Photosynthetic physiology responses and cadmium distribution of wheat seedlings under cadmium stress

To investigate the photosynthetic physiology characteristics and cadmium distribution features of wheat seedlings in response to cadmium (Cd²⁺) stress, a hydroponic experiment was performed with a wheat cultivar, Zhengnong 4108 grown under different concentrations of Cd²⁺ (0, 10, 20, 40, 80,120 mg·L⁻¹). Wheat seedling phenotype, photosynthetic pigment content, photosynthetic parameters, chlorophyll fluorescence parameters and Cd subcellular distribution were investigated. The results showed that the photosynthesis of wheat seedlings in Zhengnong 4108 is mainly limited by nonstomatal factors. The growth of wheat seedlings is inhibited when the concentration of Cd²⁺ reaches ≥20 mg·L⁻¹. The germination potential, stem thickness, fresh weight, chlorophyll a (Ch_a) content, stomatal conductance (G_s), transpiration rate (T_r), maximum photochemical efficiency of leaves (F_v/F_m), potential activity of PSII (F_v/F_o), actual photochemical efficiency of PSII (ΦPSII), photochemical quenching coefficient (qP), and photosynthetic electron transfer rate (ETR) are significantly reduced compared with CK with increasing Cd²⁺concentration (P<0.05). When the concentration of Cd²⁺ reaches≥40 mg·L⁻¹, the damage to wheat seedlings deterioates. Compared with CK, root length, chlorophyll b (Chl_b) content, chlorophyll total (Chl_a+b), carotenoid (Car) content, and net photosynthetic rate (P_n) significantly decrease (P<0.05). When the concentration of Cd²⁺ reaches ≥80 mg·L⁻¹, the intercellular CO₂ concentration (Ci) significantly increases compared with CK, while the nonphotochemical quenching coefficient (NPQ) increases and then decreases with the increase of Cd²⁺ concentration. The content of Cd in root and leaf subcellular components of wheat seedlings increases with the increase of Cd²⁺ stress concentration. Cd in leaves is mainly concentrated in the cell wall, while Cd in roots is mainly concentrated in the cell fluid. Wheat seedlings of Zhengnong 4108 can cope with low concentration Cd stress and could be potentially adopted as a wheat cultivar in arable land with safe use.