Effect of exogenous selenium supply on photosynthesis, Na+ accumulation and antioxidative capacity of maize (Zea mays L.) under salinity stress

Abstract

The mechanism of selenium-mediated salt tolerance has not been fully clarified. This study investigated the possible role of selenium (Se) in regulating maize salt tolerance. A pot experiment was conducted to investigate the role of Se (0, 1, 5 and 25 μM Na₂SeO₃) in photosynthesis, antioxidative capacity and ion homeostasis in maize under salinity. The results showed that Se (1 μM) relieved the salt-induced inhibitory effects on the plant growth and development of 15-day-old maize plants. Se application (1 μM) also increased the net photosynthetic rate and alleviated the damage to chloroplast ultrastructure induced by NaCl. The superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities were increased, and ZmMPK5, ZmMPK7 and ZmCPK11 were markedly up-regulated in the roots of Se-treated plants, likely contributing to the improvement of antioxidant defence systems under salinity. Moreover, 1 μM Se increased K⁺ in the shoots while decreasing Na⁺ in the roots, indicating that Se up-regulates ZmNHX1 in the roots, which may be involved in Na⁺ compartmentalisation under salinity. The findings from this single experiment require repetition together with measurement of reactive oxygen species (ROS), but nevertheless suggest that exogenous Se alleviates salt stress in maize via the improvement of photosynthetic capacity, the activities of antioxidant enzymes and the regulation of Na⁺ homeostasis.

Figure 1

Effects of Se on plant height (a), leaf length (b), dry weight (c,d), and growth performance (e) of non-stressed or salt-stressed maize plants. The control plants (CK) were cultured in half-strength Hoagland’s solution. Different treatments were added with or without different concentrations of Se or NaCl. The abbreviations 1 Se, 5 Se and 25 Se indicate 1 μM Na₂SeO₃, 5 μM Na₂SeO₃ and 25 μM Na₂SeO₃, respectively (details are shown in the Materials and Methods). Data are presented as the means ± SDs (n = 18). Columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 2. Effects of Se on the chloroplast ultrastructure of leaves of non-stressed or salt-stressed maize plants.

The control plants (CK) were cultured in half-strength Hoagland’s solution. Different treatments were added with or without different concentrations of Se or NaCl. The abbreviations 1 Se, 5 Se and 25 Se indicate 1 μM Na₂SeO₃, 5 μM Na₂SeO₃ and 25 μM Na₂SeO₃, respectively (details are shown in the Materials and Methods).

Figure 3

Effects of Se on electrolyte leakage (a) and LPO levels (b) in the leaves of non-stressed or salt-stressed maize plants. The control plants (CK) were cultured in half-strength Hoagland’s solution. Different treatments were added with or without different concentrations of Se or NaCl. The abbreviations 1 Se, 5 Se and 25 Se indicate 1 μM Na₂SeO₃, 5 μM Na₂SeO₃ and 25 μM Na₂SeO₃, respectively (details are shown in the Materials and Methods). Data are presented as the means ± SDs (n = 3). Columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 4

Effects of Se on SOD (a), CAT (b) and APX (c) activity levels in the leaves of non-stressed or salt-stressed maize plants. The control plants (CK) were cultured in half-strength Hoagland’s solution. Different treatments were added with or without different concentrations of Se or NaCl. The abbreviations 1 Se, 5 Se and 25 Se indicate 1 μM Na₂SeO₃, 5 μM Na₂SeO₃ and 25 μM Na₂SeO₃, respectively (details are shown in the Materials and Methods). Data are presented as the means ± SDs (n = 3). Columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 5

Real-time quantitative PCR analysis of ZmMPK5, ZmMPK7 and ZmCPK11 mRNA accumulation in the roots of maize plants treated with NaCl (0 and 100 mM) and Se (0 and 1 μM) for 2 h (a) or 24 h (b). CK: half-strength Hoagland’s solution; NaCl: half-strength Hoagland’s solution + 100 mM NaCl; 1 Se: half-strength Hoagland’s solution + 1 μM Na₂SeO₃; NaCl + 1 Se: half-strength Hoagland’s solution + 100 mM NaCl + 1 μM Na₂SeO₃. Data are presented as the means ± SDs (n = 3). For each gene expression, columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 6

Real-time quantitative PCR analysis of ZmNHX1 mRNA accumulation in the roots of maize plants treated with NaCl (0 and 100 mM) and Se (0 and 1 μM) for 2 h (a) or 24 h (b). CK: half-strength Hoagland’s solution; NaCl: half-strength Hoagland’s solution + 100 mM NaCl; 1 Se: half-strength Hoagland’s solution + 1 μM Na₂SeO₃; NaCl + 1 Se: half-strength Hoagland’s solution + 100 mM NaCl + 1 μM Na₂SeO₃. Data are presented as the means ± SDs (n = 3). Columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 7. Effects of exogenous Se on Se content in the shoots and roots of non-stressed or salt-stressed maize plants.

The control plants (CK) were cultured in half-strength Hoagland’s solution. Different treatments were added with or without different concentrations of Se or NaCl. The abbreviations 1 Se, 5 Se and 25 Se indicate 1 μM Na₂SeO₃, 5 μM Na₂SeO₃ and 25 μM Na₂SeO₃, respectively (details are shown in the Materials and Methods). Data are presented as the means ± SDs (n = 3). Columns labelled with different letters between treatments represent significant differences (p < 0.05).

Figure 8. Schematic representation of the positive role of Se on salt tolerance of maize.

In the present study, a model was developed to show that photosynthesis, antioxidant defence systems and Na⁺ accumulation were regulated by Se in maize under salt stress. The blue arrows (↑) and the red arrows (↓) represent the positive and passive role of Se, respectively.