Arbuscular Mycorrhizal Fungi Influence the Response of Citrus Rootstock Seedlings to Salinity

Arbuscular mycorrhizal (AM) symbiosis is thought to increase host resistance to salinity stress, a characteristic that could be interesting in areas where the scarcity of irrigation waters forces growers to use low-quality irrigation water. To test this hypothesis seedlings of the rootstocks Mandarin Cleopatra (Citrus reshni Hort. ex Tan.), Sour orange (Citrus aurantium (L.)) and Alemow (Citrus macrophylla Wester) were grown in a growth chamber and inoculated with a mixture of two AM fungi (Glomus intraradices and Glomus mosseae) (IP), or left non-inoculated (NIP). From forty-five days after fungal inoculation onwards plants were irrigated with nutrient solution containing 50 mM NaCl and, three months after inoculation, the growth, mineral nutrition and physiological response were analysed. AM fungi significantly increased all plant growth parameters studied, especially in the Cleopatra and Alemow rootstocks. In general, plant growth parameters were higher in salinized IP plants than in non-salinized NIP plants, demonstrating that AM colonization compensates for the growth limitations imposed by saline conditions. The water content was higher in IP plants of Cleopatra and Alemow but not in the Sour orange seedlings. Under saline conditions NIP Alemow plants had a strongly-decreased water content, while the water content of IP Alemow plants was similar to that of non-salinised plants. In all three rootstocks, NIP plants showed a greater degree of salt-induced foliar damage and chlorosis than IP plants. Although IP plants were not fertilised with phosphorus in the experiment, they had significantly higher levels of this nutrient in roots, stems and leaves than NIP plants both in salinised and control plants. The beneficial effect of mycorrhization appears to be unrelated with protection against the uptake of excess of Na+ or Cl- by the plant. Our findings confirm that AM fungi can alter host responses to salinity stress, improving the P nutrition and diminishing chlorosis and salt damage.