Maize (Zea mays L.) responses to salt stress in terms of root anatomy, respiration and antioxidative enzyme activity

Abstract

Abstract Background: Salinity is a major abiotic stress factor that affects crop yields. Roots play an important role in salt stress in plants. There are few studies, however, that analyse the response of maize to salt stress in terms of the development of root anatomy and respiration. Results: We found that salt stress treatments exhibited a significantly lower leaf relative water content (RWC), net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), stomatal conductance (Gs), transpiration rate (Tr), and catalase (CAT) activity. Our results also showed that salt-induced phytotoxicity introduced by the stress treatment caused the superoxide dismutase (SOD) activity, peroxidase (POD) activity, malondialdehyde (MDA) content, Na+ uptake and root-to-shoot Na+ translocation rate to be higher than those in plants from the control groups. It should be noted that the detrimental effects of exposure to salt stress were more pronounced in plants of the YY7 variety than in those of the JNY658. Salt-tolerant JNY658 plants, which feature a relatively low metabolic cost for root formation, responded to salt stress by developing a much higher number of root cortical aerenchyma (RCA) than plants from the respective control group, as well as a larger cortical cell size (CCS) and a lower root cortical cell file number (CCFN), all of which help to maintain the formation of biomass when exposed to salt stress. The total respiration rate (RTotal) of plants of both varieties exposed to salt stress was lower than that of plants from the respective control groups, while the alternate oxidative respiration rate (RAOX) and RAOX/RTotal were higher, where the response in the roots of JNY658 plants was significant. The net Na+ and K+ efflux rates from the roots of plants of both varieties were all higher than those from the roots of plants from the control group, where the strength of the response with respect to the net Na+ efflux rate from the roots of JNY658 plants and the net K+ efflux rate from roots of YY7 plants was remarkable. The increase in efflux rates reduced the Na+ toxicity of the root and helped to maintain its ion balance. Conclusion: These results demonstrated that salt-tolerant maize varieties incur a relatively low metabolic cost required to establish a higher RCA, larger CCS and lower CCFN significantly reduced their RTotal and, that it also increased their RAOX and RAOX/RTotal, thereby counteracting the detrimental effect of oxidative damage on root respiration of root growth. In addition, when subjected to salt stress they exhibit a lower Na+ ion uptake at the surface of their roots, which constrains the translocation of Na+ to the rest of the plant and significantly reduces the level of Na+ accumulation in leaves, thus preempting salt-stress induced impediments to the formation of shoot biomass.