| Cadmium (Cd) is a major contaminating heavy metal in the agricultural field. With the development of industry, Cd pollution becomes more and more serious, and has threatened the agricultural production and human health seriously. In this study, several rice (Oryza sativa L.) cultivars were used as materials, and effects of Cd on the growth, yield, quality, and their physiological mechanisms were investigated. The ways to reduce Cd toxicity through irrigation patterns and nitrogen (N) management were studied, so as to provide a theoretical basis and practical guidance to high yield, good quality, and safe production of rice. The main results are as follows:1. Effect of Cd on rice yield and qualityThere were no significant differences between Cd treatment and control (no Cd addition to soil) in grain yield of Yangdao 6 and Yangjing 9538 when Cd concentration in soil was 60 mg kg-1 or 90 mg kg-1. Compared with control, Cd significantly reduced the yield of the two cultivars at Cd concentration of 120 mg kg-1 or 180 mg kg-1 and that of Wuyunjing 7 at Cd concentration of 60 mg kg-1 or 120 mg kg-1 in soil. The decrease in grain yield under Cd stress was mainly attributed to the reduction in the number of panicles or/and number of spikelets per panicle. There were no significant differences in seed-setting rate, 1000-grain weight, milling quality, appearance quality, cooking quality, and content of protein between Cd treatment and control. The content of prolamine was increased, while the contents of albumin and glutelin were reduced with the increase in Cd concentrations in soil. There were no significant differences in the maximum viscosity, hot viscosity, final viscosity, break down values, and setback values of the starch profile in grains between Cd treatment and control when Cd concentrations in soil were 60 mg kg-1 and 90 mg kg-1. Cd significantly reduced maximum viscosity and break-down values, and increased hot viscosity, final viscosity and setback values when Cd concentrations in soil were 120 mg kg-1 and 180 mg kg-1. The Cd concentration in the same organ showed no significant difference among cultivars when Cd concentration in soil was the same. However, the cultivar Yangdao 6 accumulated more Cd in plants than the cultivar Yangjing 9538. The results indicate that the effect of Cd on rice yield and the accumulation of Cd in plants varies with cultivars, and high concentration of Cd could reduce the quality of nutrition and taste of rice.2. The physiological responses of rice to Cd stressCd stress increased the content of free radical in the root and leaf of rice, induced the special expression of low molecular weight isoenzyme of the superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), reduced the content of protein in roots and leaves, increased the content of organic acid in root exudates and decreased the contents of deoxyribonucleic acid (DNA) and protein in rice. The analysis of the SDS-PAGE and two-dimensional electrophoresis of proteome demonstrated that Cd stress could modify the expression of rice proteome. Cd induced ten new protein spots in roots and eight protein spots in leaves of a rice seedling, and inhibited four protein spots in the root and eight protein spots in the leaf.3. Agronomic and physiological traits of rice genotypes differing in Cd-toleranceUnder the Cd treatment, the grain yield was reduced by 6.2%-8.9% for the Cd-tolerant genotypes (Shanyou 63 and Yangjing 9538) and by 38.3%-47.1% for the Cd-susceptible ones (Yangdao 6 and Wujunjing 7) when compared with their respective CKs (no Cd addition to soil). The reduction in grain yield was mainly attributed to the reduction in panicles per pot and spikelets per panicle. The differences in seed-setting rate and 1000-grain weight were not significant between the Cd treatment and CK. The Cd treatment markedly inhibited the tillering of the Cd-susceptible genotypes, resulting in the reduction in dry matter accumulation during the whole growth period. Cd little affected the translocation of non-structural carbohydrate from culms and sheaths and harvest index. For Cd-susceptible genotypes, the Cd treatment significantly reduced root oxidation activity and photosynthetic rate, activities of SOD and CAT in leaves, and obviously increased contents of superoxidate radical and hydrogen peroxide and ethylene evolution rate of leaves, and increased the concentration of 1-aminocylopropane-1- carboxylic acid in root bleedings from the tillering to jointing stages, while the Cd treatment much less affected the above physiological traits for the Cd-tolerant genotypes. The differences in the above traits at heading and afterwards and abscisic acid content in leaves during the whole growth period were not significant between the Cd treatment and CK. The results indicate that the effects of Cd on rice growth and development are mainly during the early growth period (from tillering to jointing), and more tillers, stronger root activity and antioxidative defense system, and less ethylene synthesis in the plants during this period would be considered as agronomic and physiological traits of Cd-tolerant genotypes of rice.4. Effect of irrigation regimes during grain filling on grain quality and Cd concentrations and distributions in rice organsCompared with the well-watered (WW) irrigation, alternate wetting and moderate soil-drying (MD) increased grain yield and improve milling quality and appearance quality, and alternate wetting and severe soil-drying (SD) decreased the yield and quality when rice plants were grown in Cd contaminated soil. Both MD and SD increased the concentration and distribution proportion of Cd in the root, and decreased the concentration and distribution proportion of Cd in the stem and leaf. The MD had no significant effect on Cd concentration in the grain and milled rice, but significantly reduced Cd distribution proportion in the grain, whereas SD increased Cd concentration in the grain and milled rice. Under MD, the strong root activity and the low leaf stomatal conductance (namely the low transpiration) were the main reason for the higher concentration of root, lower concentration of Cd in the stem and leaf and lower proportion of Cd in the grain. Under SD, higher Cd concentration in the root and grain, lower concentration of Cd in the stem and leaf were closely associated with the decrease in the stomatal conductance and the increase apparent transportation rate of Cd in the straw.5. Effect of alternate wetting and soil-drying during the whole growth period on the rice yield, quality and Cd accumulationCompared with the well-watered (WW) irrigation, alternate wetting and moderate soil-drying (MD) during the whole growth period of rice increased the root activity and photosynthetic rate, and increased grain yield by 8.6-10%, and improved milling quality and appearance quality, while alternate wetting and severe soil-drying (SD) during the whole growth period of rice showed an opposite effect. Both MD and SD reduced transpiration rates of leaves, with more reduction in SD. Both MD and SD increased Cd concentration in the root, and reduced Cd concentration in the straw. MD reduced the concentration of Cd in the grain by 8.6-9.7% and in the milled rice by 27-31%. SD increased Cd concentration in the grain, whereas reduced Cd concentration in milled rice. The results indicate that a moderate wetting and drying during the whole growth period could increase the grain yield and reduce the content of Cd in the diet of rice.6. Effect of nitrogen application on the yield, quality, and the Cd concentration and distribution in rice organsUnder the same rate of N application, Cd treatments significantly reduced grain yield. Under the same concentration of Cd in soil, grain yield was increased with the increase in N rates (0-600 kg ha-1). There were no significant effects of N rates and Cd treatments on brown rice, milled rice and head rice. The Cd treatment increased gel consistency, reduced amylose content, and had no significant effect on the chalkness and protein content under the same rate of N. The concentrations of Cd in the hull, bran, and milled rice were increased with the increase of N rates. However, there was no significant difference in the concentration of Cd in the milled rice between 0N (no nitrogen application) and MN (300 kg ha-1). The distribution proportion of Cd in the hull and bran was significantly increased, whereas the distribution proportion of Cd in the milled rice was significantly decreased, with the increase of N rates. The results suggest that rational nitrogen application would not obviously increase the concentration of Cd in the milled rice and even reduced the proportion of Cd in milled rice. |
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