Negative gravitropism of Gnetum gnemon: the effect of the reaction wood and reaction phloem

Abstract

Gnetum gnemon (Gnetales) is a woody gymnosperm exhibiting morphological characteristics similar to those of woody angiosperms. This study investigated the mechanism underlying negative gravitropism observed in the inclined stems of G. gnemon at two plantation sites on Java Island, Indonesia. The following results were obtained: On the upper side of the inclined stem of G. gnemon, (1) eccentric growth occurred in both the xylem and the phloem; (2) significant tensile growth stresses were generated on the surfaces of the xylem and the inner phloem; and (3) a small microfibril angle (MFA) along with high cellulose crystallinity were observed in both the xylem and the inner phloem. Furthermore, (4) G-fibers developed predominantly on the upper side of the inclined stem, especially in the inner phloem, while no G-fibers were detected in the xylem. From these results, it can be concluded that reaction wood with a small microfibril angle (MFA) and high cellulose crystallinity in the lignified secondary-wall of the xylem fiber, along with reaction phloem containing G-fiber, generate significant tensile growth stresses on the upper side of the inclined stem of G. gnemon. In other words, although G. gnemon is a gymnosperm, it exhibits negative gravitropism through a mechanism not typical of normal gymnosperms but rather similar to that observed in angiosperms.