Growth stress controls negative gravitropism in woody plant stems

In the shoots of woody plant species, reaction-wood fibers are formed on the upper or lower side of the secondary xylem of a leaning trunk or branch wherever large, internal growth stress is generated. Negative gravitropic movement in woody plant stems is proposed to be the result of growth stress generated in the reaction-wood tissue. This study examines the interaction between bending moment due to increasing self-weight and recovery moment resulting from asymmetric growth stress, and tests a hypothesis that describes the relationship based on the structural mechanics "beam theory". Simulations of observed tree branch morphology of Magnolia kobus DC., Juniperus chinensis L., Abies saccharinensis Fr. Schum., and Prunus spachiana Kitamura f. spachiana cv. Plenarosea showed that (i) the growth stress generated in the reaction wood is sufficient to counteract the gravitropic response to increasing self-weight, and (ii) the specific directional angle of the shoot apex or preferred angle of the elongation zone plays an important role in controlling the spatial shape of the branch stem that is peculiar to plant species with large growth stress generated in the reaction-wood tissue.