Differential Responses of Buds along the Shoot to Factors Involved in Apical Dominance

Abstract

Intact and decapitated 6-node shoots of Hygrophila sp. were grown aseptically immersed in liquid half-strength Knop's solution with microelements and 2% (w/v) sucrose (control medium), and in medium with 0.1 mg l−1 benzyladenine (BA). In intact shoots grown in control medium apical dominance suppressed outgrowth of the lateral buds; in decapitated shoots buds grew out at several of the most apical nodes, increasing in size acropetally. There was a lag in outgrowth of the bud at the most apical node, attributable to its initially smaller size. Lateral shoots grew out first at basal nodes of intact shoots in BA medium, decreasing in size acropetally; in decapitated shoots in BA medium lateral shoots of approximately equal size grew out at all nodes. Differential effects of decapitation and cytokinin treatment on lateral shoot outgrowth along the shoot could be interpreted by postulating a basipetally decreasing gradient of endogenous auxin concentration in the intact shoot. Application of 20 mg l−1 indoleacetic acid (IAA) in agar to decapitated shoots completely prevented bud outgrowth for at least 7 d in control medium, inhibiting it thereafter, and inhibited bud outgrowth in BA medium, thus supporting the hypothesis. Comparison of lateral shoot outgrowth in whole decapitated shoots and severed decapitated shoots (isolated nodes) lent no support to the alternative hypothesis that there might be an acropetally decreasing concentration gradient of a bud-promoting substance in the intact shoot, and demonstrated much greater lateral shoot growth in isolated nodes. The results emphasize important correlative relationships between the parts of a shoot with several nodes.