Chloride transport and compartmentation within main and lateral roots of two grapevine rootstocks differing in salt tolerance

Abstract

Root Cl⁻transport was investigated using³⁶Cl⁻flux analysis in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt-tolerant) and K 51–40 (salt sensitive). Initial³⁶Cl⁻influx to the root was greater in Paulsen than K 51–40. This flux, attributed to the Cl⁻influx to the cytoplasm (Φₒc) increased with increasing external concentrations of Cl⁻for plants adapted to growth in 30 mM NaCl. The concentration kinetics in this high concentration range could be fit to a Michaeils–Menton equation. There was no significant difference between genotypes in Km (28.68 ± 15.76 and 24.27 ± 18.51 mM for Paulsen and K 51–40, respectively), but Paulsen had greater Vₘₐₓ(0.127 ± 0.042) compared to K 51–40 (0.059 ± 0.026 μm g⁻¹ FW min⁻¹). In Paulsen, the main root had greater contribution to³⁶Cl⁻uptake than lateral roots, there being no significant difference in lateral root influx between the genotypes.³⁶Cl⁻transport to the shoot of K 51–40 was greater than for Paulsen. It was estimated that efflux rate from the xylem parenchyma cells to the xylem vessels (Φcₓ) in K 51–40 was twice that of Paulsen. Compartmental analysis from³⁶Cl⁻efflux kinetics confirmed the larger Φₒcand the higher ratio of main to lateral root Φₒcfor Paulsen. Efflux from the cytoplasm (Φcₒ) was higher than 95 % of Φₒcindicating a high degree of cycling across the plasma membrane in roots at these high external Cl⁻concentrations. Paulsen appears to keep the cytoplasmic Cl⁻concentration in roots lower than K 51–40 via greater efflux to the vacuole and to the outside medium. The difference in salt tolerance between the genotypes can be attributed to different Cl⁻transport properties at the plasma membrane and tonoplast and particularly in Cl⁻ efflux to the xylem.