Trees try to aim their roots down and their stems up. The sense down is gravitropism, and the sense up is called negative gravitropism. When a stem is leaning and needs to change direction to grow ‘up’ the tree forms what is called ‘tension wood’ on the upper side of the stem. This pulls the tree vertical. What isn’t known is if the angle that the stem is out of vertical affects tension wood formation. Nugroho and colleagues designed a study to investigate negative gravitropism, tension wood formation and growth eccentricity in Acacia mangium seedlings inclined at different angles.
The experiments were quite simple. They tipped uniform seedlings of A. mangium at 30°, 45°, 60° and 90° from the vertical. They also made sure they had some vertical seedlings for comparison. After three months they harvested the plants and looked at the tree anatomy to see what the tension wood looked like, and how it affected growth.
What the authors found was smaller inclination angles led to 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, they bigger the tipping, the more rapidly they moved back toward the vertical.
For the anatomy, the 30°-inclined seedlings formed the narrowest region of tension wood but there were no significant differences among seedlings inclined at 45°, 60° and 90°. The 90°-inclined seedlings formed thicker gelatinous layers than those in 30°-, 45°- and 60°-inclined seedlings. These fibres could explain what is happening. Writing in Annals of Botany the authors said: “Larger amounts of newly developed gelatinous fibres and thicker gelatinous layers might generate the higher tensile stress that leads to higher speeds of stem-recovery movement in A. mangium seedlings.”
Nugroho, W. D., Nakaba, S., Yamagishi, Y., Begum, S., Rahman, M. H., Kudo, K., … Funada, R. (2018). Stem gravitropism and tension wood formation in Acacia mangium seedlings inclined at various angles. Annals of Botany, 122(1), 87–94. https://doi.org/10.1093/aob/mcy056