The Physiology And Biochemistry Of Rice Mutant To Cadmium Sensitivity And Genotypic Difference In Cadmium Accumulation In Brown Rice

At present, a great deal of arable land in China has been contaminated by cadmium, which not only inhibits the growth and development of crops, leading to reduced yield and quality, but also poses a great threat to human health via food chain due to uptake by crops. It is, therefore, imperative to reduce cadmium concentrations in edible parts of crops. Rice is one of the most important crops in the world. However, the genotypic difference on cadmium concentrations in rice grains and its mechanism remain poorly understood. Moreover, there are different points on the cadmium toxicity and tolerant mechanism of plants. The present experiments with solution culture were carried out to study the effect of Cd on growth, Cd uptake and the physiological mechanism in Cd sensitivity. Meanwhile, the genotypic difference in Cd concentration of brown rice was studied under the field conditions, which provide an economic and efficient approach to reduce cadmium concentrations in rice grains planted on the slightly or moderately contaminated soils. The main results are as follows:1. The effect of Cd on growth of Cd-sensitive mutant and wild type rice depended on Cd concentrations. Cd-sensitive mutant and wild type rice seedlings could grow well under relative low Cd treatment; however, there were apparent toxic symptom under relative high Cd treatment. The plant height, root length, root surface area, root number, root activity, dry mass were significantly reduced in both Cd-sensitive mutant and wild type rice, and all of the former were lower than those of the latter under relative high Cd treatment. Meanwhile, Cd concentrations in roots, stems, and new leaves of Cd-sensitive mutant rice were significant higher than those of the wild type. However, there was no difference on Cd concentrations in old leaves between two. Correlation analysis proved that root length, root surface area, root number, and root activity had a passively significant correlation with Cd concentrations in the plants (P< 0.01). This indicated that the greater reduction in growth was contributed to the higher Cd concentration in plants.2. Radiotracer techniques were employed to characterize ¹⁰⁹Cd²⁺ absorption and translocation to shoots in rice. The time-dependent kinetics of ¹⁰⁹Cd²⁺ uptake in rice root are biphasic, with an initial rapid stage which was completed in 20 min and represented Cd²⁺ accumulation in the apoplast followed by a longer but slower phase which represented Cd²⁺ transport across the plasma membrane. Concentration-dependent Cd²⁺ influx in rice yielded smoothed but unsaturated kinetic curves that could be resolved into linear and saturated components. This saturated part could be characterized by Michaelis-Menten equation, with approximate K_m values for Cd-sensitive mutant (2.54μmol L^-1) and wild type rice (2.37μmol L^-1), respectively. These results indicate that in rice, Cd²⁺ transport across the plasma membrane was mediated by transporters. The V_max for Cd²⁺ influx were 2-fold greater in Cd-sensitive mutant than in wild type rice, indicating that enhanced absorption into the root was one of the mechanisms involved in Cd sensitivity. After 96 h uptake period, ¹⁰⁹Cd²⁺ translocation to the shoot was approximately 1.9-fold greater in Cd-sensitive mutant than in wild type rice. Ca and La inhibited Cd²⁺ uptake in equal degree for both. Rice types and Cd uptake in the mutant rice was inhibited more markedly by Zn and Mn than that in wild type rice.3. Cd subcellular distributions and chemical forms in Cd-sensitive mutant and wild type rice werestudied using sucrose differential centrifugation and sequential chemical extraction method. The results showed that Cd was mainly kept in the cell wall and solution fraction of Cd-sensitive mutant and wild type rice. Increased Cd level in the medium caused a significant increase of Cd concentration in all fractions of roots, stems and leaves. Cd in organelies fraction of Cd-sensitive mutant rice was higher than that of wild type. In roots, stems and leaves of Cd-sensitive mutant and wild type rice, most of Cd were found in extraction of 1M NaCl, 2% HAC and 0.6 M HC1, which account for more than 65% of total Cd. And it was found that Cd-sensitive mutant, in comparison with wild type, had higher Cd in extraction of dH₂O.4. The leaf area, contents of chlorophyll, photosynthetic rate (Pn) of Cd-sensitive mutant weresignificantly reduced under relative higher Cd in the medium, however, the concentration of intercellular CO₂ concentration (Ci) was significantly increased with increasing Cd concentration. Therefore, it is suggested that Cd caused low photosynthesis due to non-stomatal limitations. Compared to chlorophyll b, chlorophyll a was more sensitive to Cd treatment. Fv/Fm, F PSII, qP, ETR of Cd-sensitive mutant were considerably decreased in the presence of relative higher Cd and qN of mutant was significant lower than that of wild type rice under high Cd treatment. It meant that mutant rice was damaged more seriously than wild type under high Cd treatment and the former had weak ability of self-protection. After the recovery for 3 days, P_N, gs and all chlorophyll fluorescence parameters could be recovered to a greater extend in the wild type rice than that in the mutant, when seedlings were treated for 12 d under 50μmol L^-1 Cd.(5) A highly significant increase in MDA, O₂ and H₂O₂ content were recorded in the plants subjected to the increasing Cd treatment. Moreover, all of them in the mutant were significant higher than that in wild type rice. We observed a much more accumulation of H₂O₂ in the mutant under relative lower Cd treatment, which induced early activity of antioxidant enzymes. However, the activity of antioxidant enzymes decreased in the mutant, such as superoxide dismutase (SOD) and catalase (CAT), which caused the accumulation of reactive oxygen species and damaged to cells. Therefore, high Cd caused the imbalance between the produce and clean of reactive oxygen species, which may be one of the reasons for Cd sensitivity of the mutant.(6) In the whole, GSH and AsA contents decreased with the increasing Cd level in the medium. NPT and PC, which could combine with Cd, increased in the leaves of both the mutant and wild type rice seedling under low Cd treatment. Moreover, the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) were high levels and the ratios of GSH/GSSG and AsA/DHA maintained relative constant in both the mutant and wild type rice. So, there was no apparent toxicity symptom in the both. However, NPT and PC did not increased, even reduced under high Cd treatment. And the activity of ascorbate peroxidase (APX) and glutathione reductase (GR), and the ratios of GSH/GSSG and AsA/DHA significantly reduced. All were lower in the mutant than in wild type rice. It means that high Cd caused a much lower redox state in the mutant, which experience more serious oxidant stress.(7) A field experiment was conducted to determine the genotypic variation in grain cadmium concentrations and the relationship between grain cadmium concentrations and yield components by using thirty-eight rice genotypes collected from the Yangtze Rever delta, China. The results showed that there were highly significant differences in yields and grain Cd concentrations among genotypes and that cadmium concentrations in brown rice of most genotypes exceeded the AMV of cereals proposed by FAO/WHO. The results also showed significant and positive correlation between Cd concentrations of brown rice and those of straw. The present experiment also showed that there were significant correlations (p<0.05 or 0.01) between Cd concentrations of brown rice and some important agronomic traits, such as grain yield, spikelets per panicle and harvest index. Thus, in referring to the AMV (Cd 0.1) of cereals proposed by FAO/WHO and yield, some genotypes with low heavy metal concentrations in grain were identified for cultivating and breeding candidates.