EFFECTS OF GROWTH REGULATING SUBSTANCES AND WATER POTENTIAL ON THE DEVELOPMENT OF SECONDARY XYLEM IN FRAXINUS

Abstract

SummaryMarked variations in secondary vascular growth of detached Fraximis excelsior L. stems were obtained with changes in water potential and in IAA and GA concentrations in the culture medium. The width of new tissue, the number of fibres and the mean fibre diameter increased with IAA concentration up to 10 mg.1−1 and generally with GA concentration up to 100 mg.1−1, but by far the greatest influence was that of water potential wbich, especially between zero and −0.1 J.g−1, significantly depressed cell division and expansion. Expansion of cambial initials to a diameter of about 6 μ appeared to be necessary before active cell division commenced.Vessel development was particularly sensitive to IAA supply and the amount of IAA required for the formation of groups of vessels increased as the number of constituent vessels increased and as the water potential was reduced. Gibberellin bad no significant effect on the number of vessel groups formed, but appeared to increase the number of vessels formed in any group. Vessel formation may occur without cell division in an initially dormant cambium. Increasing the IAA supply led to increasing vessel cross sectional areas, but progressively less effectively as the water potential was reduced; the marked reduction in vessel lumen area between zero and −0.1 J.g−1 potential suggests that vessels formed at reduced water potentials may have different metabolic patterns than those formed at zero potential from an active cambium.The significance of the depressing effect on cell division and expansion of even a slight reduction in water potential is discussed in relation to wood development in trees growing under conditions of moisture stress.