Effect of magnetized water treatment on photosynthetic and growth characteristics of Populusxeuramericanna 'Neva' seedlings under exogenous cadmium stress

In order to investigate the effect of magnetized water irrigation on plant growth and tolerance to heavy metals, a randomized block experiment was designed to explore the photosynthetic characteristics, chlorophyll fluorescence kinetic parameters and growth characteristics of 1-year-old Populus×euramericanna 'Neva' under three cadmium concentrations (0 μmol·L^-1, 50 μmol·L^-1, 100 μmol·L^-1) of magnetized water solution. The results showed that:1) plant height, stem growth and dry matter weights of roots, leaves and stems significantly dropped under cadmium stress. While root diameter and volume were enhanced under low cadmium concentration (50 μmol·L^-1, P < 0.05), root morphology parameters were inhibited under high concentration (100 μmol·L^-1). Compared with 0 μmol·L^-1 cadmium treatment, chlorophyll b and carotenoid contents decreased respectively by 12.50% and 19.27% under 50 μmol·L^-1 cadmium treatment, and 43.24% and 46.37% under 100 μmol·L^-1 cadmium treatment (P < 0.05). Net photosynthetic rate (P_n), transpiration rate (T_r) and intercellular carbon dioxide concentration (C_i) respectively decreased by 13.68%, 8.07% and 5.00% under 50 μmol·L^-1 cadmium treatment, and by 33.68%, 27.81% and 14.99% (P < 0.05) under 100 μmol·L^-1 cadmium treatment. PSⅡ potential activity (F_v/F_m), maximum photo-chemical rate (F_v/F_o), photosynthetic performance index (PIabs) and quantum yield (Φ_Eo) also dropped to varying degrees. 2) Compared with non-magnetic water treatment, magnetic water treatment enhanced the growth of plants under cadmium stress in terms of height and stem, increased dry weight of roots and leaves and also root length and surface area of the plant. The contents of chlorophyll a increased by 16.99%, 6.55% and 4.02%, and carotenoid content increased by 17.32%, 19.38% and 50.52% (P < 0.05), respectively, under 0, 50 and 100 μmol·L^-1 cadmium treatments. While P_n, G_s, C_i and WUE increased, the differences were not significant. However, T_r reduced respectively by 24.20%, 23.33% and 12.06% (P < 0.05). In addition, F_v/F_m, F_v/F_o and Φ_Eo remained stable whereas PIabs increased respectively by 8.01%, 20.63% and 31.09% (P < 0.05). In summary, magnetic treatment of water increased the contents of photosynthetic pigments, maintained the function of photosynthetic apparatus, heightened the rate of photosynthetic carbon assimilation and subsequently relieved the inhibition of plant growth by cadmium stress.