Developmental Features Accompanying the Imposition and Release of Apical Dominance in Pea

Abstract

Summary The ontogeny of the lateral shoot at the second node of the pea plant ( Pisum sativum L.) was studied to determine whether imposition and release of dominance was correlated with developmental features. The lateral shoot develops rapidly achieving maximal elongation rates 3 to 4 d after planting and then declines to a slow but measurable rate. Mature vascular tissue connecting to the main axis is not present during early growth of the shoot. As elongation slow down a single sieve tube begins to differentiate. A mature vessel is present only after the plants are 9-d-old. As the plant ages vascular development along the main axis outstrips vasculation of the lateral shoot. A single application of 6-benzylaminopurine to or decapitation of 6-d-old plants reverses this growth retardation. Increased elongation and renewed mitotic activity are noted within the first 24 h, and soon thereafter vascular tissues differentiate. These structural features suggest: 1) limited phloic and xylary connection with the main axis cannot support the vigorous growth of the lateral shoot so it becomes dominated, 2) the lateral shoot of pea is a metabolically active and a slow growing structure while apical dominance is maintained and 3) release from dominance involves an increase in the rate of mitosis and cell elongation and the stimulation of differentiation, processes that all are already in progress, albeit slowly, in the suppressed lateral shoot.