Plant materials
Bermuda grass seeds were purchased from Hunan Tianquan Grass Industry Development Co. LTD., Changsha, Hunan Province, China. The floating seedling trays were 180 holes (540mm x 340mm x 50mm) used in the experiment. The water tanks were thickened plastic logistics box with a capacity of 60 L (600mm x 400mm x 340mm). The 1/2 Hoagland nutrient solution is as follows: Ca(NO3)2・4H2O 945 mg L-1、KNO3 607 mg L-1、 (NH4)3PO4・3H2O 115 mg L-1、MgSO4・7 H2O 493 mg L-1、Fe-EDTA, adjusting the nutrient solution pH to 6.8-7.0 with 0.1M NaOH or 0.1M HCl. The culture medium was vermiculite. The total contents of N, P and K in vermiculite were 0.098, 0.243 and 9.807g kg-1 respectively.
Experimental design
The plump and consistent seeds of Bermuda grass had been in the sun for three days for improving the germination rate. The seeds were then evenly placed on vermiculite in a hydroponic seedling tray with 10 seeds per hole. The seedling trays were placed in 15 water culture tanks respectively containing 50 L 1/2 Hoagland nutrient solution. When the seedling entered the growth stage, only three plants were left in each hole. After one week of seedling thinning, the experimental treatment began.
There were four treatments with three replicates. The four treatments were CK, 0.4 mg kg-1 Cd (Cd), 0.4 mg kg-1 Cd and KNO3 (COK), 0.4 mg kg-1 Cd and (NH4)3PO4 (CNP). The nutrient availability of each treatment was shown in table 1, in which the available P and available K were all water-soluble nutrients. The content of available K in COK was twice than that in CD, and the content of available P in CNP was three times than that in CD. The nutrient solution used for plant culture was 1/2 Hoagland nutrient solution. The treatment time was 15 d. During the experiment, the floating seedling tray was taken out every day. The amount of 1/2 Hoagland nutrient solution evaporated on that day was added to the water tank, and then the floating seedling tray was put back into the water tank. In order to avoid the effects of other factors, no pesticide and fertilizer were applied during the experiment.
Morphological indices
Nine plants were selected for each treatment and washed carefully with tap water and distilled water respectively. The samples were put into the oven and dried to constant weight at 80℃. The dry weights of roots, stems, leaves and the biomass were determined.
Determination of chlorophyll content and chlorophyll fluorescence parameters
The relative chlorophyll content (SPDA) of the fully expanded leaves of each treatment was determined by SPDA-502 Plus portable chlorophyll meter (Konica Minolta Japan). The new leaves of PSII maximum photochemical quantum efficiency (Fv/Fm) and PSII potential activity (Fv/Fo) after full dark adaptation were determined using rapid plant stress measurement instrument (OS-30p+ Opti-sciences, America).
Determination of pH and available Cd in water
10 ml water samples were taken, pH was determined by pH meter (Ray-magnetic PHS-3G), and the content of water-soluble Cd was measured by ICP-MS.
Determination of N, P and K in plant organs
The plant samples were carefully cleaned with tap water and distilled water respectively, all samples were put into the oven and heated to 105℃ for fixation, to stay for half an hour, then the samples were dried to a constant weight at 80℃. Plant samples from different treatments were divided into root, stem and leaf, and crushed by grinder. Samples of crushed roots, stems and leaves were digested by heating with concentrated H2SO4-H2O2 (analytical reagent). 10 ml digestion solution was used to determine the content of N and P in different organs by flow analyzer (AA3, German SEAL). The other 10 ml digestion solution was used to determine K content in various organs of plants by atomic absorption spectrophotometer (NOVAA350 German Jena Company).
Cd determination of soil and plants
The leaves and stems are the aboveground parts, and the root system was underground part. Take 2.6 finely ground root, stem and leaf samples were heated and digested with concentrated HNO3 and HClO4 (3:1 v/v, analytical reagent). ICP-MS was used to determine the total Cd content of roots, stems and leaves.
Statistical analysis
All the results were the mean ± standard deviations (SD) of three replications. Microsoft Excel 2016 was used for data statistics and tabulation, and SPSS 20.0 was used for normal distribution and T test. Data was analyzed using SPSS 20.0 for the ANOVA test and LSD multiple comparative analysis. The figures were drawn using Origin 9.0.