Species-specific growth responses to cd stress in intercropped system
In kenaf, intercropping did not significantly alter plant height in comparison to monoculture regardless of Cd dose. However, the values were higher compared to monoculture (Fig. S1-A). For rapeseed, the height of the plant decreased by 20.19% and 14.19% at soil Cd concentrations of 0 and 15 mg kg− 1, respectively, compared to monoculture. At a Cd concentration of 40 mg kg− 1 in the soil, intercropping resulted in a non-significant decrease in plant height compared to monoculture. This differential response suggests species-specific interactions in the intercropping system. The elevated Cd concentration in the soil impeded the growth of kenaf and rapeseed plants, as evidenced by a decrease in the dry weight of both the shoot and root (Fig. 1A-B). Intercropping kenaf and rapeseed effectively mitigated the reduction in biomass caused by Cd stress and increased shoot dry weight by 18.19%, 17.92%, and 35.26% (Fig. 1A) and root dry weight by 18.47%, 16.47%, and 28.26% (Fig. 1B) in kenaf plants cultivated under 0, 10, and 40 mg kg− 1 soil Cd, respectively, Compared to monoculture. In rapeseed, the dry weight of shoots increased by 26.15%, 31.32%, and 25.66%, and the dry weight of roots increased by 7.69%, 27.78%, and 15.38% at Cd concentrations of 0, 10, and 40 mg kg− 1 in the soil, respectively, compared to the monoculture. These results show that intercropping effectively mitigates the reduction in biomass caused by Cd stress in both kenaf and rapeseed and evaluate the potential of intercropping as a strategy to improve plant growth and productivity under Cd stress.
Furthermore, in intercropping, kenaf stem diameter increased by 19.49%, 22.03%, and 13.08% in soil containing 0, 15, and 40 mg kg− 1 of Cd, respectively, compared to monoculture (Fig. S1-B). However, intercropped rapeseed stem diameter decreased by 13.55%, 15.73%, and 23.74% compared to monocropped plants under 0, 15, and 40 mg kg− 1 Cd in soil, respectively, suggests species-specific responses to Cd contamination and intercropping interactions. The study observed no difference in leaf surface area between monocropping and intercropping kenaf plants (Fig. S1-C). Rapeseed intercropped plants had 25.58%, 36.45%, and 9.92% higher leaf surface area than monocultured plants under 0, 15, and 40 mg kg− 1 Cd in soil, respectively. This varying effects on leaf surface area between kenaf and rapeseed highlights the potential of intercropping to influence canopy development and light interception, which are critical for photosynthesis and plant productivity.
Photosynthetic pigment dynamics in kenaf and rapeseed under cd stress through intercropping
The photosynthetic pigments of kenaf and rapeseed decreased with increasing Cd content in the soil. This indicates the negative effects of Cd stress on photosynthetic efficiency of both species (Fig. S2). Intercropping leads to an increase in Chla, Chlb, and Cart content in Cd-stressed kenaf and rapeseed plants compared to monoculture. Intercropping enhanced the content of Chla by 6.25% (Fig. S2-A), Chlb by 14.98% (Fig. S2-B), and Cart by 38..80% (Fig-S2C) in kenaf under 15 mg kg− 1 Cd in soil. In rapeseed, intercropping resulted in a 4.58% increase in Chla, 16.19% increase in Chlb, and 21.31% increase in Cart at Cd concentration of 15 mg kg− 1, compared to monoculture. At 40 mg kg− 1 Cd in soil, intercropping increased the Chla, Chlb, and Cart contents of kenaf by 5.66%, 12.30%, and 3.99%, respectively, in comparison to monoculture. Likewise, in rapeseed, intercropping exhibited significant increases in Chla (11.31%), Chlb (11.12%), and Cart (27.67%) compared to the monoculture at a concentration of 40 mg kg− 1 Cd. In conclusion, both kenaf and rapeseed showed an increase in photosynthetic pigments when intercropped, although the extent of this increase vary between species. This insight into species-specific responses provides valuable information for adapting intercropping strategies to specific plant species and environmental conditions.
In addition, we measured the SPAD index of intercropped and monocultured rapeseed and kenaf plants to corroborate photosynthetic pigment analysis. As soil Cd increased, SPAD values decreased (Fig. S2-D), however, kenaf and soybean intercropping increased SPAD values at all soil Cd levels. This observation agrees with the results of the photosynthetic pigment analysis.
Resilience of photosynthetic processes in intercropped kenaf and rapeseed under cd stress
In kenaf, intercropping resulted in a significant increase in Pn by 2.22%, 10.92%, and 8.82% and Gs by 31.81%, 26.31%, and 11.76% at soil Cd concentrations of 0, 15, and 40 mg kg− 1, respectively, compared to monoculture (Table 1). Intercropping had no significant effect on Ci in kenaf at all tested Cd concentrations, but values were higher than in monoculture. In addition, Tr exhibited a notable rise of 21.75% at a soil Cd concentration of 0 mg kg− 1. However, there were no significant alterations in Tr at soil Cd concentrations of 15 and 40 mg kg− 1, in comparison to the monoculture. In rapeseed, intercropping significantly increased Pn by 11.54%, 23.35%, and 8.21%, Gs by 35.92%, 63.95%, and 33.33% at 0, 15, and 40 mg kg− 1 soil Cd, and Ci by 6.30% and 1.85% at 0 and 15 mg kg− 1 soil Cd, respectively, when compared to monoculture. Intercropping had no significant effect on Ci in rapeseed at 40 mg kg− 1 soil Cd. Moreover, Intercropping resulted in an increase in Tr under all tested Cd treatments. However, the increase was statistically significant only at the Cd concentration of 0 mg kg− 1 soil, when compared to monoculture (Table 1). Overall, these results show that intercropping improves photosynthetic performance and physiological processes related to photosynthesis in both kenaf and rapeseed, even under Cd stress conditions. This illustrate the potential of intercropping as a sustainable strategy to improve the resilience and productivity of crops in polluted environments.
Table 1
The impact of intercropping on the photosynthetic gas exchange parameters of kenaf and rapeseed cultivated in soil contaminated with Cd.
Crop | Treatment (mg.kg− 1) | Cropping pattern | Pn (umol CO2 m− 2 s− 1) | Gs (mol m− 2 s− 1) | Ci (umol/mol) | Tr (mmol H2O m− 2 s− 1) |
Kenaf | 0 | Mono | 9.01 ± 0.35a | 0.22 ± 0.008b | 168.71 ± 1.30b | 6.30 ± 0.02b |
Inter | 9.21 ± 0.16a | 0.29 ± 0.009a | 171.57 ± 2.45b | 7.67 ± 0.23a |
15 | Mono | 7.51 ± 0.38cd | 0.19 ± 0.001c | 184.96 ± 4.23a | 6.22 ± 0.01b |
Inter | 8.33 ± 0.42b | 0.24 ± 0.027b | 185.90 ± 1.52a | 6.45 ± 0.24b |
40 | Mono | 7.03 ± 0.07d | 0.17 ± 0.004d | 183.07 ± 1.95a | 5.68 ± 0.01c |
Inter | 7.65 ± 0.11c | 0.19 ± 0.001c | 185.10 ± 1.51a | 5.86 ± 0.11c |
rapeseed | 0 | Mono | 8.49 ± 0.33b | 1.03 ± 0.012b | 213.49 ± 1.32d | 13.91 ± 0.17ab |
Inter | 9.47 ± 0.24a | 1.40 ± 0.048a | 226.94 ± 1.25a | 14.67 ± 0.07a |
15 | Mono | 7.45 ± 0.26cd | 0.86 ± 0.009c | 222.51 ± 2.29bc | 13.26 ± 0.08b |
Inter | 9.19 ± 0.51a | 1.41 ± 0.011a | 226.63 ± 1.66ab | 13.76 ± 0.08ab |
40 | Mono | 7.19 ± 0.21d | 0.75 ± 0.017d | 220.39 ± 3.99c | 11.40 ± 0.12c |
Inter | 7.78 ± 0.30c | 1.00 ± 0.008b | 222.36 ± 2.46c | 12.01 ± 1.61c |
Oxidative stress responses in intercropped kenaf and rapeseed under cd contamination
The contrasting responses of kenaf and rapeseed to intercropping in terms of oxidative stress levels illustrate species-specific differences in stress tolerance mechanisms. High soil Cd concentration caused oxidative stress in kenaf and rapeseed, but intercropping led to significant reduction in O2− and MDA levels in kenaf compared to monocultures (Fig. 2). In comparison to monoculture, intercropping reduced the O2− content by 10.84% and 28.67% and the MDA content by 41.10% and 15.59% at 15 and 40 mg kg− 1 soil Cd, respectively. Intercropping had no significant effect on the O2− and MDA content of kenaf at 0 mg kg− 1 soil Cd; nonetheless, values were lower than those of monoculture. Additionally, intercropping increased kenaf H2O2 content by 30.39%, 27.93%, and 58.20% compared to monoculture. In rapeseed, intercropping significantly increased O2− levels by 26.64% and 15.47% at 15 and 40 mg kg− 1 soil Cd, respectively, and decreased O2− levels by 8.36% at 0 mg kg− 1 soil Cd compared to monoculture. Additionally, intercropping led to a decrease in H2O2 content and an increase in MDA content in rapeseed at all soil Cd concentration. However, these changes were not statistically significant compared to monoculture.
Species-Specific Antioxidant Responses in Cd-Stressed Kenaf and Rapeseed through Intercropping
In this study it was observed that intercropping of kenaf and rapeseed increased the activities of enzymatic antioxidant compared to monoculture in both crop species (Fig. 3). In kenaf, SOD activity was increased by 72.32%, 93.19% and 15.48%, POD increased by 54.21%, 79.49%, and 49.80% and CAT increased by 31.79%, 29.35%, and 28.72% at 0, 15 and 40 mg kg− 1 soil Cd, respectively compared to the monoculture. In rapeseed, the activity of SOD increased by 23.45%, 27.78% and 23.60% and POD increased by 36.71%, 52.03% and 70.70% at 0, 15 and 40 mg kg− 1 soil Cd, respectively compared to the monoculture. No significant difference was observed in CAT activity of monocultured and intercropped plants while values of the CAT activity was higher in intercropped plants compared to the monocultured plants. While both species showed an increase in enzymatic antioxidant activities with intercropping, the magnitudes of these increases differed, indicating potential differences in stress tolerance mechanisms between kenaf and rapeseed. Furthermore, the content of non-enzymatic antioxidants (AsA and GSH) was also increased in intercropping system compared to the monoculture system. In kenaf, intercropping of kenaf and rapeseed significantly increased AsA content by 18.75% and 9.88% at 0 and 40 mg kg− 1 of Cd in soil and GSH content increased by 29.51% and 53.27% at 15 and 40 mg kg− 1 soil Cd, respectively, compared to their monoculture. In case of rapeseed, AsA content significantly increased by 31.35% and 47.73% at 15 and 40 mg kg− 1 of Cd in soil, respectively in intercropping system compared to the monoculture system. On contrary, no significant difference observed in the GSH content of rapeseed at various Cd concentrations between the intercropping and monoculture systems.
Cadmium uptake and translocation dynamics in intercropped kenaf and rapeseed
The concentration of Cd in kenaf and rapeseed tissues increased as the amount of Cd in the soil increased (Table 2). Intercropping significantly increased the Cd content in kenaf shoots and roots by 48.12 and 35.49% at 15 mg kg− 1 soil Cd, respectively, and by 16.14 and 10.73% at 40 mg kg− 1 soil Cd, respectively, when compared to monoculture. Intercropping also increased Cd content in shoots and roots of rapeseed. Compared to monoculture, rapeseed shoot Cd content increased by 17.77% and 15.33% and root Cd content increased by 21.59% and 17.04% at soil Cd concentrations of 15 and 40 mg kg− 1, respectively. Both kenaf and rapeseed exhibited higher Cd accumulation in the roots compared to aboveground parts, indicating preferential Cd uptake and accumulation in the root system.
The phytoextraction of Cd in kenaf and rapeseed was measured by estimating BCF, TF, and PEE values (Table S1). Intercropping significantly enhanced kenaf and rapeseed BCF values in soil to 15 and 40 mg kg− 1 Cd compared to monoculture. In kenaf, intercropping resulted in a non-significant increase in TF by 8.97% at 15 mg kg− 1 soil Cd and 4.76% at 40 mg kg− 1 compared to monoculture. In contrast, at soil Cd concentrations of 15 and 40 mg kg− 1, intercropped rapeseed had lower TF values than monoculture with non-significant difference. Furthermore, compared to monoculture, intercropping significantly increased the PEE of kenaf plants when they were exposed to soil containing 15 and 40 mg kg− 1 Cd. Compared to monoculture, intercropping decreased rapeseed PEE at 15 mg kg− 1 soil Cd and increased it at 40 mg kg− 1. The overall increase in Cd accumulation in plant tissues under intercropping suggests possible differences in the mechanisms of Cd uptake and translocation between the two species. Understanding these species-specific responses is essential for optimizing phytoremediation strategies in contaminated environments.
Table 2
The uptake and distribution of Cd in the shoot and root of kenaf and rapeseed plants cultivated under monoculture and intercropping systems at varying levels of Cd in the soil.
Treatment (mg.kg-1) | Cropping pattern | Kenaf | rapeseed |
| Shoot | Root | Shoot | Root |
0 | Mono | 2.49 ± 0.19e | 2.85 ± 0.14e | 1.99 ± 0.17e | 2.15 ± 0.06e |
| Inter | 1.18 ± 0.07e | 1.198 ± 0.22e | 2.43 ± 0.31e | 3.22 ± 0.08e |
15 | Mono | 24.19 ± 1.52d | 30.99 ± 0.66d | 77.15 ± 1.86d | 81.04 ± 1.35d |
| Inter | 35.83 ± 1.67c | 41.99 ± 1.52c | 90.86 ± 0.94c | 98.54 ± 1.34c |
40 | Mono | 53.09 ± 1.64b | 83.81 ± 0.57b | 160.94 ± 3.37b | 166.14 ± 5.09b |
| Inter | 61.66 ± 1.51a | 92.8 ± 1.49a | 185.62 ± 1.28a | 194.46 ± 1.56a |
The values in the table are represented as the mean ± standard deviation (n = 3). The presence of distinct letters within a column denotes a statistically significant difference (p ≤ 0.05). Mono: monoculture, Inter: Intercropping.
Soil pH and available cd content under kenaf and rapeseed intercropping
Intercropping of kenaf and rapeseed resulted in lower soil pH values than monoculture at Cd concentrations of 0, 15, and 40 mg kg− 1 (Fig. 4A), although there was no significant difference at 0 and 15 mg kg− 1 soil Cd. The intercropping of kenaf and rapeseed at a soil Cd concentration of 40 mg kg− 1 resulted in the most significant reduction in soil pH, compared to monoculture. The pH of the soil was greater in rapeseed monoculture than in kenaf monoculture, although the difference was not statistically significant at 0 and 15 mg kg− 1 Cd. The significant increase in available Cd in soil under intercropping compared to monocropping at higher soil Cd concentrations indicates that intercropping can influence mobility and availability of Cd in the soil.
The pH of the soil in the rapeseed monoculture was significantly higher than that of the kenaf monoculture at 40 mg kg− 1 Cd. In addition, intercropping significantly increased soil available Cd content compared to monoculture at soil Cd concentration of 15 and 40 mg kg− 1. The soil available Cd content in both monocultured and intercropped soil increased as the concentration of Cd in the soil increased (Fig. 4B). The significant reduction in soil pH in the intercropping system suggests a potential interaction between Cd contamination and intercropping effects on soil acidity, which is valuable for understanding the dynamics of soil health in contaminated environments.
Cd-induced variations in soil enzyme activities under intercropping and monoculture system
current investigation found that increasing soil Cd enhanced soil urease and catalase activity while decreased sucrase activity in monoculture and intercropping rhizosphere (Fig. 5). Intercropped rhizosphere had significantly higher urease and catalase activity than monocultured rhizosphere. Soil urease activity in intercropped rhizosphere increased by 227.27%, 57.28% and 49.70% compared to kenaf monocultured rhizospherel and by 13.12%, 3.75% and 9.32% compared to rapeseed monocultured rhizosphere at 0, 15, 40 mg kg− 1 Cd in soil (Fig. 5A). The catalase activity in kenaf and rapeseed intercropped rhizosphere was increased by 28.57%, 26.09% and 29.41% compared to kenaf monoculture and by 22.73%, 12.84% and 29.41% compared to rapeseed monoculture rhizosphere at 0, 15 and 40 mg kg− 1 soil Cd, respectively (Fig. 5B). In contrast, intercropped rhizosphere showed significant decrease in soil sucrase activity by 30.72%, 39.70% and 43.86% compared to kenaf monoculture rhizosphere and 19.51%, 18.41% and 1.16% compared to the rapeseed monoculture rhizosphere at 0, 15 and 40 mg kg− 1 Cd soil respectively (Fig. 5C). In summary, this finding suggests that Cd contamination influences soil enzyme dynamics, indicating the potential for soil enzyme activities to serve as indicators of Cd contamination levels in soil.