| Heavy metal contamination in soil has become a serious problem, and cadmium (Cd) in particular is of increasing international concern, as it is readily taken up by plants and its toxicity to humans is much higher than others. A number of strategies have been proposed for the successful management of the Cd-contaminated agricultural soils. Phytoremediation is one of the strategies to cope with the problem of cadmium toxicity. But only a few species have been reported having the capacity of heavy metal hyper-accumulation, and their successful use in phytoremediation is even less documented to date. One approach applicable on slightly contaminated soils is aiming to screen and use low Cd-accumulating crop varieities. The selection of plant genotypes with less uptake in root and transport to edible parts is a reasonable approach to alleviate adverse effects of Cd contamination in soil. Precautionary measures are required to be taken into account to reduce Cd accumulation in soybean to alleviate the risk of health hazards from Cd-polluted soils as the crop is very important in China. In view of these considerations, a series of experiments were conducted at Zhejiang University, Hangzhou, China during 2003-2005, to identify the genotypic difference in Cd tolerance and to determine interaction between Cd and Al, Na and K in their effect on growth and nutrient dynamics when the plants were exposed to Cd toxicity. The main results are as follows:(1) In the first experiment, 23 soybean genotypes were grown at 0 and 2.0 μmol L~-1 Cd in nutrient solution culture to compare the difference in Cd toleranc. The growth, chlorophyll content, photosynthesis rate and Cd concentration in plants were measured. It was found that there was a big difference among genotypes in their response to Cd toxicity. Evaluation of genotypes through the recorded parameters and their T/C ratio (ratio of the parameter under Cd treatment / control) revealed that genotype S951-3 performed relatively better than other genotypes. On the other hand, Q17-3 showed high sensitity to Cd toxicity.(2) In the second experiment, the two genotypes with different Cd tolerance were used to investigate the mechanisms of Cd tolerance in soybean. It was found that under Cd stress especially at 5.0 μmol L~-1, S951-3 showed less growth inhibition. The treatment of 5.0 μmol L~-1 Cd reduced growth, chlorophyll content and photosynthetic rate in both genotypes, but the extent of reduction differed between the two genotypes. Cadmium and MDA contents, and activity ofSOD and POD enzymes were significantly increased with Cd treatment, and the increase differed significantly between Q17-3 and S951-3, being higher in Q17-3.(3) In the third experiment, synergistic effects of AI and Cd was studied using two soybean varieties viz. Liao-l and Zhechun 2, differing in Al tolerance. There were six treatments pH 6.5;pH 4.0;pH 6.5 + 1.0 umol L"1 Cd;pH 4.0 + 1.0 umol L'1 Cd;pH 4.0 + 150 umol L"1 Al;pH 4.0 + 1.0 umol L"'Cd + 150 umol L'1 Al. The low pH (4.0) and Al treatments caused a marked reduction in the growth, chlorophyll content and photosynthetic rate. Significantly higher MDA content, SOD and POD activities were detected in the plants exposed to both Al and Cd than in those exposed to Al treatment alone. A drastic enhancement of SOD and POD activity was observed in the plants exposed to 150 umol L"1 Al relative to the control without Al addition. Al-sensitive cultivar Zhechun 2 had a considerably higher activity than Al-tolerant Liao-l. Cadmium addition increased Al concentration in the plants exposed to Al stress, and the tolerant cultivar had relatively lower concentration. Combined application of Cd and Al enhanced their accumulation in roots, but caused a reduction in shoots. The concentrations of all the 10 nutrients determined (P, K, Ca, Mg, Fe, Mn, Cu, Zn and B), except Mo were also increased when plants were exposed to low pH relative to pH 6.5. Al addition caused a reduction in the concentration of most nutrients in both plant parts;but K, Mn and Zn in roots were increased. Al-sensitive genotype Zhechun 2 had lower nutrient concentration than Al-tolerant genotype Liao 1.(4) In the fourth experiment, effects of NaCl (50 mmol L'1 1) and Cd (lumol L"1) as sole and combined on growth, chlorophyll content, photosynthesis and fluorescence changes in leaves were studied using two soybean genotypes, Huachunl8 and NGB. The plant height was lowest in both genotypes under 50 mmol L'1 NaCl treatment. Huachun 18 suffered a more serious decline than NGB for net photosynthetic rate (Pn) under salinity and combined stress (NaCl + Cd), suggesting Huachun 18 is relatively a salt sensitive genotype. The reduction of Pn caused by salt stress in Huachun 18 was mainly due to the decrease of total chlorophyll content and photosynthetic efficiency (Fv/Fm), whereas that in NGB was caused by the modification of gs. The combined stresses did not induce a further suppression in photosynthesis and fluorescence in both genotypes relative to salt or Cd stress alone. Higher Cd concentration was found in both seeds and pods when the plants were exposed to salt stress. Higher changes in the concentration of Zn2+, Ca2+, Mg2+, K+ and Na+ were detected in pods of Huachun 18 under salt stress, whilethere was little change for NGB. The results suggested that there was a marked difference in the effect of salinity-Cd interaction on growth and nutrient uptake in two genotypes of soybean.(5) The last experiment was conducted to determine the effect of K on alleviating Cd toxicity in soybean. There were six treatments: no Cd or K. applied;Cd only @ 1.0 |imol L"1;K only;Cd + K. Cadmium reduced growth, chlorophyll content and fluorescence, photosynthesis and stomatal conductance markedly in both genotypes. Enhanced MDA content, SOD and POD activity were also detected in plants exposed to Cd than those with K. alone or combined with Cd. Higher activity of SOD and POD was found in cv. Liao-1 than in cv. Zhechun3. Apparently, K and Cd behaved antagonistically, indicating that K could be a candidate for Cd detoxification in crops planted under polluted environments.
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