Nitric Oxide is Required for Aminolevulinic Acid-Induced Salt Tolerance by Lowering Oxidative Stress in Maize (Zea mays)

Although some investigations show that 5-aminolevulinic acid (ALA) participates in plant stress tolerance, the role of nitric oxide (NO) in ALA-induced improvement in tolerance to salt stress in plants necessitates to be understood. So, a NO scavenger, 0.1 mM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO); was applied jointly with ALA. Of four ALA concentrations tested in a preliminary experiment, 20 mg L−1 was chosen to apply as seed treatment or foliage spray to saline-stressed maize seedlings. Salinity (100 mM NaCl) significantly led to reduction in plant biomass, PSII maximum efficiency (Fv/Fm), total chlorophyll, leaf potassium (K+) and calcium (Ca2+) as well as leaf water potential in both cultivars, but it led to enhanced contents of proline, hydrogen peroxide (H2O2), malondialdehyde (MDA), electrolyte leakage (EL) and leaf sodium (Na+) as well as internal NO and nitric oxide synthase (NOS). ALA treatments improved plant growth, activities of antioxidant enzymes, and leaf Ca2+ and K+, but it led to further increases in leaf NO and NOS activity as well as ALA levels. ALA-induced salt tolerance was completely abolished by cPTIO application by eliminating the internal NO generation. This evidently showed that ALA triggered the synthesis of NO through activation of NOS, which in turn improved salt tolerance of the maize plants.