Radial and axial water transport in the sugar beet storage root

To evaluate the contribution of transcellular, apoplastic and symplastic pa thways to water movements, horizontal (axial pathway) and vertical (radial pathway) sugar beet root (Beta vulgaris L.) slices were studied. Volume flo ws (J(v)) were measured under hydrostatic and/or osmotic gradients, using a computer-based data-acquisition system, When tissues were tested under hyd rostatic gradients (0.3 MPa m(-1)) a much more important permeability was o bserved in the axial pathway, as compared with the radial one. Negative pre ssure 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 employe d solution) an osmotic flux, sensitive to HgCl2, was observed. The inhibito ry effect of mercurial compounds was reversed by beta-mercaptoethanol while [C-14] mannitol unidirectional fluxes were not affected by mercurial agent s. In the axial pathway, the presence of a mannitol gradient did not develo p 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 trans fers in the radial pathway. On the other side, water and solutes are transp orted by a hydrostatic gradient in the xylem vessels. In general, these res ults extend and adapt to a storage root the 'composite transport model' fir st proposed by Steudle et al.