Stem gravitropism and tension wood formation in Acacia mangium seedlings inclined at various angles

Background and Aims In response to a gravitational stimulus, angiosperm trees generally form tension wood on the upper sides of leaning stems in order to reorientate the stems in the vertical direction. It is unclear whether the angle of inclination from the vertical affects tension wood formation. This study was designed to investigate negative gravitropism, tension wood formation and growth eccentricity in Acacia mangium seedlings inclined at different angles. Methods Uniform seedlings of A. mangium were artificially inclined at 30 degrees, 45 degrees. 60 degrees and 90 degrees from the vertical and harvested, with non-inclined controls, 3 months later. We analysed the effects of the angle of inclination on the stem recovery angle. the anatomical features of tension wood and radial growth. Key Results Smaller inclination angles were associated with earlier stem recovery while stems subjected to greater inclination returned to the vertical direction after a longer delay. However, in terms of the speed of negative gravitopism towards the vertical, stems subjected to greater inclination moved more rapidly toward the vertical. There was no significant difference in terms of growth eccentricity among seedlings inclined at different angles. The 30 degrees-inclined seedlings formed the narrowest region of tension wood but there were no significant differences among seedlings inclined at 45 degrees. 60 degrees and 90 degrees. The 90 degrees-inclined seedlings formed thicker gelatinous layers than those in 30 degrees-, 45 degrees- and 60 degrees-inclined seedlings. Conclusion Our results suggest that the angle of inclination of the stem influences negative gravitropism, the width of the tension wood region and the thickness of gelatinous layers. Larger amounts of gelatinous fibres and thicker gelatinous layers might generate the higher tensile stress required for the higher speed of stem-recovery movement in A. mangium seedlings.