Rootstocks influence the salt tolerance of Kinnow mandarin trees by altering the antioxidant defense system, osmolyte concentration, and toxic ion accumulation

Rootstocks are a vital element of commercial citrus production. A varied level of salinity tolerance has been described for citrus rootstock cultivars, however, the effects of the rootstock can be scion-dependent. Our objective was to determine if rootstock selection has an influence on the salt-tolerance of 2-year-old Kinnow mandarin plants grafted onto two salt-sensitive (Carrizo citrange and Sanchton citrumello) and two salt-tolerant (Rangpur lime and Rubidoux) rootstock cultivars. Kinnow mandarin scion was grafted on these four rootstocks and irrigated with water containing 0, 30, 60, 90, or 120 mM NaCl. At the end of the experiment (90 days of salinization) growth, leaf gas exchange and chlorophyll fluorescence parameters, leaf and root Cl and Na concentrations, and parameters related to the antioxidant system were measured. Under salinity stress, plants grafted on salt-tolerant rootstocks had less reduction in plant biomass, total chlorophyll content, photosynthesis rate, stomatal conductance, and internal CO2 concentration as compared to the plants grafted on salt-sensitive rootstocks. Similarly, plants with salt-tolerant rootstocks had lower reactive oxygen species, suggesting enhanced activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase), and higher accumulation of osmolytes (proline and glycinebetaine). In addition, for plants with salt-tolerant rootstocks the levels of Na and Cl were higher in roots while lower in leaves; the opposite was the case for plants grafted on salt-sensitive rootstocks. Therefore, the selection of the rootstock can enhance the salt-tolerance potential by enhancing antioxidant activities, increasing concentrations of osmoprotectants, and limiting leaf levels of Na and Cl. Kinnow mandarin plants grafted on Rangpur lime or Rubidoux showed more salt tolerance than those on Carrizo citrange or Sanchton citrumello.