| Soil pollution with heavy metals has become one of the serious environmental issues in the last decades.Among all heavy metals,cadmium(Cd)has gained an increasing attention owing to its high toxicity,strong carcinogenicity and easy enrichment migration.The soil Cd concentration in the Chengdu Plain,as an important grain production base in the Southwest,China,significant increased in recent years due to mining,fertilizer and pesticide abuse.Cd in soils not only deteriorates the soil fertility,reduces crop yield and quality,but also endangers human health through food chain.However,the process of Cd-contaminated soil remediation is time-consuming and costly.Therefore,producing safe food from polluted soil has become an urgent problem to be solved.Cd uptake by rice presences a significant genotypic difference and selecting Cd excluder's cultivars is the key to solve the problem of grain safety production in light and moderate-Cd polluted soil.In this study,31 samples of japonica rice and 8 indica rice were used as materials to evaluate their potential capacity for Cd uptake.The main results were as follows:1.In the pot experiment with 5.0 mg kg-1 Cd stress,the Cd concentrations in brown rice ranged from ns(undetectable)to 1.23 mg kg-1 among the rice cultivars with 9 japonica rice varieties Cd content belongs to rice Cd security.The 39 rice cultivars were divided in to low accumulation lines of japonica rice,high Cd accumulation lines of japonica rice,relatively accumulation lines of indica,and high Cd accumulation lines of indica depending on the Cd concentration of brown rice.The low accumulation lines of japonica rice included Wu Yunjing 21,Yijianzhongqing,Nanjing 94140,Yueguang,Changjing 11-5,Yan 9029 and Yang Yujing 116;the high Cd accumulation lines of japonica rice were Nanjing 9108,Dianheyou 106,Wu Yunjing 27,Ning D212,D46,Dianheyou 34,Huai 6188,Chujing 27,and Dianza 31;The relatively accumulation lines of indica,and high Cd accumulation lines of indica included Dexiang4103 and Yixiangyou 2115;While the high Cd accumulation lines of japonica rice included Ya 3A-1188,F 498 and Gangyou 725.The Cd concentrations in brown rice in indica were all exceeded the national safety standard(Cd<0.2 mg kg-1).However,the Cd concentrations in low accumulation lines of japonica were lower than that in food safety national standard,indicating these varieties could be considered as Cd safety rice germplasm resources for Cd-polluted farmlands.2.Under Cd stress,the biomass and grain yield of the tested rice varieties were obviously inhibited with the increasing of Cd concentration.The biomasses in the low Cd uptake rice were higher than those of high Cd uptake varieties,while an opposite trend showed for the yield.The contents of Cd in roots,stems,leaves,spikes and other organs of rice were increased with the increase of Cd concentration(0-50 mg kg-1).They were in low accumulation lines of japonica rice significantly lower than that of indica rice and high Cd uptake japonica rice.On the whole,the Cd concentration in the root was the highest among the organs.Although the enrichment factor(BCF)in low-Cd-accumulating japonica rice was 0.90-5.64 and 0.01-8.50 in root,they were lower than 0.60 in brown and fine rice,which indicated the edible part of the japonica rice excluder's cultivars had a lower Cd enrichment ability.3.The contents of chlorophyll a(Chl a)and chlorophyll b(Chl b)in the leaves of rice increased and then decreased with the increase of Cd concentration.While the content of carotenoid(Car)decreased with the increase of Cd concentration.When Cd concentration reached 50 mg kg-1,the descending rate of WuYunjing 21(84.19%)and Yijianzhongqing(80.44%)were both higher than that of the high Cd accumulation japonica rice and hybrid indica rice.With the increase of Cd concentration,the net photosynthetic rate(Pn)and intercellular carbon dioxide concentration(Ci)of each rice varieties decreased gradually,while the stomatal conductance(Gs)decreased to the minimum and then increased.However,the minimum Gs of the four japonica(257.60?mol·m-2·s-1-286.90 pmol·m-2·s-1)and the two hybrid indica(242.70?mol·m-2·s-1 and 250.41?mol m-2·s-1)were when the Cd concentration reached 10 mg kg-1,15 mg kg-1,indicating the decrease of photosynthetic rate was mainly due to the decrease of stomatal conductance at low Cd concentration,and the photosynthesis mainly belonged to stomatal limitation and the decrease of photosynthetic rate was due to the destruction of mesophyll cells in leaves,leading to the decrease of photosynthetic activity and the decrease of photosynthetic rate.4.The content of malondialdehyde(MDA)in the leaves of rice increased with the increase of Cd concentration at the tillering stage.When Cd concentration reached 50 mg kg-1,the contents of MDA in the Wuyunjing 21(43.97 ?g g-1)and Yijianzhongqing(45.02?g g-1)were significantly higher than that of the high Cd accumulation japonica rice(7.53 ?g g-1 and 38.52 ?g g-1)and hybrid indica rice(36.70 ?g g-1 and 38.26 ?g g-1).The results showed that Cd excluders japonica rices were more affected by Cd stress and the membrane lipid peroxidation were stronger.Under the Cd stress,the activity of superoxide dismutase(SOD)in the rice varieties at tillering stage varied with the increase of Cd concentration.However,when the Cd concentration exceeded 15 mg kg-1,the SOD activity of all tested rice varieties significantly lower than the control.While the activity of catalase(CAT)and peroxidase(POD)increased first and then decreased with the increase of Cd concentration.But the maximum value of CAT and POD in Yijianzhongqing and Wu Yunjing 21 were higher thanthat in high Cd accumulation rices.Therefore,under low Cd stress,the ratio of SOD/CAT and SOD/POD decreased,but increased under medium and high Cd stress,which indicated that the antioxidant enzymes of rice could resist the damage of low and medium cadmium to a certain extent,but ultrahigh Cd concentration will cause serious damage to rice. |
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