Insights into physio-biochemical responses of maize to salt shock stress and removal of the stress at the whole-plant level

Maize (Zea mays) is one of the world's three major food crops but sensitive to salinity at the seedling stage. Salinity/salt stress usually occurs due to gradually increased NaCl or under sudden exposure to NaCl, of which the latter is called salt shock (SS). However, little is known about physio-biochemical responses of maize to SS at the whole-plant level. The purpose of this study was to characterize the physio-biochemical response events of maize under SS. The experiments were conducted with four maize foundation parent inbred lines of Huangzao4, Chang7-2, Zheng58 and Ye478 under 150 mM NaCl for SS and after removal of SS in the nutrient solutions. The main findings were that the maize lines had no clear phase-order-response to SS, which suffered from the combined effects of osmotic stress, water deficiency, and Na⁺ accumulation-induced toxicity once SS occurred, and that SS-tolerant maize lines showed (1) timely increased activities of antioxidant enzymes (SOD, POD, CAT and APX) and stronger superoxide anion radical-mediated signalling in roots at the beginning of SS, (2) a slow Na⁺ transport rate from roots to shoots especially in the early SS stage, and (3) opening of leaf stomata, and fine cell membrane integrity during SS. The related mechanisms of SS tolerance of maize were proposed and discussed.