Davood Khoshbakht

Effect of exogenous spermidine and nitric oxide on growth and gas exchange parameters in Iranian mandarin Bakraii (Citrus reticulata × Citrus limetta) under salinity stress

A greenhouse study was conducted to evaluate the effect of polyamines and nitric oxide on improving the resistance of citrus [(Bakraii (Citrus reticulata × Citrus limetta)] against salt stress in three separate experiments [first experiment: the role of various polyamines (Putrescin, spermin and spermidine) at 0 (control), 0.5 and 1 mM; second experiment: nitric oxide (sodium nitroprusside as nitric oxide donor) at 0 (control), 0.25, 0.5 and 1 mM concentrations; third experiment included the most effective polyamine and sodium nitroproside obtained from the first and second experiment under salt stress conditions as foliar applications. The results showed that with increasing salinity level, leaf number, plant height, leaf area, fresh and dry weight, relative leaf water content, total protein content, soluble sugars, starch, proline, Ca²⁺, Mg²⁺ and K⁺ in leaves and roots, leaf chlorophyll content, chlorophyll fluorescence rate, gas exchange parameters: net CO₂ assimilation rate, stomata conductance and transpiration) were decreased. Salinity increased Na⁺, Cl^–, proline, ion leakage, catalase, superoxide dismutase, ascorbate peroxidase and peroxidase, enzyme activity, hydrogen peroxide and peroxidation rate of lipids. The results of the first and second experiments showed that the best type of polyamine was spermidine and the best concentration for spermidine and sodium nitroprusside were 0.5 mM. SNP and Spd considerably reduced  the leaf Na⁺ and Cl^– concentartion, but improved those of Ca²⁺, Mg²⁺ and K⁺ in the salt stressed plants. Exogenous SNP and Spd induced growth improvement in citrus plants reduced electrolyte leakage, malondialdehyde, hydrogen peroxide levels, and increased relative water content, chlorophyll fluorescence parameters, induced activities of some key antioxidant enzymes such as ascorbate peroxidase, catalase, superoxide dismutase and peroxidase, photosynthetic rate, intercellular CO₂ concentration, stomatal conductance and transpiration rate under saline stress. The present study showed that foliar application of SNP and Spd alone or in combination mitigated the adverse effects of salinity, while the combined of two compounds proved to be even more effective in alleviating the adverse effects of NaCl stress.