Effect of exogenous gibberellin A_4/7 on tracheid production, longitudinal growth and the levels of indole‐3‐acetic acid and gibberellins A₄, A₇ and A₉ in the terminal shoot of <i>Pinus sylvestris</i> seedlings

Abstract

Gibberellin A 4/7 (GA 4/7 ) was applied twice weekly to 2‐year‐old Pinus sylvestris (L.) seedlings in each of two years, starting close to budbreak and ending after shoot elongation, but before cambial activity ceased. In 1988, the GA 4/7 was injected into the 1987 terminal shoot (0, 0.2 or 2 mg seedling −1 application −1 ), while in 1990 it was applied as a soil drench (0, 10 or 50 mg seedling −1 application −1 ). In the 1988 experiment, GA 4/7 treatment promoted diameter growth, and tended to increase both longitudinal growth and the indole‐3‐acetic acid (IAA) level in the 1988 terminal. In the 1990 experiment, GA 4/7 treatment increased tracheid production, longitudinal growth, and the cambial region IAA concentration in the 1990 terminal, but did not affect its pith diameter, needle number, needle dry weight, or needle IAA level. Tracheid production in the previous‐year's terminal was also promoted in both experiments. The 50 mg GA 4/7 soil drench markedly elevated the concentrations of GA 4 , GA 7 and GA 9 in the needles and cambial region of the 1990 terminal, while the 10 mg treatment raised the GA 4 level in the cambial region, providing evidence that GA 4 and GA 7 applied to the roots reaches the shoot system. The results support the hypothesis that the exogenous GA 4/7 ‐induced stimulation of tracheid production in the terminal shoot of intact plants is mediated through an increase in the IAA level in the cambial region. However, per se activity of GA 4 , GA 7 or their metabolites cannot be ruled out.