Relative Elemental Rates of Elongation and the Protoxylem-Metaxylem Transition in Hypocotyls of Soybean Seedlings

Abstract

The transition from helical to reticulate wall patterns in the conducting elements of the xylem was located in etiolated soybean hypocotyls in conjunction with the distribution of elongation in the axis. The most acropetal reticulate elements are interpreted as the most acropetal metaxylem. Evidence is given that these elements experience strain as they mature. Predicted strain correlated significantly with maximum relative elemental rate of elongation (RELEL), which ranged from 0.08 to 0.12-hr-1. RELEL varied from 0.03 to 0.06hr-1 at the point of transition from helical to reticulate xylem element wall patterns. Estimates of strain to be experienced in the reticulate elements varied from 0.11 to 0.13 in untransplanted plants and from 0.04 to 0.11 in plants transplanted to the same, fully watered medium. Transplanting suppressed elongation in the region of maximum RELEL for ca. 12 hr and caused an upward migration of the region of depressed rates on the outside of the axis that accounts for hook-opening. Attention is called to the fact that this region of depressed growth corresponds with the onset of transverse enlargement of the hypocotyl axis.