinus sylvestris shoots

Abstract

Gibberellin A4/7 (GA4/7) was applied in lanolin or ethanol around the circumference at the midpoint of the previous‐year terminal of dormant Pinus sylvestris seedlings. After cultivating the seedlings under environmental conditions favorable for growth for up to 10 weeks, cambial growth was measured as the radial widths of xylem and phloem, and the level of indole‐3‐acetic acid (IAA) was determined by combined gas chromatography‐mass spectrometry using [136](IAA) as the internal standard. In intact seedlings, both 1 mg GA4/7 g−1 lanolin and 50 mg GA4/7 I−1 ethanol increased phloem production and the cambial region IAA level in the current‐year terminal, without significantly altering its longitudinal growth. In the previous‐year terminal, 1 mg GA4/7 g−1 lanolin promoted phloem production at the application point and increased the cambial region IAA level above this point, whereas 50 mg GA4/7 I−1 ethanol stimulated the production of both xylem and phloem at the treatment site and elevated the cambial region IAA level beneath it. Laterally applied GA4/7 at 50 mg I−1 ethanol stimulated xylem and phloem production in debudded previous‐year terminals treated at the apical cut surface with 1 mg IAA g−1 lanolin, but not in those treated with plain lanolin. However, the promotion of cambial growth in debudded terminals treated apically with 1 mg IAA g−1 lanolin and laterally with 50 mg GA4/7 I−1 ethanol was not associated with an elevated IAA content in the cambial region. The results indicate that exogenous GA4/7 can promote xylem and phloem production provided an IAA source is present, and that it or a metabolic product acts directly, rather than indirectly by stimulating longitudinal growth and/or raising the cambial region IAA level.