Distinct physiological responses of tomato and cucumber plants in silicon-mediated alleviation of cadmium stress

Abstract

The alleviative effects of silicon (Si) on cadmium (Cd) toxicity were investigated in cucumber (Cucumis sativus L.) and tomato (Solanum lycopersicum L.) grown hydroponically. The growth of both plant species was inhibited by 100 μM Cd, but Si application counteracted the adverse effects on growth. Si application significantly decreased the Cd concentrations in shoots of both species and roots of cucumber. The root-to-shoot transport of Cd was depressed by added Si in tomato whereas it was increased by added Si in cucumber. The total content of organic acids was decreased in tomato leaves but increased in cucumber roots and leaves by Si application under Cd stress. Si application also increased the cell wall polysaccharide levels in the roots of both species under Cd toxicity. Si-mediated changes in levels of organic acids and cell wall polysaccharides might contribute to the differences in Cd transport in the two species. In addition, Si application also mitigated Cd-induced oxidative damage in both species. The results indicate that there were different mechanisms for Si-mediated decrease in shoot Cd accumulation: in tomato, Si supply decreased root-to-shoot Cd transport; whereas in cucumber, Si supply reduced the Cd uptake by roots. It is suggested that Si-mediated Cd tolerance is associated with different physiological responses in tomato and cucumber plants.

Keywords: Cd transport; antioxidant defense; cadmium; cell wall polysaccharide; organic acid; silicon.

FIGURE 1

Effects of silicon (Si) on dry weights of root (A) and shoot (B), the second leaf area (C) and total number of leaves (D) in tomato and cucumber plants under cadmium (Cd) stress. Bars with different letters means significant difference at P < 0.05 according to one-way ANOVA. Lower-case and upper-case letters indicate significance for tomato and cucumber, respectively.

FIGURE 2

Effects of Si on changes in fourier transform infrared spectra of root cell walls in tomato (A) and cucumber (B) under Cd stress.

FIGURE 3

Fluorescence of leaves as affected by Cd and PTS treatments.

FIGURE 4

Effects of Si on TBARS and H₂O₂ contents in leaves (A,C) and roots (B,D) in tomato and cucumber under Cd stress. Bars with different letters means significant difference at P < 0.05 according to one-way ANOVA (Lower-case and upper-case letters indicate significance for tomato and cucumber, respectively). TBARS, thiobarbituric acid reactive substances.

FIGURE 5

Effects of Si on the activities of SOD (A,E), CAT (B,F), APX (C,G) and GR (D,H) in leaves (A–D) and roots (E–H) in tomato and cucumber under Cd stress. Bars with different letters mean significant difference at P < 0.05 according to one-way ANOVA (Lower-case and upper-case letters indicate significance for tomato and cucumber, respectively). SOD, superoxide dismutase; CAT, catalase; APX, ascorbate peroxidase; GR, glutathione reductase.

FIGURE 6

Effects of Si on GSH (A,B) and AsA (C,D) contents in plants under Cd stress. (A) GSH content in leaves; (B) GSH content in roots; (C) AsA content in leaves; (D) AsA content in roots. Bars with different letters mean significant difference at P < 0.05 according to one-way ANOVA (Lower-case and upper-case letters indicate significance for tomato and cucumber, respectively).