Rates of apoplastic diffusion in wheat leaves

summary Since the movement of water in leaves beyond the veins is not apoplastic, solutes excluded from the symplast move in the apoplast by diffusion. Estimates of the diffusivities (D) of sulphorhodamine G (SR) in the apoplast of wheat leaves are presented, measured by the analysis of advancing profiles of fluorescence of SR over a range of times from 7 to 90 min. The greatest values of D were found from vein sheaths, over the surface of fibre extensions where present, to and along the inner periclinal walls of the epidermises. The value along the lower epidermis (D = 8 × 1(T8 cm2 s1) was consistently twice that in the upper epidermis. SR diffused radially into the fibre walls of the vein extensions at a much lower rate (D= 10−10 cm2 s−1). At these rates the visible fluorescence occupied much of the apoplast between the veins after about 1 h. These findings suggest that useful generalizations may be gained from investigations of the spread of natural solutes of the transpiration stream in cell wall material.