Na+ Efflux from Apple Dwarfing Rootstocks is Associated with High-Salt Resistance of Their Scions

In apple (Malus) cultivation, the adaptability of rootstocks has a fundamental influence on multiple abiotic-stress resistance properties of their scions. Here, we investigated the high-salt resistance of apple rootstocks, by treating 2-year-old apple trees consisting of the ‘Fuji’ cultivar grafted onto three different dwarfing rootstocks (SH6, M9-T337 and G935) with a high-salt solution for 30 days. We found that the high-salt stress caused leaf yellowing and growth inhibition to all apple plants, and that the most serious reduction in relative chlorophyll content, plant height and biomass accumulation was associated with the SH6 rootstock. In addition, based on the salt injury index, the high-salt resistance of the ‘Fuji’ scion on the three dwarfing rootstocks was ranked as G935 > M9-T337 > SH6. In addition, analysis of mineral elemental composition showed that the Na+ content in ‘Fuji’ leaves on the G935 rootstock increased less than twofold compared to the control, whereas those on the SH6 rootstock increased nearly 20-fold, accompanied with reduced K and Fe element absorption under high-salt treatments. Expression analysis of nine Na+ transport-related genes indicated that in the salt overly sensitive (SOS) pathway, the SOS1, SOS2 and SOS3 genes showed considerably greater induction by high-salt stress in G935 roots than in SH6 roots. Given the Na+ content detected in roots of different rootstocks, we propose that more efficient Na+ extracellular efflux in G935 roots, rather than intracellular Na+ compartmentalization, contributed to the improved characteristics of the G935 rootstock under high-salt conditions.