Impact of soil pressure and compaction on tracheids in Norway spruce seedlings

We tested the effect of soil compaction on Norway spruce seedlings in terms of the size and theoretical volume flow rate of the tracheids. The results show that soil pressure limits growth in the diameter of the lumens of tracheids in all parts of seedlings studied. The tracheids of the roots with primary xylem had larger lumens than those of the roots and shoots with secondary xylem in both unloaded and loaded seedlings. This corresponds to the higher cumulative theoretical volume flow rate of the tracheids from roots with primary xylem than those from roots and shoots with secondary xylem. Although the volume flow rate of tracheids, according to the Hagen-Poiseuille law, was directly proportional to the quadratic power of the capillary diameter (tracheid lumen), the cumulative curve of the theoretical hydraulic volume flow rate was higher or relatively comparable in loaded seedlings. An explanation for these findings is that there were higher gradients of water potential values in roots and leaves in loaded seedlings because the lengths of the conductive pathways were 27% shorter than in unloaded seedlings. We hypothesise that trees have adapted to different stresses by shortening their conductive pathways to maintain a transpiration rate similar to that of non-stressed trees. These results concerning the impact of soil compaction on tracheid diameter and volume flow rate improve our understanding of the growth and functioning of different conifer organs and the mechanisms underlying the efficiency of water transport through the root xylem to the shoot.