Evidence of sodium toxicity for the vegetative growth of maize (Zea mays L.) during the first phase of salt stress

Salt stress reduces plant growth, especially that of glycophytic plants such as maize (Zea mays L.). According to the two-phase model proposed by MUNNS (1993), the first phase of salt stress is caused by osmotic effects, which are postulated to be responsible for the growth reduction. In a second phase, Na+ accumulates and becomes toxic. In this study, we tested this model for a relatively salt-resistant maize genotype (Pioneer 3906) with particular attention to possible ionspecific effects apart from the osmotic stress during the first phase of salt stress. We induced osmotic stress in hydroponic culture by application of 100 mM NaCl or by equivalent osmotic potential adjusted with polyethyleneglycol (PEG 6000). The growth of the 4u, leaf 5, 7, and 9 d after stress application of both treatments was strongly reduced in comparison to their corresponding controls. This indicates a prevalence of osmotic stress in the first phase of growth reduction. In comparison to the PEG treatment, a significant reduction in leaf growth under NaCl treatment was observed, which reveals an additional ion effect. An experiment using various salt solutions (NaCl, Na₂SO₄ and MgCl₂ ) with equivalent ion concentrations and identical osmotic potential adjusted with PEG identified Na+ toxicity during the first phase of salt stress. In conclusion, we propose a modification of the two-phase model of salt stress for maize.