4.1 Effects of optimized water management treatments on Cd and As uptake and accumulation in rice
The highest and lowest plant organ Cd concentrations were found in the Drained and Flooded treatments, respectively, but the highest and lowest plant organ As concentrations were found in the Flooded and Drained treatments, respectively. These results are similar to those reported by Arao et al. (2009), Hu et al. (2013), Wan et al. (2019), and Mlangeni et al. (2019). For both soils, the plant organ Cd concentrations were ranked in the following order: roots > other aboveground organs > grains, and this same ranking was previously reported by Xie et al. (2015). The As contents of the rice plant parts followed the pattern: roots > leaves > stems > rachises > grains in both soils, as was found by Wang et al. (2006) and Smith et al. (2008).
For reducing the Cd concentration of rice grains, the optimized water treatments F5D3, F5D5, and F5D7 reduced concentrations of Cd in grains by 80.0%, 64.3%, and 18.5%, respectively, in W soil and by 90.8%, 82.4%, and 76.1%, respectively, in H soil, compared with the Drained treatments, indicating that the treatment with the most days of flooding (F5D3) was more efficient than those with fewer days of flooding (both F5D5 and F5D7) at reducing grain Cd content. This result is probably attributable to the differences in soil solution Cd concentrations and soil Eh among the different treatments, which is supported by the positive and significant correlations between the Cd contents of grains and soil solution (R2 = 0.98, p < 0.001) and between grain Cd content and soil Eh (R2 = 0.68, p = 0.03), findings that are in agreement with those of Han et al. (2018). Optimized water management could lead to changes in soil Eh that influence the redox state of soil and determine the bioavailability of Cd (EI-Naggar et al., 2018). After soil flooding, accompanied by a decrease in the soil Eh (Fig. 3), soil microbes respire using oxidized components, such as SO42−, NO3−, and Mn (VI/III) and Fe (III) species, and these species receive electrons and are reduced to S2−, NO2−, Mn2+, and Fe2+ (de Livera et al., 2011). CdS then forms from a combination between S2− and Cd, which is insoluble in water (Porter et al., 2004). When soil is drained, with higher Eh, sulphate concentrations in the soil solution increased (Shaheen et al., 2016), and Cd sulphate is quite soluble (Porter et al., 2004). The concentration of dissolved Cd in the soil solution is directly correlated with the amount of absorption by plants (Lofts et al., 2004; Zhang et al., 2016). Thus, the phytoavailability of Cd could be manipulated using optimal water management. In addition, Cd accumulation in grains was strongly related to the Cd accumulations in all detected organs, (especially that in roots [R2 = 0.99, p < 0.0001]), root-to-stem Cd TFs (R2 = 0.99, p < 0.0001), and stem-to-rachis Cd TFs (R2 = 0.88, p = 0.0008), indicating that the accumulation of Cd in grains was closely related to that in other organs and to the translocation of Cd from roots to shoots. The accumulation of Cd in rice grains is determined by xylem-mediated Cd translocation from root to shoot and phloem-mediated Cd transport (Uraguchi et al., 2009; Kato et al., 2010). Thus, the efficient reduction of Cd content in grains could be achieved by restraining Cd accumulation in roots and inhibiting Cd transport from roots to shoots.
For reducing As concentrations in grains, the optimized water treatments F5D3, F5D5, and F5D7 reduced concentrations of As in grains by 77.4%, 77.8%, and 86.7%, respectively, in W soil and by 73.1%, 77.2%, and 80.6%, respectively, in H soil compared with the Flooded treatment. This indicated that the treatment with the most days of drainage (F5D7) was more efficient than those with fewer days of drainage (both F5D3 and F5D5) at reducing grain As content. These results were mainly related to the differences in soil solution As content, soil Eh, and pH under the different water management treatments. This is supported by the positive and significant relationships between grain As content and the soil solution As concentration (R2 = 0.92, p = 0.0001) and between grain As content and soil pH (R2 = 0.88, p = 0.0008); the negative and significant relationship between grain As content and soil Eh (R2 = -0.82, p = 0.03), which was also observed in a previous study (Marin et al., 1993), also supports this. A significant negative relationship between the Eh and pH in soils was also found in this study (R2 = -0.74, p = 0.01) (Table 2), as has previously been found by Rinklebe et al. (2016). In treatments involving more days with anaerobic conditions, the soil solution As concentration was higher than in those involving fewer days with aerobic conditions in both soils (Fig. 2b). The soil solution As concentration was positively related to soil pH but negatively related to soil Eh; these same relationships were reported in a previous study (Marin et al., 1993). When soil is drained and the accompanying increase in soil Eh and decrease in soil pH occurs (Fig. 3), As(V) becomes the predominant inorganic As species. As (V) readily combines with Fe and Al (hydr)oxides (Goldberg, 2002), and the resulting forms of As are less mobile and not easily absorbed by plants. When soil is flooded, soil Eh decreases and soil pH increases (Fig. 3), and As is released from those Fe and Al (hydr)oxides as As (V), which is then reduced to As (III) and readily taken up by plants. A study by Norton et al. (2012) found that the As content of rice grown in drained soil was usually 10 times lower than that of rice grown in flooded soil. Soil pH was positively and significantly correlated with the soil solution As concentration, and the variation in pH explained 84.1% of the variation in soil solution As concentrations (Table 2); this is consistent with the findings of Yamaguchi et al. (2011), who found that the solid/solution distribution ratio for inorganic As (III and V) decreased significantly with an increase in pH (from 5.5 to 7.0 and above). Thus, alternating between flooding and drainage in paddy fields plays an important role in the variation in soil solution As concentrations owing to its effects on soil pH and Eh, even in terms of As availability to rice plants (Takahashi et al., 2004; Arao et al., 2009; Li et al., 2009). Therefore, the most efficient means to alleviate As accumulation in rice would be to maintain aerobic conditions during the growth season (Xu et al., 2008).
Compared with the W soil, Cd and As accumulation in rice in the H soil was higher, which probably is related to the higher total Cd and As contents in H soil (which result mainly from industrial activities and domestic pollution), resulting in a higher soil solution Cd concentration. However, recent studies have indicated that some soils are enriched in Cd but have low Cd bioavailability. In acidic soils with relatively low total Cd contents, rice plants absorbed considerable amounts of Cd and accumulated higher amounts of Cd in their grains (Wen et al., 2019). Based on a GIS analysis of soil geochemical survey data, Xia et al. (2019) also noted that the spatial patterns of Cd concentrations in rice were not consistent with the soil Cd levels. These quite distinct results imply that the soil Cd and As pollution evaluation methods need to continue improving through best practices and take into consideration the bioavailability of Cd and As.
To evaluate the degree of trade-off between soil solution As and Cd concentrations and identify the optimal water management strategy to simultaneously minimize the concentrations of both, we calculated the “trade-off value” in accordance with a report by Honma et al. (2016). The minimum “trade-off value” was observed in the F5D3 treatments in both soils, with a value of 0.86 and 0.26 in soils W and H, respectively. Thus, the F5D3 water treatment was most effective for the simultaneous reduction of total dissolved Cd and As in both soils in this study. Among the three optimized water treatments that we tested, the F5D3 treatment also was the most efficient at concurrently reducing both the Cd and As contents of grains from both soils. Therefore, F5D3 makes the simultaneous reduction of Cd and As uptake and accumulation by rice achievable by keeping the soil solution Cd and As concentrations low. Honmat et al. (2016) found that a treatment with three days of flooding and five days of drainage was the most efficient for the simultaneous reduction of Cd and As concentrations in grains. Clearly, the optimal water management strategy identified by one or two studies is likely not applicable across all different soil types (e.g., soils with different pH values, organic matter contents, cation exchange capacities, and Cd and As contents). It is relatively difficult to reduce Cd and As simultaneously in rice grains through water regimes alone in soil polluted with both Cd and As.
4.2 Effects of optimized water management treatments on the AA concentrations of organs and factors associated with the variation in individual AAs in husks
In the F5D3 treatment, the total concentration of EAAs and NEAAs in husks and total EAAs in roots in W soil, as well as the total concentration of the NEAAs in roots in both soils, were lower compared with those of the Flooded and Drained treatments, and lower concentrations of Cd and As in rice organs were also detected in the F5D3 treatment compared with those detected in the Flooded or Drained treatments, which indicates that it might not be necessary for rice to synthesize abundant AAs to chelate metal ions in the F5D3 treatment. Studies in several species have indicated that many kinds of AAs (as Thr, Val, Pro, Glu, Gly, and Lys, etc.) play vital roles in plant responses to various abiotic stresses (Bowne et al., 2012; Witt et al., 2012; Obata and Fernie, 2012; Zhao et al., 2019; Yuan et al., 2020). Plant defense systems are activated under heavy metal stress, improving the synthesis of defense-related AAs. For example, His is involved in nickel chelation (Kramer et al., 1996); Pro is a radical scavenger that may participate in metal-ion chelation (Sharma and Dietz, 2006) as it is often detected in plants suffering from heavy-metal stress (Matysik et al., 2002); Glu and Gly are separately responsible for the antioxidant and heavy-metal detoxification pathways, respectively (Wu et al., 2016; Yuan et al., 2020).
The total AA concentration in husks was higher than those in the stems and roots within the same treatment (Fig. 4), but also most of the materials needed for grain filling come from the photosynthesis of the husk, and the development of the husk directly affects grain filling, and AAs in the husk certainly regulate the levels of AAs in grains. Thus, we analyzed the correlation between the changes in amino acid levels in the husks and other variables (Table S2). The results indicated that the variations in Asp-a, Glu-a, and Gly contents were closely correlated with the rachis-to-grain TFs for Cd, Cd, and As, respectively; the variation in Cys content was positively and significantly correlated with grain As content and rachis-to- grains TFs, with the variations of those variables explaining 85.5–90.4% of the variation for those AAs, as they did in Dwivedi et al. (2012). Glu and Gly are components of glutathione and phytochelatins, which are respectively responsible for the antioxidant and heavy-metal detoxification pathways (Wu et al., 2016).The improvement of Glu synthesis is a significant influencing factor for Cd accumulation in rice grains and may thus alleviate Cd toxicity by forming the Glu-Cd complex (Yuan et al., 2020) or synthesizing phytochelatins (PCs) and PC-Cd complexes (Pál et al., 2018). PCs have a [(γ-Glu-Cys)n]-Gly structure and can form low molecular weight (LMW) complexes with Cd (PC-Cd) (Cobbett, 2000). The variation in Phe content displayed a close correlation with the variation in soil Eh and the variation in the TFs of Cd from stems to rachises, but it was negatively and significantly correlated with the variation in rachis As contents. The latest research shows that the change in grain Phe contents is closely related to the distribution of Cd in grains. Glu might act as the first line of defense, and Phe as the second defensive line to constrain Cd transport under Cd-stress conditions (Yuan et al., 2020). The different responses observed for AA synthesis in rice organs under optimal water management in Cd- and As-polluted soils may be helpful in maintaining the balance related to water resource saving and to the yield and quality of rice. Further studies should build on what we have demonstrated here and further elucidate the mechanisms underlying the interaction between water management and Cd and As accumulation in rice with respect to AA synthesis.