The Effect of Applied Growth Hormones on Cambial Division and the Differentiation of the Cambial Derivatives

When indole-3-acetic acid (IAA) is applied to woody shoots cambial division is stimulated and the cambial derivatives differentiate to produce xylem tissue. When gibberellic acid (GA) is applied, cambial division occurs but the resultant derivatives on the xylem side of the cambium remain undifferentiated. The relative levels of applied IAA and GA are important in determining whether mainly xylem or phloem tissue is produced. High IAA/low GA concentrations favour xylem formation, whereas low IAA/high GA concentrations favour phloem production. The new phloem tissue produced as a result of hormone treatment is fully differentiated, containing sieve elements and sieve plates. IAA is important in promoting the elongation of the cambial derivatives to produce xylem vessel and fibre elements, though in the case of xylem fibres applied GA causes further elongation. IAA is an important factor in determining vessel diameter in the ring-porous species Robinia pseudacacia, high levels of applied IAA giving wide springwood-type vessels and low levels giving narrow 'summerwood' vessels.