Exogenous salicylic acid alleviates salt stress by improving leaf photosynthesis and root system architecture in cucumber seedlings

Exogenous salicylic acid (SA) can alleviate salt stress in plants. Previous researches have been focused on SA's role in leaf morphological and physiological characteristics, however, little information is available on its role in both leaf photosynthesis and root system architecture (RSA). A hydroponic study was conducted to analyze biomass, leaf photosynthesis, RSA parameters and expression of RSA genes in cucumber seedlings grown under four treatments, including control (CK), 50 mM NaCl (NaCl), 0.3 mM SA (SA) and 50 mM NaCl plus 0.3 mM SA (NaCl + SA). The results showed that compared with CK, both NaCl and SA treatments severely reduced relative growth rate (RGR) and leaf photosynthetic parameters (F-v/F-m, Phi(II) and Pn). However, compared with NaCl treatment, NaCl + SA treatment significantly increased RGR, leaf photosynthetic (F-v/F-m, Phi(II) and Pn) and RSA (total root length, root surface area, number and total length of primary and secondary LR) parameters. In addition, compared with CK, although both NaCl and SA treatments down-regulated expression of GL2 gene and up-regulated expression of RHD2 gene, NaCl + SA treatment notably up-regulated expression of NAC1, NAC2, GL2, EXP and down-regulated expression of RHD2. It is concluded that exogenous SA alleviated salt stress via effectively promoting leaf photosynthesis and RSA in cucumber seedlings. SA-mediated increment in expression of RSA genes, total length and number of primary and secondary LR could contribute to the improved RSA.