The three-dimensional distribution of bordered pits across growth rings of stem segment in Platycladus orientalis (Cupressaceae) seedlings

The dimensions and distribution pattern of bordered pits are critical to the xylem water transport function of trees. However, studies on the three-dimensional distribution pattern of pits across xylem growth rings are still limited. Here, Micro-Computed Tomography (Micro-CT) was used to acquire xylem images of two-year-old Platycladus orientalis seedlings. We analyzed the tracheid lumen diameter of 665 tracheids, pit volume, and pit diameter of 11 810 inter-tracheid pits from three radial profiles. The volume ratio of inter-tracheid pits was about 0.90–0.96%. The average pit diameter was 6.8 μm and the average pit volume was 58.9 μm3, and more than 80% of the pits had a diameter of 5–8 μm and a volume of 20–90 μm3. Both the pit diameter and the pit volume increased gradually in the radial direction from the second-year growth ring to the current-year growth ring, and the pit diameter and volume were significantly higher in earlywood than in latewood. The distribution pattern of the tracheid diameter was similar to that of the pit diameter, and the wider the tracheids, the larger the pits. The distribution of bordered pits in P. orientalis may be a critical trait explaining the xylem embolism spread pattern in this species.