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

Abstract

Root Cl⁻ transport was investigated using ³⁶Cl⁻ flux analysis in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt-tolerant) and K 51–40 (salt sensitive). Initial ³⁶Cl⁻ influx to the root was greater in Paulsen than K 51–40. This flux, attributed to the Cl⁻ influx to the cytoplasm (Φ ₒc) 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 V ₘₐₓ (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 to ³⁶Cl⁻ 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 (Φ cₓ) in K 51–40 was twice that of Paulsen. Compartmental analysis from ³⁶Cl⁻ efflux kinetics confirmed the larger Φ ₒc and the higher ratio of main to lateral root Φ ₒc for Paulsen. Efflux from the cytoplasm (Φ cₒ) was higher than 95� % of Φ ₒc indicating 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.