Table 3.

Impact of differing plasmodesmal (pd) microchannel radii^a, predicted by the Hagen-Poiseuille Law (see Equation 1), on plasmodesmal numbers required to support the observed volume flow rate (R_v)^b of phloem sap, of specified viscosity^c, symplasmically unloaded from SE/CC complexes to vascular parenchyma cells of developing wheat grains at a transcellular hydrostatic pressure differential of 1.0 MPa across the interconnecting plasmodesmata of known length^d.

Microchannel radius (nm)	PredictedeRv per microchannel (× 104 nm3 s−1)	Microchannel numbers supporting observed Rv (× 104)f	Microchannel numbers per pdg	Pd numbers supporting observed Rv (× 103)h
0.5	2.46	472,313	33	143,125
1.0	39.3	29,524	17	17,367
1.5	199	5832	11	5302
2.0	629	1845	8	2306
4.0	10,057	115	4	288
8.0	160,916	7	2	35.0
10.0	392,900	3	1	10.0

Plasmodesmal number estimates are compared with an observed value for developing wheat grains at the SE/CC complex and vascular parenchyma cell interface of 44,000 × 10³ per grain (Wang et al., 1995).

^aEstimates of microchannel radii of plasmodesmata (for details, see Section Is Symplasmic Sieve Element Unloading Dominated by Bulk Flow?)

^bObserved by volume flow rate (R_v) of 11.6 × 10¹³ nm³ s⁻¹ (Fisher, 1990).

^cAveraged phloem sap viscosity of 2 × 10⁻⁹ MPa s (Mullendore et al., 2010).

^dCell wall thickness and hence plasmodesmatal length between SE/CC complexes and vascular parenchyma cells in developing wheat grains—500 nm (Fisher and Cash-Clark, 2000a).

^eR_v predicted using the nominated data sets in the Hagen-Poiseuille Law (Equation 1).

^fMicrochannel numbers supporting observed R_v derived as the ratio of observed by volume flow^a to predicted flow rate per microchannel^e.

^gMicrochannel numbers per plasmodesmata were estimated assuming that their equators (diameters) are positioned on a circumference of half the internal radius (i.e., 10.5 nm—Wang et al., 1995) of plasmodesmata interconnecting SE/CC complexes to vascular parenchyma cells and that they occupy 50% of this circumference.

^hPlasmodesmal numbers supporting the observed R_v derived as the ratio of microchannel numbers supporting observed R^f_v and microchannel numbers per plasmodesmata^g.