Root apoplastic transport and water relations cannot account for differences in Cl- transport and Cl-/NO3 - interactions of two grapevine rootstocks differing in salt tolerance

Grapevine is moderately sensitive to salinity and accumulation of toxic levels of Cl- in leaves is the major reason for salt-induced symptoms. In this study, apoplastic Cl- uptake and transport mechanism(s) were investigated in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt tolerant) and K 51-40 (salt sensitive). Increased external salinity caused high Cl- accumulation in shoots of the salt sensitive K 51-40 in comparison to Paulsen. Measurement of (NO3)-N-15 (-) net fluxes under high salinity showed that by increasing external Cl- concentrations K 51-40 roots showed reduced NO3 (-) accumulation. This was associated with increased accumulation of Cl-. In comparison to Paulsen, K 51-40 showed reduced NO3 (-)/Cl- root selectivity with increased salinity, but Paulsen had lower selectivity over the whole salinity range (0-45 mM). To examine if root hydraulic and permeability characterisations accounted for differences between varieties, the root pressure probe was used on excised roots. This showed that the osmotic Lp(r) was significantly smaller than hydrostatic Lp(r), but no obvious difference was observed between the rootstocks. The reflection coefficient (sigma) values (0.48-0.59) were the same for both rootstocks, and root anatomical studies showed no obvious difference in apoplastic barriers of the main and lateral roots. Comparing the uptake of Cl- with an apoplastic tracer, PTS (3-hydroxy-5,8,10-pyrentrisulphonic acid), showed that there was no correlation between Cl- and PTS transport. These results indicated that bypass flow of salts to the xylem is the same for both rootstocks (0.77 +/- A 0.2 and 1.05 +/- A 0.12 %) and hence pointed to differences in membrane transport to explain difference in Cl- transport to the shoot.