Plant growth
A comparison was performed between the roots and leaves with regard to biomass (dry weight), leaf area, and the length of fresh treetop (Fig. 1) of ‘Neva’ seedlings under different treatments. The results showed that in comparison to the controls (M0, NM0), Cd stress treatment (M100, NM100) inhibited the biological characteristics of ‘Neva’. Compared with the M0 and NM0 treatments, the results for the M100 and NM100 treatments were as follows: leaf biomass decreased by 19.72% and 33.46% (Fig. 1A), respectively; root biomass decreased by 26.46% and 38.32% (Fig. 1B), respectively; leaf area decreased by 32.83% and 29.18% (Fig. 1C), respectively, and this difference was significant (P<0.05); and new-tip length decreased by 9.64% and 9.90% (Fig. 1D), respectively, but this difference was not significant. Moreover, under the NM100 treatment, the decrease in root biomass was the most significant, followed by the decrease in leaf biomass. Additionally, comparison of the NM0 and NM100 treatments showed that the leaf biomass, roots biomass, and growth of new treetop and leaf areas in magnetized plants were all promoted under Cd stress. The change in leaf biomass under the M0 treatment was small compared with the NM0 treatment (0.39%), and the change in leaf biomass was 21.11% higher in the M100 than the NM100 treatment, although this difference was not significant (P>0.05). Between the M0 and NM0 treatments, the root biomass increased by 9.26% and 30.26%, but this difference was not significant (P>0.05); the new-tip was also significantly enhanced by 19.79% and 20.13% (P<0.05). In the M100 treatment, the root biomass increased the most, followed by the leaf biomass. In the magnetic treatments, the leaf area of the plants increased significantly (P<0.05) at rates of 33.23% and 26.37% for the M0 and NM0 treatments, respectively.
Photosynthetic pigment contents
Comparison of the M0 and NM0 treatments revealed decreases in the contents of Chl a (Fig. 2A), Chl b (Fig. 2B), and Car (Fig. 2C) under Cd stress. Specifically, compared with the M0 treatment, the contents of Chl a, Chl b and Car in the M100 treatment decreased by 38.65%, 34.74%, and 31.11%, respectively (all P<0.05), with Chl a showing the greatest decrease followed by Chl b. Compared with NM0, the contents of Chl a, Chl b, and Car in NM100 decreased by 9.27%, 18.54%, and 20.57%, respectively (all P<0.05). In a comparison of NM0 and NM100, we found that magnetic treatment stimulated the synthesis of photosynthetic pigments. Specifically, compared with NM0, the contents of Chl a, Chl b, and Car in M0 increased by 51.26%, 46.25%, and 35.48%, respectively (all P<0.05). Additionally, the photosynthetic pigment contents under M100 improved by 2.28%, 17.17%, and 17.50% compared with NM100, respectively (all P<0.05).
NH4+-N, NO3--N and TN contents
The contents of NH4+-N (Fig. 3A), NO3--N (Fig. 3B), and TN (Fig. 3C) were lower in leaves exposed to Cd conditions than in the control. In particular, compared with M0, NH4+-N, NO3--N and TN contents in M100 decreased by 64.04%, 31.25% and 27.90%, respectively, with significant differences (P<0.05). Moreover, NH4+-N, NO3--N and TN contents in the NM100 treatment decreased by 61.32%, 18.41% and 25.80% compared with the values in the NM0 treatment, respectively, and these differences were significant (P<0.05). The content of NH4+ showed the largest decrease, of 61.32-64.04%, under Cd stress. Additionally, magnetic treatment was beneficial for nitrogen accumulation in leaves. Compared with NM0, the contents of NH4+-N, NO3--N and TN in M0 increased by 65.08%, 50.49% and 7.08%, respectively. Furthermore, NH4+-N, NO3--N and TN contents in the M100 treatment increased by 53.48%, 26.81% and 4.05% compared with that in the NM100 treatment, respectively, and the differences in NH4+-N and NO3--N were significant (P<0.05). Therefore, under magnetic treatment, the increase in NH4+-N was highest and ranged from 53.48 to 65.08%.
However, nitrogen accumulation in roots was slightly different from that in leaves (Fig. 3). Compared with the M0 and NM0 treatments, the contents of NH4+-N and NO3--N in the M100 and NM100 treatments decreased significantly (P<0.05) by 11.19% and 75.07% and 28.69% and 10.74%, respectively, with NH4+-N showing the largest percent decrease (Fig. 3A) at an average of 43.13%. These changes were opposite to what was observed for the TN content (Fig. 3C), which increased by 32.15% and 5.00% in the M100 and NM100 treatments, respectively, with significant differences between the M100 and M0 treatments (P<0.05). Compared with the NM0 and NM100 treatments, the M0 and M100 treatments caused an increase in the accumulation of NH4+-N and TN in the roots, with NH4+-N increasing significantly (P>0.05) by the largest percent. Specifically, the NH4+-N content in M0 was 1.98 times higher than that in NM0, and the content in M100 was 9.62 times higher than that in NM100. The percent increases in TN contents in the M100 and NM100 treatments were relatively small, at 20.32% and 51.43%, respectively, and this difference was significant (P<0.05). Compared with the changes observed in NH4+-N and TN levels, the NO3--N content in roots decreased under magnetic treatment (Fig. 3B), by 8.14% and 26.62% in NM0 and NM100, respectively, with the largest percent decrease observed under the NM100 treatment, which reached a remarkable level (P<0.05).
Activities of key enzymes in nitrogen metabolism
Different effects on the activities of nitrogen metabolism enzymes were found in the M100 and NM100 compared with the control treatments of M0 and NM0 (Fig. 4). Cd significantly stimulated NR activity (Fig. 4A), which increased by 16.81% and 67.43% in the M100 and NM100 treatments, respectively. GS and GOGAT activities also increased in the M100 and NM100 treatments, by 9.09% and 7.23% and by 16.69% and 9.23%, respectively (Fig. 4C, D), and the differences in NR and GOGAT were significant (P<0.05). In contrast, Cd stress slightly inhibited the activity of NiR, which decreased by 3.39% and 7.61% in the M100 and NM100 treatments, respectively (Fig. 4B), though the differences were not significant (P>0.05). When comparing between the NM0 and NM100 treatments, we found that the activities of nitrogen metabolism enzymes increased after magnetic treatment, with the activities of NR, NiR, GS and GOGAT all increasing by 52.47%, 5.58%, 16.69% and 19.28%, respectively, between M0 and NM0 and the differences in NR, GS and GOGAT being significant (P<0.05). Compared with NM100, the activities of NR, NiR, GS and GOGAT in M100 increased by 6.38%, 10.39%, 22.55% and 27.43%, respectively, and the differences in NiR, GS and GOGAT were significant (P<0.05).
The nitrogen metabolism enzymes also responded differently to Cd stress (Fig. 4), which resulted in increased NR, GS and GOGAT activities (Fig. 4A, C, D). Compared with the M0 and NM0 treatments, activities under the M100 treatment increased by 36.49%, 32.10% and 30.55%, respectively, and those under the NM100 treatment increased by 21.50%, 17.17% and 11.16%, respectively. Cd stress had the most stimulating effect on the activity of NR, with the largest percent increase of an average of 29.00%. NR and GS activities were significantly different between treatments (P<0.05), and NiR activity was inhibited under Cd stress (Fig. 4B). Compared with the control treatments, NiR activity in the M100 and NM100 treatments decreased slightly by 1.00% and 5.75%, respectively, but this difference was not significant (P>0.05). Magnetic treatment also enhanced the activities of NR, GS, and GOGAT compared with those in seedlings without magnetic treatment. Specifically, compared with NM0, the activities of NR, GS, and GOGAT under M0 increased by 5.02%, 43.06% and 4.81%, respectively, and the differences in GS and GOGAT activity were significant (P<0.05). Furthermore, NR, GS, and GOGAT activities under M100 increased by 17.98%, 61.28% and 23.10% compared with those in NM100, respectively, and the differences in NR and GS were significant (P<0.05). Therefore, magnetic treatment enhanced GS activity, which showed the largest increase of an average of 52.17%. However, compared with NR, GS and GOGAT, NiR activity decreased after magnetic treatment, and a comparison with the NM0 and NM100 treatments showed that NiR activity in M0 and M100 decreased by 8.80% and 4.12%, respectively, though the differences were not significant (P>0.05).
Free amino acid contents
Analysis of Cys, Glu, Gln, and Gly levels in leaves (Fig. 5) indicated that the contents of Cys, Gln, and Gly decreased under Cd stress compared with the controls (Fig. 5A, C, D). Overall, the contents of Cys, Gln, and Gly in the M100 treatment decreased by 24.69%, 34.32% and 33.89% compared with that in the M0 treatment, respectively, and these differences were significant (P<0.05). Compared with NM0, the contents of Cys, Gln, and Gly in NM100 decreased significantly (P<0.05) by 20.97%, 32.27% and 88.46%, respectively. Moreover, the percent decrease in Gly in leaves was the most significant, reaching an average of 61.18%. Cd stress was beneficial for Glu synthesis (Fig. 5B), which increased by 200.05% in M100 compared with that in M0 and increased by 4.53% in NM100 compared with that in NM0; these changes were significantly different (P<0.05). NM0 and NM100 comparison indicated that magnetic treatment promoted the synthesis of Cys and Gln but inhibited that of Glu and Gly, and the differences between treatments were significant (P<0.05). Compared with NM0 and NM100, the contents of Cys and Gln increased by 109.74% and 73.58% under the M0 treatment, respectively, and by 99.88% and 11.83% under the M100 treatment, respectively. The percent content of Cys increased the most due to the magnetic treatment, by an average of 104.81%. Compared with that in NM0, Glu and Gly contents in M0 decreased by 84.15% and 84.60%, respectively, and compared with NM100, Glu and Gly contents in M100 decreased by 54.50% and 11.83%, respectively. Gly showed the smallest percent decrease by 48.22% on average. Overall, significant differences between treatments were observed for amino acid contents (P<0.05).
Different free amino acids in roots behaved differently under Cd stress. Compared with that in M0, contents of Cys, Glu, Gln and Gly in M100 increased significantly by 67.90%, 7.64%, 0.43% and 5.32%, respectively (Fig. 5), with the largest percent increase observed for Cys and the smallest for Gln. Compared with NM0, Cys, Gln and Gly contents in NM100 decreased by 9.55%, 53.27% and 76.53%, respectively (Fig. 5A, C, D), and the results for Gln and Gly were significantly different (P<0.05). The Glu content in NM100 was 81.06% higher than in NM0 (Fig. 5B), and these differences were significant (P<0.05). Comparison of NM0 and NM100 showed an increasing trend in Cys, Glu, Gln, and Gly contents under magnetic treatment. Specifically, the increase in Cys under M0 treatment was as small as 5.95%, whereas the contents of Glu, Gln, and Gly under this treatment increased dramatically and were 7.83, 3.62, and 6.22 times higher than in the NM0 treatment, respectively. The contents of Cys, Glu, Gln, and Gly in the M100 treatment were 0.97, 4.25, 8.94, and 31.42 times higher than in the NM100 treatment, respectively. Additionally and remarkably, Gly showed the largest percent increase under the magnetic treatment, with an average value 18.82 times higher than under the nonmagnetic treatment. Significant differences among the four groups were found for the contents of Glu, Gln, and Gly (P<0.05).
Contents of the elements K, Ca, Na and Mg
Compared with the control treatments of M0 and NM0, Cd stress treatments exerted different effects on the four mineral elements K, Ca, Na, and Mg in the leaves of ‘Neva’ (Fig. 6). In particular, M100 and NM100 induced an increase in K contents of 52.78% and 8.58% when comparing with M0 and NM0, respectively (Fig. 6A), and the K contents between M100 and M0 were significantly different (P<0.05). Compared with M0, the Ca content in M100 decreased by 9.93% (Fig. 6B). In contrast, the Ca content was significantly increased in NM100 compared with that in NM0 (P<0.05), with a percent increase of 17.12%. M100 treatment induced an increase in Na content by 9.05% when compared with the M0 treatment (Fig. 6C), although compared with NM0, NM100 inhibited Na absorption, which decreased by 9.29%; however, the differences in Na were not significant (P>0.05). Cd stress had a small effect on the Mg content in leaves (Fig. 6D), which increased in the range of 0.17-2.86%. Compared with NM0 and NM100, M0 and M100 significantly inhibited the absorption of K by leaves (Fig. 6A, P<0.05), with decreases of 52.95% and 33.80%, respectively. Conversely, these treatments benefited the accumulation of Ca and Mg in leaves (Fig. 6B, C), which increased by 32.33% and 0.09% in M0 compared with that in NM0, respectively, and by 1.69% and 2.86% in M100 compared with that in NM100, respectively. In addition, Ca contents in M100 and M0 showed significant differences, yet the difference in Mg content was not significant (P>0.05). Compared with NM0, the Na content in M0 decreased by 8.33% (Fig. 6D); compared with NM100, the Na content in M100 increased by 9.25%. Significant differences were not observed between these treatments (P>0.05).
A comparison of M0 and NM0 revealed that Cd induced an increase in K, Ca and Na contents in roots (Fig. 6A, B, D). Specifically, compared with M0, K, Ca and Na contents in M100 increased by 3.84%, 104.64% and 12.37%, respectively, and compared with NM0, K, Ca and Na contents in NM100 increased by 2.13%, 49.31% and 4.17%, respectively. Moreover, a significant difference in Ca contents between treatments was found (Fig. 6B, P<0.05). In contrast, the Mg content decreased by 1.11-4.53% under Cd stress, but these differences were not significant (Fig. 6C, P>0.05). Compared with the NM0 and NM100, the M0 and M100 treatment significantly promoted absorption of K and Ca by roots, with percent increases of 17.83% and 38.75% for M0 and 19.81% and 90.18% for M100, respectively. The Ca content showed the greatest increase in roots, with an average of 64.47%. Although the contents of K and Ca increased, M0 and M100 led to a decrease in uptake of Na and Mg by roots, with percent decreases of 11.95% and 6.87% for M0 and 5.29% and 3.73% for M100, respectively, although these differences were not significant (P>0.05).
Trace element contents
According to the results for the four trace elements of Fe, Mn, Zn, and Cu in leaves (Fig. 7), Cd stress promoted the absorption of Fe, Mn, and Zn compared (Fig. 7A, B, C). Specifically, compared with M0, uptake of Fe, Mn, and Zn in M100 increased by 9.83%, 7.07%, and 21.04%, respectively. Additionally, compared with NM0, the contents of Fe, Mn, and Zn in NM100 increased by 1.03%, 11.52%, and 5.64%, respectively. The change in Fe content was markedly different between M0 and M100 (Fig. 7A, P<0.05). In addition, the Cu content under M100 treatment decreased by 15.78% compared with M0 (Fig. 7D), whereas the Cu content in NM100 decreased by 3.65% compared with NM0; these differences were not significant (P>0.05). Comparison of NM0 and NM100 revealed that magnetic treatment promoted the accumulation of Fe, Mn, Zn, and Cu in leaves. Notably, compared with NM0, Fe, Mn, Zn, and Cu contents in M0 increased by 5.27%, 42.14%, 19.62%, and 84.11%, respectively, with a significant difference in Cu content between the two treatments (P<0.05). Additionally, the Fe, Mn, Zn, and Cu contents in M100 increased by 14.44%, 36.45%, 37.05%, and 49.59% compared with NM100, respectively, and significant differences were observed. Therefore, magnetic treatment led to the largest percent increase in Cu with an average of 66.85%, followed by the percent increase in Mn with an average of 39.30%.
The results indicated that Cd stress inhibited the accumulation of Fe and Zn in roots (Fig. 7A, C) under the M0 and NM0 treatments. Specifically, the contents of Fe and Zn respectively in M100 decreased by 13.16% and 32.16% compared with M0, and these contents in NM100 decreased by 34.11% and 34.09% compared with NM0. Moreover, the Zn content showed the largest percent decrease, with an average content of 33.13%, and the contents of Fe in M100 and M0 were significantly different (P<0.05). In contrast, Cd stress promoted Mn uptake in leaves (Fig. 7B) and led to an improvement of 7.02% and 4.17%, although the differences were not significant (P>0.05). The Cu content in M100 increased by 30.49% (Fig. 7D), whereas the Cu content in NM100 slightly decreased by 2.92%, and significant differences were not observed (P>0.05). Comparison of NM0 and NM100 indicated that magnetic treatment promoted the accumulation of Fe but reduced Mn, Zn, and Cu contents in roots. Fe contents in the M0 and M100 treatments increased by 24.91% and 64.69% compared with NM0 and NM100, respectively, but the differences between M100 and NM100 were not significant. Compared with NM0, the contents of Mn, Zn, and Cu in M0 decreased by 42.24%, 23.17%, and 38.40%, respectively, and these differences were significant. Finally, the contents of Mn, Zn, and Cu in M100 decreased by 40.66%, 20.93%, and 20.78% compared with NM100, respectively. The Mn content decreased the most in roots, with an average of 41.45%.