Interaction between xylem rays and vessel elements

Increasing attention has recently been given to the connections between the two systems of secondary vascular tissues. Due to the dependencies described thus far between developing vessel elements and xylem rays, it was decided to analyze the influence of ray proximity on the tangential dimension of developing vessel element, as well as the effect of vessel element development on the width of neighboring ray. We hypothesized that rays act as obstacles to the tangential enlargement of vessel elements, thereby influencing their final tangential dimension, and that the enlargement of vessel elements is associated with localized changes in the width of the rays they encounter during growth. Two anatomical techniques were used to obtain tangential and transverse sections of the secondary xylem of Norway maple and pedunculate oak. In maple wood, the tangential dimension of vessels in contact with rays was measured, while in oak wood, the width of rays in contact with fully developed vessels was recorded. The anatomical and statistical analyses revealed a reduction in the tangential dimension of vessel that contacts ray, and showed that the tangential growth of vessel element is associated with a local decrease in the width of the ray it encounters. These findings indicate that rays generally act as obstacles to vessel element growth, independent of whether growth is intrusive or symplastic. Considering their biomechanical properties, we propose that rays may play a buffering role within the secondary xylem differentiation zone – limiting not only the tangential intrusive growth of vessel elements but also affecting their overall tangential enlargement. In addition, three theoretical explanations were introduced to account for the observed reduction in the average width of rays.