Radial and axial water transport in the sugar beet storage root

To evaluate the contribution of transcellular, apoplastic and symplastic pathways to water movements, horizontal (axial pathway) and vertical (radial pathway) sugar beet root (Beta vulgaris L.) slices were studied. Volume flows (J(v)) were measured under hydrostatic and/or osmotic gradients, using a computer-based data-acquisition system, When tissues were tested under hydrostatic gradients (0.3 MPa m(-1)) a much more important permeability was observed in the axial pathway, as compared with the radial one. Negative pressure gradients (tensions) were as effective as positive ones in inducing a net water movement. After the establishment of a concentration gradient in the radial pathway (obtained by adding 300 M m(-3) mannitol to the employed solution) an osmotic flux, sensitive to HgCl2, was observed. The inhibitory effect of mercurial compounds was reversed by beta-mercaptoethanol while [C-14] mannitol unidirectional fluxes were not affected by mercurial agents. In the axial pathway, the presence of a mannitol gradient did not develop a sustained osmotic flux. After an initial J(v) in the expected direction, the J(v) reversed and moved in the opposite way. It is concluded that, in the sugar beet root, water channels play a significant role in water transfers in the radial pathway. On the other side, water and solutes are transported by a hydrostatic gradient in the xylem vessels. In general, these results extend and adapt to a storage root the 'composite transport model' first proposed by Steudle et al.