| In recent years, soil were contaminated very seriously by heavy metals cadmium with the rapid development of industry and agriculture, which affected the quality and safety of rice. Root is the gateway for heavy metal pollutants in soil. In order to reduce the heavy metals'activity and mobility in soil, stable chelating with heavy metals is formed through chelating of root exudation and some free heavy metalion in the rhizosphere, meanwhile, heavy metal pollutants can be adsorbed and embedded by the root exudation and then precipitated outside the root. These mechanisms are the embodiment of heavy metal tolerance of plants. At present, the roots exudation have been researched in mostly corn, wheat and soyabean at home and abroad, however, researches of alleviating Cd stress by roots exudation have been carried out fewer. Therefore,14 rice varieties with materials and species were used in this study. A hydroponic experiment was carried out to study the roots exudation organic acid, amino acid root and Cd contents under different Cd levels. The mechanism of Cd tolerance in rice root was discussed. The main results were as follows:(1) For Cd concentration changing and Cd treatment time extending, each rice root dry weight difference obvious. Under the condition of 0.5 mg/L and lmg/L Cd treatment, the root dry weight of 14 rice varieties decreased with the increasing of Cd. Comparing with the control, the dry weight of them did not have obvious difference, all the Cd content of root achieved significant level. When Cd concentration was 3mg/L, root dry weight decreased significantly, the Cd content of root reached maximum. Comparing with II-32B,G46A,R128, the root dry weight changes of Chuan nong you 527 was relatively stable, and for the Cd content of root, it had relatively stronger ability of low accumulation of Cd.(2)For Cd concentration changing, each rice root exudating organic acids difference obvious.The total organic acids in root exudates of II-32B, R128, and Chuan nong you 527 were higher than the corresponding other rice. The secretion of organic acids generally were as follows:Oxalic acid >Malic acid> Citric acid> succinic acid> malonic acid> acetic acid, and oxalic acid and malic acid were the most. When Cd concentration was 0.5mg/L, the secretion of organic acids of rice were significantly increased compared to control, and the Cd accumulation of root also was increased. The secretion of organic acid and the Cd accumulation were positively correlated. When Cd concentration was 3 mg/L, Cadmium affected the normal growth and development of rice materials, the rice root were damaged, root organic acids secretion was negatively correlated with Cd accumulation. With the increase of Cd accumulation of root, the root organic acids secretion decreased. A certain amount of Cd stimulated the secreting of organic acids, but when that certain value was over beyond, the secretion reduced, which showed a low-Cd for the root organic acids secretion, while high-Cd for inhibition. This indicated that the secretion of organic acids in rice roots was related with its Cd-resistant mechanisms. For Cd concentration changing or Cd treatment time extending, compared to other rice, II-32B, G46A, R128 and Chuan nong you 527 could secrete more organic acid, and performed relatively stably.(3)For Cd concentration changing, each rice root exudating amino acids difference obvious.From the changing trend of amino acid:under any Cd concentration in the roots exudation, glutamic acid had the most amount, followed by aspartic acid, serine acid, histidine acid, threonine acid, glycine acid, tyrosine acid, arginine acid, alanine acid and valine acid, and all the above amino acids of the total roots exudation had the same percents basically, but the secretion amount methionine acid, isoleucine acid, leucine acid and Phenylalanine acid were very little. From the total amount of amino acid secretion:with the Cd concentration and the processing time changing the total aoumt of Amino acid and each amino acid amount changes were basically the same. When the Cd concentration was 0.5 mg/L, the total amino acid exudation of root were higher than the control group. When the Cd concentration was increased to 3 mg/L, rice was very sensitive to high concentrations of Cd stress, the root were damaged and amino acid secretion was significantly decreased, and the secretion of all rice were significantly lower than the control group. Under the high concentration of Cd stress, rice root could secrete just a small amount of amino acids, and a variety of Amino acid was not detected. Low concentrations of Cd promoted the secretion of amino acids, while high concentrations of Cd inhibited amino acid secretion. It showed that Cd stress did not affect the variety of amino acid exudation from roots much, while the changing rate of amino acid in the secretion of was impacted greater. In contrast, no matter Cd concentration changed or Cd treatment time extended, compared to other rice, II-32B, G46A, R128 and Chuan nong you 527 could secrete more amino acid, and performed stablly. |
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