Improved salt-tolerance in Citrus macrophylla mutant rootstocks

Increasing salinity tolerance is a major goal in citrus breeding programs. The difficulties of preventing salt damage through traditional breeding methods has given rise to alternative techniques such as mutagenesis by gamma rays. In previous works, five mutants (MM3B, MM4B, MM5B, MM2A and MM3A) from Citrus macrophylla, obtained through the exposure of seeds to gamma radiation, were selected for their ability to cope with salt stress in vitro. In the present experiment, the selected mutants, along with Citrus macrophylla shoots as control, were submitted to 60 mM NaCl (MM3B, MM4B and MM5B mutants) or to 80 mM NaCl (MM2A and MM3A), in their respective culture media. After 8 weeks of exposure, proliferation rates, shoot length and damaged leaves were recorded to assess the effects produced by salinity in the culture medium. Also, full ex-plants were stored for further physiological analysis. Chlorophylls, soluble sugars, starch, proline and the nutrient composition were quantified. In the saline conditions assayed, all the mutants showed better productivity than Citrus macrophylla. Furthermore, MM5B showed less leaf damage and higher proline levels in the saline stress conditions than Citrus macrophylla. The amount of toxic ions increased in the salinity conditions assayed, although, in light of the productivity and observation of damage, the mutants were less sensitive to their effect. Furthermore, NO3- accumulation of was also higher in mutant plants than in Citrus macrophylla in the saline conditions, which conferred an additional advantage against stress.