A comparative study on salt resistance and Na+ transport characteristics of two apple dwarfing rootstocks

【Objective】This study aimed to clarify the differences in Na+ distribution and transport between salt-resistant and -sensitive dwarfing rootstocks of apples under high-salt stress, so as to provide reference for exploring the salt resistance mechanism and breeding of resistant apple dwarfing rootstocks.【Methods】Two-year-old plants of salt-resistant dwarfing apple rootstock G935 (Malus domestica) plants, salt- sensitive dwarfing rootstock SH6 (M. honanensis), and their double- rootstock grafted Fuji (DRG) were used as experimental materials. Branches were removed, and plants with basically the same growth vigor were selected. Potted plant irrigated with deionized water were used as the control, and those irrigated with a 200 mmol·L-1 NaCl solution were for high-salt stress treatment. The salt damage of each plant were observed. The relative chlorophyll content (RCC), net photosynthetic rate (Pn), relative growth rate (RGR), and the contents of Na + and K+ and the Na + /K+ ratio in different tissues (root, stem base, stem and leaf tissues) were measured. Moreover, Na + staining was performed on root slices of the two rootstocks, and the transport and distribution characteristics of Na+ were comparatively analyzed.【Results】Following 21 days of exposure to high- salt stress treatment, the leaf margins of SH6 plants exhibited characteristic symptoms of salt injury, including leaf chlorosis, marginal scorching and partial leaf shedding. These morphological symptoms were accompanied by a significant decline in key physiological parameters, e.g. RCC, Pn, and RGR. In contrast, the leaves of G935 plants showed no obvious salt damage and the RCC and Pn did not significantly decrease, but their relative growth rate significantly decreased. The DRG plants displayed a pronounced bilateral asymmetry in response to salt stress. The SH6- side leaves of the DRG plants developed severe browning and abscission, and their RCC and Pn were significantly lower than those of the G935 side leaves. The detection of Na+ content in different tissues showed that under the salt stress treatment, the Na + content in the roots of SH6 plants was lower than that of G935 plants, while the Na+ content in the leaves of SH6 plants was 7.5 times that in the leaves of G935 plants, and the Na+ content in the leaves of the SH6 side of DRG plants was 4.5 times that in the leaves of the G935 side. Analysis of K+ contents showed that in SH6 plants no significant change was observed in the root and stem base tissues compared to the control, whereas a significant decrease was detected in the stem and leaf tissues. In G935 plants, K+ was redistributed under salt stress, with a significant decrease in the roots but no significant change in the stem base and stem. Notably, this was associated with a significant accumulation of K+ in the leaves. In the SH6 sector of DRG plants, K+ was relatively constant in the roots and scion stem, but there was a significant reduction in the stem base and leaves, particularly, the K+ content in the leaves, which decreased by 23.16% compared to the control. However, all of the root, stem base, and scion stem tissues exhibited a significant decrease in K+ content in G935 rootstock, with reductions of 8.07%, 13.56%, and 15.00%, respectively; while no significant change was recorded in the leaves. The observation of Na+ staining in root tip sections showed that the Na + fluorescence intensity of the root tips of the G935 rootstock was 1.71 times that of the SH6 rootstock before the salt stress treatment. After the salt stress treatment, the fluorescence intensity of the SH6 rootstock was significantly higher than that of the G935 rootstock. Moreover, compared with the G935 rootstock, the Na + in the root tips of the SH6 rootstock was mainly distributed in the middle column tissue.【Conclusion】Dwarfing rootstock G935 exhibited a high salt tolerance with remarkably poorer mobility of Na+ from the roots to the above-ground portion and a lower Na+ /K+ ratio, while in salt-sensitive dwarfing rootstock SH6, Na+ penetrates more through the endodermis and enters the vascular tissue in the stele, and is thus more transported to the above-ground part, which might be the primary factor accounting for the difference in salt resistance between the two apple dwarfing rootstocks.