Table 4.

Comparison of estimated transport rates of sucrose by diffusion and by bulk flow through a specified range of plasmodesmal microchannel radii each with a length of 500 nm for reported sucrose concentrations (C) in SEs^a and vascular parenchyma cells^b of developing wheat grains.

Microchanne radius (nm)	Transport rates (× 10−10 nmol s−1) by:
	Diffusion at a ΔC (mM) of:		Bulk flow at a C (mM) of:
	200	400	450	600
0.5	1.64	3.28	0.22	0.14
1.0	6.53	13.1	1.77	2.36
1.5	14.7	29.4	8.95	11.9
2.0	26.2	52.3	28.2	37.7
4.0	105	209	453	603
8.0	418	837	7,241	9,655

Rates of sucrose diffusion were estimated using Fick's First Law^c (Equation 3) for two trans-plasmodesmal differences in sucrose concentration (ΔC). Bulk flow rates of sucrose were estimated as the product of estimated volume flow rates (see Table 3 and Equation 1) and SE sucrose concentrations (C)^a.

^aSucrose concentrations of SE sap collected from exuding pedicels of developing wheat grains ranged from 450 to 600 mM (Fisher and Gifford, 1986).

^bSucrose concentrations detected in frozen tissue slices of developing wheat grains containing vascular parenchyma cells ranged from 200 to 260 mM (Fisher and Wang, 1995). These concentrations are assumed to be representative of cytosolic sucrose concentrations in vascular parenchyma cells abutting SE/CC complexes. This assumption is an approximation as it respectively accepts that cytosolic and vacuolar sucrose concentrations are in equilibrium and that the sucrose concentration in all vascular parenchyma cells is identical.

^cD_sucrose is 0.52 × 10⁹ nm² s⁻¹ in water at 25 °C. ΔC values are derived from differences between SE^a and vascular parenchyma^b sucrose concentrations.