Soil pH, biomass and Cd content of peanut
Soil pH
Figure 1 shows that, in this study, application of slaked lime significantly increased soil pH in nearly all growth stages (p < 0.05). The following general pH trend with increasing application rates of slaked lime during the growth stages was observed: C1500 > C1200 > C900 > C600 > C300 > C0. Among the soil characteristics, soil pH is considered as an important index that impact Cd uptake by crops, since pH can obviously affect the speciation and solubility of Cd in soil liquids15. The use of slaked lime can neutralize excessive H+ concentrations in soil solutions and decrease Cd solubility33, but there were no observable differences among the different growth stages.
Effects of slaked lime application on soil pH values. The values are means (± SD) of three replicates. Bar groups with different capital letters indicate significant differences (p < 0.05) among the growth stages. Bars with different lowercase letters indicate significant differences (p < 0.05) among the six slaked lime treatments for the same growth stage.
Compared with C0, soil pH increased by 3.3, 12.92, 16.88, 25.0, and 28.96% at the seeding stage; 5.86, 20.08, 22.18, 28.45, and 34.1% at the pod-pin stage; 4.74, 7.01, 19.79, 23.92, and 27.84% at the pod bearing stage; and 1.4, 2.2, 14.37, 23.35, and 24.95% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively. Slaked lime is an alkaline substance that can increase soil pH by increasing metal precipitation and ion adsorption34, increasing soil adsorption and promoting changes from soluble metals to residuals35. In the present study, the pristine soil pH was between 4.5–5.5, which was considered to be a strong acid soil. Lower soil pH may cause high metal ion availability because of the greater competition between free metal ions and protons for binding on soil surfaces36. Application of slaked lime could induce changes in carbonate or hydroxide precipitation or by enhancing adsorption sites by inducing deprotonation at the soil surface37.
Biomass of peanut
As shown in Table 2, for the root biomass, there were no significant effects from the slaked lime applications in the seeding, pod-pin and pod bearing stages, with the exception of the maturity stage (p < 0.05). Compared with C0, root biomass increased by 22.95% at the maturity stage for the C300 treatment. Under slaked lime applications, there were significant differences in shoot biomass at all growth stages except for the seeding stage (p < 0.05). Compared with C0, shoot biomass increased by 61.64% at the pod-pin stage, 58.4% at the pod-bearing stage for the C1500 treatment, and increased by 4% at the maturity stage for the C300 treatment. Pod biomass describes peanut productivity38. The biomass in shells and kernels showed an increasing trend under higher application rates of slaked lime at the maturity stage. Compared with C0, shell biomass increased by 23.08, 54.03, 57.97, 60.6, and 79.18% and kernel biomass increased by 0.30, 28.39, 44.89, 46.78, and 77.23% for the C300, C600, C900, C1200, and C1500 treatments, respectively.
Cd toxicity directly affects net CO2 assimilation and the transportation of photosynthetic electrons and indirectly reduces chlorophyll concentrations (by indirectly inhibiting chlorophyll biosynthesis) or by preventing the photosynthetic process at different levels of its tissues, the lipid and protein ingredients of thylakoid membranes, and chloroplast fine structure39. It has been widely demonstrated that addition of slaked lime to acidic soils very strongly and directly enhances crop productivity by improving the soil physical–chemical and biological characteristics, which ultimately boost the mobility and availability of essential crop nutrients33.
Soil nutrients that are crucial for plant growth include OM, N, P and K. The nutrient contents in heavy-metal-contaminated soil comprise a pivotal factor that affects seed germination and plant settlement40. Agegnehu et al.41 and Melo et al.42 reported that soil amendments lead to higher plant uptake of C, N, P and K, as well as increased availability of soluble organic carbon (SOC) in soil due to changes in soil pH. Therefore, chlorophyll contents and plant yields were improved.
Slaked lime contains large amounts of the Ca nutrient. Ca is an essential main element for plant growth and accounts for 0.1–5% of dry plant biomass7. It is the second messenger in signal transition and plays a key role in crop cell supersession, signal shift, and nutrient uptake over cell membranes43. Peanut is a calciphilic crop, and peanut yields in acidic soil are often limited by a lack of Ca rather than by a lack of other plant nutrients. In peanut, exogenous application of Ca2+ can improve plant stress resistance by enhancing PSII efficiency, boosting the activities of antioxidant enzymes (e.g., SOD and CAT), and decreasing MDA content under stress14,44.
Cd content of peanut
Cd uptake clearly decreased in nearly all growth stages when the rates of slaked lime application increased (p < 0.05) (Fig. 2). Compared with C0, the Cd contents in roots decreased by 17.07, 22.84, 52.16, 67.79, and 53.85% at the seeding stage; 23.62, 30.32, 58.02, 55.39, and 70.55% at the pod-pin stage; 3.13, 26.65, 61.31, 63.68, and 71.27% at the pod-bearing stage; and 19.39, 29.01, 30.90, 40.23, and 53.79% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively (Fig. 2a).
Effects of slaked lime application on the total Cd contents of (a) roots, (b) shoots, (c) shells, and (d) kernels for peanuts. The values are means (± SD) of three replicates. Bar groups with different capital letters indicate significant differences (p < 0.05) among the growth stages. Bars with different lowercase letters indicate significant differences (p < 0.05) among the six of slaked lime treatments for the same growth stage.
For the Cd contents in shoots, obvious reductions were found with increasing application rates of slaked lime in nearly all growth stages (p < 0.05). Compared with C0, the Cd contents in shoots decreased by 7.36, 15.20, 42.52, 43.47, and 57.96% at the seeding stage; 18.32, 44.67, 52.90, 63.93, and 72.52% at the pod-pin stage; 6.64, 28.05, 41.03, 50.94, and 62.64% at the pod bearing stage; and 13.09, 23.68, 29.11, 30.08, and 30.92% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively (Fig. 2b).
Similarly, the Cd contents in shells showed a decreasing trend that was similar to those for roots and shoots in the maturity stage (p < 0.05), and these contents decreased by 5.76, 23.02, 41.73, 42.45, and 43.17% for the C300, C600, C900, C1200 and C1500 treatments compared to C0, respectively (Fig. 2c).
Peanut kernels contain large amounts of protein and oil and are a crucial oil source in the food processing industry, therefore, pollution by heavy metals in peanut kernels is an important concern for food security and international business45. Figure 2d shows that the Cd concentrations in peanut kernels were greater than the maximum approved levels defined by the National Food Safety Standard of Contaminants in Foods (GB2762-2017). However, the Cd contents markedly decreased with increasing rates of slaked lime and decreased by 8.57, 43.81, 43.81, 47.62, and 50.48% in the C300, C600, C900, C1200 and C1500 treatments, respectively, when compared to C0. In addition, the process of absorbing Cd by peanut shells and seeds is almost simultaneous. Therefore, Cd accumulation was similar between peanut shells and kernels. Furthermore, previous study has shown that mechanisms of Cd immobilization affected by slaked lime are principally due to the changes in soil Cd availability and crops root absorption instead of internal translocation in plants12.
Generally, one of the most effective remediation options for eliminating Cd is the improvement of soils and minimization of metal bioavailability. The bioavailability of metals can be minimized via chemical and biological immobilization by using a series of inorganic compounds, which include clay minerals, P compounds, and lime, as well as organic amendments46.
Slaked lime is employed to mitigate soil acidity; simultaneously, it has gained wide acceptance as a popular choice in the scientific community due to its ability to decrease heavy metal toxicity in soils. For many years, liming has been accepted as a conventional treatment to reduce Cd contents in the edible parts of farmland crops46.
The results indicated that peanuts exhibited good adsorption capacity for Cd, and the differences in uptake among the different tissues at various growth stages may be related to the application rates of slaked lime. Soil pH affected Cd transportation from soil to roots and improved metal absorption onto soil particles15. Due to the competition between Ca and Cd for uptake by crop roots, the higher Ca concentrations in lime strongly reduced Cd adsorption by farmland crops. It has been accepted that competition between Ca2+ and Cd2+ is one of the crucial influences for suppressing Cd mobility and therefore remarkably enhanced Ca concentrations and decreased Cd accumulations in crops33. In addition, increasing soil pH usually results in the formation of more negatively charged sorption sites, hydroxyl species of positive metal ions, and sedimentation of Cd2+ as Cd(OH)2 and leads to a reduction in soil Cd availability12. Previous studies have indicated that Cd addition in the soil did not inhibit the formation of external hyphae and mycorrhizal colonization47,48,49, which maybe the reason of Cd translocation excessive, this mechanism will continue to be discussed in future studies.
Effect of Cd absorption by peanut
Cd accumulation in peanut
The effects of slaked lime on Cd accumulation in the tissues of peanuts at various growth stages are presented in Table 3. With the application of slaked lime, at the pod-bearing stage, Cd accumulations in shoots and roots of peanuts were higher in the C300 treatment than in the C0 treatment, however, it has a decreasing trend with the increasing of slaked lime. This result indicated that slaked lime at quantities of 300 kg ha−1 had not reduced the content of Cd markedly in roots and shoots at the pod-bearing stage. Pod bearing is a crucial stage in which peanuts absorb nutrients to increase the number of pods and kernel filling of peanuts50. However, lower slaked lime levels reduced Cd immobilization34.
The lowest Cd accumulations in roots were observed for the C1200 treatment (seeding stage), C1500 treatment (pod-pin stage), C1200 treatment (pod bearing stage) and C1500 treatment (maturity stage). Moreover, the lowest Cd accumulations in shoots were for the C900 treatment (seeding stage), C1500 treatment (pod-pin stage), C1500 treatment (pod bearing stage) and C900 treatment (maturity stage). With respect to the shells and kernels, the lowest Cd accumulations were for the C1500 and C300 treatments in the maturity stage, respectively. These results demonstrate that there were significant differences in the rates of slaked lime application among the growth stages.
Peanut is considered to be a higher metal accumulator among crops and is thereby applicable for the absorption of heavy metals, with its roots usually gathering much higher concentrations51. In contrast, with the addition of slaked lime, the biomass and Cd content (Table 2 and Fig. 2) in shoots were obviously higher than those in roots. This result indicated that higher Cd accumulations in shoots than in roots. This may be due to the high genotypic variations in Cd accumulation between cultivars12.
BCF and TCF value of Cd in peanut
The bioconcentration factor (BCF) is generally defined as the proportion of metal content in crop tissues to that in soil. The translocation factor (TCF) measures the ability of a crop to transport metals from roots to shoots15.
BCFs obviously decreased for different tissues when the rates of slaked lime application increased (p < 0.05) (Table 4). Compared with C0, BCFs for the roots decreased by 22.46, 32.44, 59.00, 74.87, and 64.88% at the seeding stage; 29.72, 40.16, 64.17, 66.73, and 79.33% at the pod-pin stage; 3.16, 33.03, 67.08, 65.16, and 73.84% at the pod bearing stage and 19.82, 42.91, 47.37, 53.82, and 64.72% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively. The BCFs for the shoots, compared with C0, decreased by 13.38, 25.53, 50.70, 55.81, and 68.13% at the seeding stage; 24.84, 52.59, 59.52, 73.14, and 80.58% at the pod-pin stage; 6.60, 33.88, 49.76, 53.62, and 65.96% at the pod bearing stage and 13.62, 38.69, 45.98, 45.98, and 47.30% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively. In addition, compared with C0, BCFs for the shells and kernels decreased by 29.43, 60.00, 74.72, 79.62, and 83.40% and by 18.75, 45.00, 50.00, 53.75, and 58.75% at the maturity stage for the C300, C600, C900, C1200 and C1500 treatments, respectively.
The accumulation efficiency of metals is characterised by the BCF, and higher BCFs of metals in aboveground sections may lead to hazards for grazers and dissemination of metals in the environment via wind or water15. Therefore, management steps and rational methods for polluted crop treatments, such as liming, should be employed to decrease the hazards that contaminate the food cycle or environment52.
The higher TCFs, the higher Cd content uptake and translocation by plants53. A TCF value above 1 indicates a high capacity for plants to translocate metals54. The results of the TCFs obtained are presented in Table 5. Except for the C1500 treatment in the seeding stage, the TCF values were above 1 for nearly all treatments, which demonstrated the higher tolerance to Cd for peanut in its growth stages. As a conventional common trace metal passivating agent, the efficiency of slaked lime application is related to the application rates55. As shown in Table 5, there were significant differences among the application rates of slaked lime during the growth stages (p < 0.05). Higher Ca content in lime reduces crop uptake of accumulated Cd due to competing Ca and Cd uptake in plant roots56. Previous studies have indicated that the competition between Ca2+ and Cd2+ was one of the crucial reasons for the inhibition of Cd migration, thereby significantly enhancing the Ca content in plants and decreasing the accumulation of Cd in plants33,56,57. Besides, too high soil pH can lead to the dissolution of heavy metals. Within a certain range, the increase of soil pH will enhance the uptake of heavy metals, however, the mobility of heavy metals decreases due to the formation of hydroxyl complexes under strong alkaline conditions, which ultimately leads to metal precipitation33,58,59. This indicated that the optimal amount of slaked lime in this experiment was 900 kg ha−1. De Maria and Rivelli60 reported higher Cd accumulations in roots and older leaves of sunflower at the blossom bud stage, while Cd remigrated at the maturity stage. These results indicated that heavy metal accumulations in the roots in the early growth stages and the transfer to shoots at more mature stages may be a strategy that was utilized by sunflowers to address heavy metal stress. We speculated that peanut, which is an oil crop, may have a similar ability as sunflower.
Effects of slaked lime on the physiological and biochemical characteristics of peanut
Chlorophyll content
Chlorophyll content is an important index for indicating environmental stress in plants and reflects plant tolerance. In addition, chlorophyll content has been considered to be a reliable stress-induced biomarker to assess heavy metal-induced phytotoxicity in a variety of farmland crops61.
Figure 3a shows that the application of slaked lime significantly enhanced chlorophyll contents except for the pod-pin stage (p < 0.05) and that the variations in chlorophyll content were related to the application rates of slaked lime. Compared with C0, the chlorophyll contents increased by 10.53% for the C300 treatment at the seeding stage; 1.3, 3.93, 16.62, and 12.12 and 11.65% for the C300, C600, C900, C1200, and C1500 treatments at the pod-bearing stage, respectively, and by 0.68% for the C1200 treatment at the maturity stage.
Effects of slaked lime application on the contents of (a) chlorophyll and (b) malondialdehyde (MDA) in peanut leaves. The values are means (± SD) of three replicates. Bar groups with different capital letters indicate significant differences (p < 0.05) among the growth stages. Bars with different lowercase letters indicate significant differences (p < 0.05) among the six slaked lime treatments for the same growth stage.
Higher chlorophyll contents were observed in the seeding stage than in the pod-pin stage (Fig. 3a). The main reason is that the chlorophyll contents of peanut increased at the initial stage under mild and moderate Cd stress but decreased as the Cd contents of peanut increased45. Subsequently, slaked lime increased soil pH and reduced Cd availability by improving the surface areas and numbers of special sorption sites for Cd and constituted insoluble metal compounds of higher stability in soil, which decreased the impact of Cd stress on the plants62. Also, application of slaked lime improved the quantity of nutrients for peanuts and enhanced the mineral metabolism of plant Fe (a co-factor related to chlorophyll biosynthesis) and Mn transport63. Furthermore, the correlation between chlorophyll contents and application rates of slaked lime may offer reliable information for the reliability of the rate of slaked lime that is determined to be necessary for application to polluted soil to obtain the expected standard of photosynthesis efficiency and CO2 sequestration64.
MDA content
In general, Cd-induced oxidative problems increased MDA content by generating higher reactive oxygen species (ROS) levels; therefore, observation of MDA in crop tissues could be considered as a criterion of oxidative harm. Obviously, slaked lime significantly influenced the MDA content of peanuts except at the seeding stage (p < 0.05) (Fig. 3b). This effect may be due to the enhanced antioxidant defence system in peanut under Cd stress under application of slaked lime65.
On the one hand, compared with C0, the MDA contents decreased by 47.94, 49.15, 46.85, and 48.18% and by 47.04, 26.48, 33.96, and 22.74% for the C600, C900, C1200, C1500 treatments at the pod-pin and pod bearing stages, respectively. Furthermore, the MDA contents decreased by 31.22, 53.96, 60.85, 51.32 and 66.67% for the C300, C600, C900, C1200, C1500 treatments at the maturity stage, respectively, compared to C0.
Slaked lime acts as a soil conditioner that further increases OM, Ca and major nutrient (e.g., N, P and K) concentrations that are essential for plant growth50. OM and nutrition (particularly N and P) may be related to the combination of polyphenols present and increased activity of phenylalanine ammonia-lyase, which would increase phenolic concentrations and lead to stress alleviation for the crop. Phenolics may be related to the alleviation of metal-induced stress, and flavonoids and phenolics play the role of energetic antioxidants that support ROS alleviation50. K aids in the reduction of O2− and decreases the danger of OH− generation under metal stress66. N provides nutrition for peanuts and facilitates the generation of organic acid–metal complexes in peanut tissues, which mitigate Cd poisoning67.
CAT, POD and SOD activity
Antioxidant enzymes, such as CAT, POD and SOD, represent the physiological reactions of crops to reject oxidative stress that is caused by many abiotic stress components34.
Liming substantially influenced antioxidant enzyme activities (p < 0.05) in the leaves of peanut under Cd-polluted soil68. Figure 4a shows that, with applications of slaked lime, CAT activities showed significant differences at various growth stages except for the pod-pin stage (p < 0.05). Compared with C0, CAT activities for the pod bearing stage increased by 82.98, 85.96, 97.13, 106.38 and 153.51% for the C300, C600, C900, C1200 and C1500 treatments, respectively. However, CAT activities decreased by 17.64, 48.78, 34.38, 67.84 and 57.71% and by 12.34, 18.69, 27.99, 32.55, and 37.03% at the seeding and maturity stages for the C300, C600, C900, C1200 and C1500 treatments, respectively, compared to C0.
Effects of slaked lime application on the contents of (a) catalase (CAT), (b) peroxidase (POD) and (c) superoxide dismutase (SOD) in peanut leaves. The values are means (± SD) of three replicates. Bar groups with different capital letters indicate significant differences (p < 0.05) among the growth stages. Bars with different lowercase letters indicate significant differences (p < 0.05) among the six slaked lime treatments for the same growth stage.
Compared with C0, notable reductions (p < 0.05) were observed for POD activities in the C300 (22.58%), C600 (21.67%), C1200 (51.35%), and C1500 (17.09%) treatments at the seeding stage; C600 (29.87%), C900 (36.99%), C1200 (32.4%), and C1500 (30.9%) at the pod-pin stage; and C600 (48.47%) and C1200 (38.42%) treatments at the pod bearing stage, respectively. However, compared with the other stages, POD activities significantly decreased (p < 0.05) at the maturity stage, and the highest and lowest values were observed for the C300 and C1500 treatments, respectively (Fig. 4b).
Furthermore, for the SOD activities, we observed no remarkable differences (p < 0.05) among all treatments at the seedling stage (Fig. 4c). Compared with C0, SOD activities increased by 37.37, 98.73 and 129.73% for the C300, C600 and C1200 treatments, respectively, but decreased by 1.06 and 91.09% for the C900 and C1500 treatments at the pod-pin stage, respectively. Moreover, the highest SOD activity was recorded for C0, whereas other treatments with slaked lime application led to 17.11, 54.74, 47.02, 84.07 and 29.62%, and to 9.13, 19.57, 22.29, 47.36, 12.54% reductions in SOD activities at the pod bearing and maturity stages, respectively.
The activities of antioxidant enzymes, such as SOD and CAT, in crops can alleviate oxidative stress and neutralize ROS toxicity caused by metals50. Within crop cells, SOD provides the front line of protection by transforming O2− into H2O2. Next, SOD further detoxifies and/or transforms to H2O and O2 with the involvement of CAT. Moreover, POD is regarded as another antioxidant enzyme that transforms H2O2 into O2 and H2O69.
It was previously mentioned that OM and the main nutrients, such as N and P, are the main components which alter SOD and CAT activities70. Essential nutrients, mainly K, can promote the activity of these enzymes when the crop is under metal stress. K aids in the transportation of water and nutrition via the xylem and may further promote the involvement of many other latent enzymes61. Therefore, with the application of slaked lime, supplementing with K increased H2O2 detoxification due to increased SOD and CAT activity50.
Correlation analysis
Figure 5 shows the correlations between the application rates of slaked lime and peanut biomass, Cd content of peanut, and chlorophyll content, MDA content and antioxidant enzyme activity of leaves. Lime application rates (L ar) were significantly positively correlated with soil pH, and a noteworthy negative correlation was found between the Cd and L ar contents of peanut tissues for all growth stages. This result agreed with previous studies that indicated that liming addition in Cd-contaminated soil increased soil pH71, and with another study that reported that soil pH was negatively correlated with heavy metal availability in crops72.
Correlation analysis between the indexes for peanut in the different growth stages. pH soil pH, CAT catalase contents in leaves, POD peroxidase contents in leaves, SOD superoxide dismutase contents in leaves, MDA malondialdehyde contents in leaves, RB root biomass, SB shoot biomass, SLB shell biomass, KB kernel biomass, Chl chlorophyll contents in leaves, R Cd Cd contents in roots, S Cd Cd contents in shoots, SL Cd Cd contents in shells, K Cd Cd contents in kernels, and Lar lime application rate. ** and * indicate a significant correlation at p < 0.01 and p < 0.05 levels, respectively.
With the exception of the seeding stage, L ar was significantly negatively correlated with MDA, which may be due to the activation of the antioxidant defence system, while the impact of slaked lime on MDA was not significantly different at the seeding stage (Fig. 3b). Cd is a nonessential, toxic heavy metal that can promote ROS generation by inhibiting the antioxidant defence system and transportation of electrons in photosystem II. Overgeneration of ROS induces production of higher MDA levels73. Significant positive correlations were noted between L ar and SLB and KB at the maturity stage. Previous studies have shown that slaked lime significantly increased soil pH, alleviated the toxicity of Cd to the photosystem, boosted chlorophyll content, and improved yields45.
Moreover, there were obvious differences between L ar and enzyme activities at different growth stages. Figure 5 shows that L ar was significantly negatively correlated with CAT, POD (seeding stage), POD (pod-pin stage), CAT, and SOD (maturity stage) and was significantly positively correlated with CAT (pod bearing stage). These results indicated that there were significant correlations among the L ar, soil pH, Cd contents in peanut tissues, MDA contents and antioxidant enzyme activities (e.g., CAT, POD, and SOD) at different growth stages.
Source: Ecology - nature.com
