Figure 2. Scaling of sieve element (SE) conductivity and osmotic pressure potential enable Münch-type pressure flow in trees.
( A) Axial widening of phloem cells along the stem of individual trees of Norway spruce (blue), European ash (orange), and bitter willow (gray). Average values of cell diameter contain up to 30% non-SE cells. Axial widening of SEs toward the bottom of a tree translates to decreased hydraulic resistance, and the scaling relationship was found to enable optimal xylem-phloem water exchange that is critical to drive flow in tall trees. ( B) SE resistance of 44 different tree species with sampled individuals grown to their typical maximum height. Negative scaling of resistance with transport distance illustrates the anatomical optimization of SEs for phloem transport in trees. ( C) Measurements of osmolality indicated the presence of an osmotic pressure gradient along the phloem of 18 m high Scots pine trees, which changed in accordance with the xylem water potential. The differences in osmotic potential were calculated to be large enough to overcome the xylem water pressure potential and establish a phloem turgor pressure gradient that drives mass flow according to the Münch mechanism across the diel cycle 56. Frames ( A– C) are based on data from Petit and Crivellaro 57, Liesche et al. 60, and Paljakka et al. 61, respectively.