Radial and axial water transport in the sugar beet storage root

To evaluate the contribution of transcellular, apoplastic and symplasticpathways to water movements, horizontal (axial pathway) and vertical(radial pathway) sugar beet root (Beta vulgaris L.)slices were studied. Volume flows (Jv) were measuredunder hydrostatic and/or osmotic gradients, using a computer-baseddata-acquisition system. When tissues were tested under hydrostaticgradients (0.3 MPa m^-1) a much more importantpermeability was observed in the axial pathway, as compared with the radialone. Negative pressure gradients (tensions) were as effective as positiveones in inducing a net water movement. After the establishment of aconcentration gradient in the radial pathway (obtained by adding 300 Mm^-3 mannitol to the employed solution) an osmoticflux, sensitive to HgCI2, was observed. The inhibitory effect of mercurialcompounds was reversed by -mercaptoethanolwhile [¹⁴C] mannitol unidirectional fluxes were notaffected by mercurial agents. In the axial pathway, the presence of amannitol gradient did not develop a sustained osmotic flux. After aninitial Jv in the expected direction, theJv reversed and moved in the opposite way. It isconcluded that, in the sugar beet root, water channels play a significantrole in water transfers in the radial pathway. On the other side, water andsolutes are transported by a hydrostatic gradient in the xylem vessels. Ingeneral, these results extend and adapt to a storage root the 'compositetransport model' first proposed by Steudle etal.