| It is known to all that arsenic (As) is one of the most harmful agricultural pollutants. Excessive arsenic in soil is not only harmful to plant growth, but also damaging human health through the food chain. Vegetables are essential in people's daily life, and because of the large consumption, even the low level of pollution can cause great harm, so the security of vegetables is of great importance on people's health. In order to improve agricultural products'safety and to reduce arsenic threat on human health, it is important to select the vegetable varieties suitable for growing in the arsenic polluted areas and to understand the response and mechanism of differrent As-tolerant vegetable varieties to arsenic. In this study, different genotypes of common melon and vegetables, such as cucumber (Cucumis sativus L.), pepper(Capsicum annuum L.), cowpea (Vigna unguiculata L.), and tomato(Cyphomandra betacea L.) were used to study the response and mechanism of different As-tolerant vegetable varieties and the impact of phosphorus (P) on arsenic stress. The main results were as follows:1. Arsenic stress response of different vegetable varieties was studied through soil pot culture. The results showed that genetic variation in tolerance and uptake of arsenic existed among different vegetable varieties. C2 (Cucumber NO.2), C4, C14, P1 (pepper NO.1), P3, and tomoto were selected as the As-tolerant genotypes, featuring as the significant increasing of biomass and yield under arsenic stress to 1.15-2.85 times, 1.06-2.51 times, respectively, compared with the controls. The As-sensitive genotypes were C3, C8, C9, C15, P2, and cowpea, with the characterization of the significant inhibit of biomass and yield under As stress, decreasing to 51%-91% and 27%-57%, respectively, compared with the controls. The As concentrations in C5, C6, C7, and P1 exceeded the National Food Safety Standards (0.05mg/kg FW). By comprehensive consideration of As accumulation and food security in the case of this study, C14, P3, and tomato were selected as the suitable genotypes for planting in the low As-polluted farmland, C8, P2, and cowpea could be used as indicator plants in As polluted areas. 2. Genotypic variations in As speciation, kinetics of As uptake and root exudates of vegetables were investigated via solution culture experiment. The results showed that arsenic in plants existed mainly as arsenite, with percentage of 66.79%-98.32%. The transformation of arsenate to arsenite was regarded as one of the mechanisms of arsenic detoxicity. The percentage of arsenate in leaves (13.78%-33.21%) was significantly higher than that in roots (1.68%-25.17%), indicating that leaves were more sensitive to arsenic. Generally, the As-tolerant varieties had a higher transformation percentage.As-tolerant genotypes showed greater ability of As resistant than that of As-sensitive. Kinetic study of As uptake showed that the sensitive varieties had lower Km and Cmin under arsenite stress, indicating a higher arsenite affinity. While in contrast, the As-tolerant varieties had a higher arsenate affinity.There were different types and quantities of organic acids in root exudates among genotypes of vegetables, and the tolerant varieties usually exuded more organic acids than the sensitive ones. The main organic acids exuded from cucumber were malic acid, succinic acid and acetic acid, which, as compared with the control, increased respectively to 1.20-1.97 times,1.46-2.36 times and 1.21-1.44 times for the tolerant,1.02-1.48 times, 1.10-1.89 times and 1.07-1.15 times for the sensitive. For pepper, acetic acid and malic acid exuded from P3 increased to 1.48 times and 1.05 times, respectively, while P2 only exuded acetic acid. Malic acid, succinic acid and acetic acid exuded from cowpea increased to 1.53 times,1.38 times and 1.26 times respectively, while malic acid and acetic acid for tomato increased to 2.89 times and 2.90 times, respectively.3. The impacts of P on arsenic stress for different cucumber genotypes were investigated via solution culture experiment. The results showed that P mitigated arsenic stress, but promoted As migration rate (shoot/root accumulation), so food safety was potential threatened at As concentration of 1mg/L. Arsenic decreased uptake and migration of P to 45%-68% campared with the control, and reduced the migration rate of P to 2.65-3.26, while 4.33-5.02 in the control, with the effect of arsenite being stronger than arsenate. Similarly, P suppressed As uptake and accumulation, but promoted As migration, with the increase of migration rate of 0.12-0.19, while 0.05-0.09 in no P treatments. Arsenic speciations of leaves and roots in the As-tolerant varieties were not influenced by As valence sourses and P treatment. However, As-sensitive varieties could decrease arsenic toxicity by increasing arsenite ratios through P uptake. Meanwhile, the As-stress reduced by increasing the activities of enzymatic antioxidants (superoxide dismatase-SOD, catalase-CAT) and the contents of non-enzymatic antioxidants (glutathione-GSH, soluble protein). In single arsenic stress, with the role of CAT and GSH, cucumbers were not hurt. However, in As stress without P, cucumbers were damaged even with an activity or content increase of SOD, CAT and GSH. |
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