Growth, ionic homeostasis, and physiological responses of cotton under different salt and alkali stresses

Abstract

To better understand the mechanism of salt tolerance, we analyzed cotton growth and the ionomes in different tissues under different types of salt-alkali stress. Cotton was exposed to the soil salt and alkali stresses, NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃, in a pot study. Salt and alkali stress significantly inhibited cotton growth, significantly reduced root length, surface area, and volume, and significantly increased relative electrical conductivity (REC) and malondialdehyde (MDA) content but also significantly increased antioxidant enzyme activities, and proline (Pro) content. The REC in leaves was higher under salt stress than under alkali stress, but the effects on Pro were in the order Na₂CO₃ + NaHCO₃ > NaCl > Na₂SO₄. Principal component analysis showed a significant difference in ion composition under the different types of salt-alkali stress. Under the three types of salt-alkali stress, concentrations of Na and Mo increased significantly in different organs of cotton plants. Under NaCl stress, the absorption of Ca was inhibited, the transport capacity of P, Mg, and Cu was reduced, and the ion balance was maintained by promoting the uptake and transport of Zn, Mn, Al, and Mo. Under Na₂SO₄ stress, the absorption of P and Ca was inhibited, the transport capacity of Mg, B, and Cu was reduced, and the ion balance was maintained by promoting the uptake and transport of S, Zn, Fe, Mo, Al, and Co. Under Na₂CO₃ + NaHCO₃ stress, the absorption of P and S was inhibited, the transport capacity of Mg and B was reduced, but that of Al and Fe increased, and the ion balance was maintained by promoting the uptake and transport of Mn, Mo, Ni, and Co. The relative expression of GhSOS1 and GhNHX1 in leaves increased significantly under salt stress but decreased under alkali stress. These results suggest that cotton is well-adapted to salt-alkali stress via the antioxidant enzyme system, adjustment of osmotic substances, and reconstruction of ionic equilibrium; neutral salt stress primarily disrupts the ion balance, whereas alkali stress decreases the ability to regulate Na and inhibits the absorption of mineral elements, as well as disrupts the ion balance; and the changes in the expression of salt tolerance-related genes may partially explain the accumulation of Na ions in cotton under salt-alkali stress.

Figure 1

Leaf, stem, root, and total biomass (g/plant) of cotton plants under different types of salt–alkali stress: (a) NaCl (CS), (b) Na₂SO₄ (SS), and (c) Na₂CO₃ + NaHCO₃ (AS). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (**p < 0.01).

Figure 2

Root length (cm), surface area (cm²), and volume (cm³) of cotton plants under different types of salt–alkali stress (NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (*p < 0.05; **p < 0.01). (a–c) indicate the root length in NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (d–f) indicate the surface area in NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (g–i) indicate the root volume in NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively.

Figure 3

Malondialdehyde (MDA) content (nmol/g fresh weight (FW)) and relative electrical conductivity (REC) (dS/m) in cotton leaves under different types of salt–alkali stress (NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (**p < 0.01). (a–c) indicate the MDA content in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (d–f) indicate the REC in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively.

Figure 4

Activities (U/g fresh weight (FW)) of the antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in cotton leaves under different types of salt–alkali stress (NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (*p < 0.05; **p < 0.01). (a–c) indicate the SOD in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (d–f) indicate the POD in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (g–i) indicate the CAT in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively.

Figure 5

Proline content (U/g fresh weight (FW)) in cotton leaves under different types of salt–alkali stress (NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (**p < 0.01). (a–c) indicate the Proline content in cotton leaves under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively.

Figure 6

Principal component analysis (PCA) of tissue ionome variation in cotton under different types of salt–alkali stress (Control (CK), NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)), with the loadings of mineral elements to the PC1 and PC2. (a) Leaf ionome variation among samples and the loadings of elements to the PC1 and PC2; (b) Stem ionome variation among samples and the loadings of elements to the PC1 and PC2; (c) Root ionome variation among samples and the loadings of elements to the PC1 and PC2.

Figure 7

Hierarchical cluster analysis of the leaf ionomes in cotton plants under different types of salt–alkali stress (Control (CK), NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). The relative values are indicated by color intensity in the legend in the upper right.

Figure 8

Hierarchical cluster analysis of the stem ionomes in cotton plants under different types of salt–alkali stress (Control (CK), NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). The relative values are indicated by color intensity in the legend in the upper right.

Figure 9

Hierarchical cluster analysis of the root ionomes in cotton plants under different types of salt–alkali stress (Control (CK), NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). The relative values are indicated by color intensity in the legend in the upper right.

Figure 10

Leaf, stem, and root K/Na ratios of cotton plants under different types of salt–alkali stress: (a) NaCl (CS), (b) Na₂SO₄ (SS), and (c) Na₂CO₃ + NaHCO₃ (AS). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (**p < 0.01).

Figure 11

Pearson’s coefficients of correlation between the element Na and other elements in cotton leaves under different types of salt–alkali stress. (a) NaCl; (b) Na₂SO₄; and (c) Na₂CO₃ + NaHCO₃. Blue, negative correlation; red, positive correlation.

Figure 12

Pearson’s coefficients of correlation between the element Na and other elements in cotton stems under different types of salt–alkali stress. (a) NaCl; (b) Na₂SO₄; and (c) Na₂CO₃ + NaHCO₃. Blue, negative correlation; red, positive correlation.

Figure 13

Pearson’s coefficients of correlation between the element Na and other elements in cotton roots under different types of salt–alkali stress. (a) NaCl; (b) Na₂SO₄; and (c) Na₂CO₃ + NaHCO₃. Blue, negative correlation; red, positive correlation.

Figure 14

Relative expression (RQ) of the genes GhSOS1 and GhNHX1 in leaf and root of cotton under different types of salt–alkali stress (NaCl (CS), Na₂SO₄ (SS), and Na₂CO₃ + NaHCO₃ (AS)). Columns with bars represent the mean ± standard error (n = 3). Asterisks indicate a significant difference between the control (CK) and the salt–alkali stress (*p < 0.05; **p < 0.01), and ns indicates no significant difference. (a–c) indicate the GhSOS1 gene relative expression in cotton leaf under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (d–f) indicate the GhNHX1 gene relative expression in cotton leaf under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (g–i) indicate the GhSOS1 gene relative expression in cotton root under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively. (j–l) indicate the GhNHX1 gene relative expression in cotton root under NaCl, Na₂SO₄, and Na₂CO₃ + NaHCO₃ stress, respectively.