Arsenic fractionation
Among different As fractions, L-As, Ca-As, OM-As and R-As increased while Fe-As and Al-As decreased by increasing the PR level under As stress in three different textured soils (Fig. 1). L-As increased by 17.1, 30.4 and 47.9% while, 35.7, 54.9 and 70.9% in sandy soil, 19.2, 43.2 and 64.4% while 11.7, 39.9 and 52.6% in loamy soil, and 22.8, 34.3 and 60.0% while 21.3, 53.3 and 68.0% in clayey soil with PR5, PR10 and PR20 at As60 and As120, respectively compared to respective As treatment without PR (Fig. 1a). Ca-As increased with PR supplementation, and maximum increase was 63.6 and 72.3% in sandy, 62.0 and 49.0% in loamy soil, and 60.1 and 35.8% in clayey soil at As60 and As120, respectively with PR20 compared to respective As treatment without PR (Fig. 1b). PR20 increased OM-As fraction by 50.0 and 85.0% in sandy soil, 55.0 and 54.3% in loamy soil, and 78.7 and 36.9% in clayey soil at As60 and As120, respectively compared to respective As treatment without PR (Fig. 1c). R-As increased by 46.2 and 23.3% in sandy soil, 57.9 and 36.5% in loamy soil, and 78.1 and 51.6% in clayey soil with PR20 at As60 and As120, respectively compared to respective As treatment without PR (Fig. 1d). In contrast, Al-As decreased with PR supplementation and maximum decrease was 42.8 and 44.6% in sandy soil, 37.3 and 42.7% in loamy soil, and 26.8 and 41.4% in clayey soil with PR20 at As60 and As120, respectively compared to respective As treatment without PR (Fig. 1e). Fe-As decreased by 36.7 and 31.7% in sandy soil, 44.5 and 43.6% in loamy soil, and 38.2 and 35.7% in clayey soil at As60 and As120, respectively with PR20 compared to respective As treatment without PR (Fig. 1f). Although, PR5, PR10 and PR20 had a marked effect on As fractionation in all the three textured soils but the effect was more profound at highest PR level (PR20), and in sandy textured soil.
Labile As percentage
The percentage of total As in labile form varied significantly (p ≤ 0.05) with PR in all the three soil textural types (Fig. 2). PR supplementation under As stress increased L-As percentage. At As60, L-As percentage increased by 16.9, 30.8, 48.0% in sandy, 19.1, 43.1 and 64.0% in loamy, while 21.8, 34.5 and 58.2% in clayey soil with PR5, PR10 and PR20, respectively compared to As treatment without PR. At As120, L-As percentage increased by 36.0, 55.2 and 68.1% in sandy, 11.5, 39.6 and 52.3% in loamy soil while 22.3, 54.6 and 70.0% in clayey soil by PR5, PR10 and PR20, respectively compared to As treatment without PR.
Soil soluble P concentration
Soluble P concentration in soil increased significantly (p ≤ 0.05) with increasing the level of PR under As stress (Fig. 3). However, the increase in soluble P was relatively higher at As60 compared to As120. PR supplementation increased the soluble P in sandy soil by 34.5, 58.6 and 144.8% at As60 while, 24.1, 41.4 and 103.4% at As120 with PR5, PR10 and PR20, respectively compared to respective As treatment without PR. In loamy soil, soluble P increased by 31.6, 52.6 and 78.9% at As60 while, 18.4, 32.6 and 59.2% at As120 by PR5, PR10 and PR20, respectively compared to respective As treatment without PR. In clayey soil, the increase in soluble P was 22.4, 50.0 and 63.1% at As60 while, 17.4, 30.4 and 49.3% at As120 with PR5, PR10 and PR20 compared to respective As treatment without PR. When comparing the soil textural types, maximum soluble P was found in clayey followed by loamy and sandy soil in descending order.
Labile As: soluble P ratio
L-As: soluble P ratio decreased with increasing PR supplementation level, particularly at As60 (Fig. 4). L-As: soluble P ratio decreased by 12.9, 17.7 and 39.5% in sandy, 9.3, 6.0 and 8.2% in loamy while 0, 10.0 and 2.2% in clayey soil at As60 with PR5, PR10 and PR20, respectively compared to As treatment without PR. At As120, L-As: soluble P ratio was affected by As and PR but no clear trend was observed. When comparing the soil textural types, maximum reduction in L-As: soluble P ratio with the supplementation of PR was found in sandy soil followed by loamy and clayey soils in descending order. Labile As: soluble P ratio was negatively correlated with achene yield (Fig. 5) in sandy soil (R2 = 0.8024), loamy soil (R2 = 0.9345) and clayey soil (R2 = 0.7599).
Plant growth and yield characteristics
Plant growth and yield characteristics of sunflower in term of plant height, head diameter, head weight, and number of achenes head− 1 reduced significantly (p ≤ 0.05) by both levels of As contamination, leading to a reduction in achene yield of 23.3, 20.9 and 17.6% at As60 while 46.7, 50.0 and 34.4% at As120 in sandy, loamy and clayey soils, respectively compared to control (Table 1). PR supplementation at different levels released P into soil solution which reduced As uptake, and improved plant growth characteristics. Achene yield improved by 7.3, 14.6 and 18.3% in sandy, 4.3, 9.4 and 12.8% in loamy, and 5.8, 7.8 and 14.5% in clayey soils at As60 with PR5, PR10 and PR20, respectively compared to As60 without PR. At As120, achene yield improved by 19.3, 29.8 and 52.6% in sandy, 9.4, 18.9 and 25.7% in loamy, and 7.3, 12.2 and 21.9% in clayey soil with PR5, PR10 and PR20, respectively compared to As120 treatment without PR.
Table 1
Growth and yield response of sunflower (Helianthus annuus L.) to phosphorus under arsenic toxicity in three texturally different soils
As levels (mg kg− 1) | PR (g kg− 1) | Plant height (cm) | Head diameter (cm) | Head weight (g) | Number of achenes head− 1 | Achene yield (g plant− 1) |
S | L | C | S | L | C | S | L | C | S | L | C | S | L | C |
Control | PR-0 | 95.3b | 105.8a | 102.4a | 8.4ab | 8.7a | 8.6a | 27.9ab | 31.6a | 31.1a | 338c | 452a | 398b | 10.7c | 14.8a | 12.5b |
As-60 | PR-0 | 67.6de | 79.5cd | 81.7c | 5.9ef | 6.6de | 7.0cd | 12.7de | 20.2c | 21.6c | 244e | 367bc | 306cd | 8.2d | 11.7bc | 10.3c |
PR-5 | 75.2d | 83.7c | 87.6bc | 7.4c | 7.1cd | 8.0b | 16.1d | 22.2bc | 24.3bc | 272de | 389b | 314cd | 8.8d | 12.2b | 10.9bc |
PR-10 | 87.3bc | 90.6b | 91.2b | 7.8bc | 7.6c | 8.2ab | 22.9bc | 25.3b | 27.3ab | 297d | 409ab | 336c | 9.4bc | 12.8b | 11.1bc |
PR-20 | 88.5bc | 91.7b | 94.2b | 8.2ab | 8.4ab | 8.3ab | 24.3bc | 27.1ab | 27.7ab | 316cd | 422ab | 352bc | 9.7cd | 13.2ab | 11.8bc |
As-120 | PR-0 | 53.5f | 65.3e | 70.4d | 5.3fg | 6.1e | 6.6de | 9.7e | 14.9d | 19.3cd | 196f | 289d | 242e | 5.7ef | 7.4de | 8.2d |
PR-5 | 66.4de | 73.2d | 74.3d | 6.3e | 6.6de | 7.5c | 12.7de | 16.9d | 21.7c | 227e | 322cd | 270de | 6.8e | 8.1d | 8.8d |
PR-10 | 72.5d | 78.1cd | 77.4cd | 6.8d | 7.2cd | 7.9b | 17.3cd | 19.7c | 23.7bc | 249de | 349c | 292d | 7.4de | 8.8d | 9.2cd |
PR-20 | 83.4c | 81.1c | 82.8c | 7.1cd | 7.4c | 7.7bc | 20.8c | 21.7c | 24.9b | 264de | 397b | 304cd | 8.7d | 9.3cd | 10.0c |
Mean values are of three replicates (n = 03). In each column, values with different letters differ significantly from each other at specific p ≤ 0.05 value following Duncan’s Multiple Range Test. (Control: no external As; As-60: 60 mg As kg− 1 soil; As-120: 120 mg As kg− 1 soil; PR0: no addition of phosphate rock; PR-5: 5 g phosphate rock kg− 1 soil; PR-10: 10 g phosphate rock kg− 1 soil; PR-20: 20 g phosphate rock kg− 1 soil. S: sandy soil; L: loamy soil; C: clayey soil). |
Physiological characteristics
H2O2 concentration in the sunflower leaves increased by 56.5, 43.5 and 35.6% at As60 while 89.6, 75.8 and 83.4% at As120 in sandy, loamy and clayey soils, respectively compared to control (Table 2). PR supplementation at all levels decreased H2O2, maximum reduction with PR20 at both levels of contamination. MDA concentration increased by 135.2, 152.6 and 132.7% at As60 while 225.2, 235.5 and 247.8% at As120 in sandy, loamy and clayey soils, respectively compared to control (Table 2). PR supplementation under As stress protected the plants against oxidative damage as evidenced by marked reduction in MDA concentration, more prominent effect with PR20. Maximum GSH was found in control which decreased by 42.9 and 72.2% in sandy, 36.5 and 69.6% in loamy and 30.6 and 69.5% in clayey soils at As60 and As120, respectively compared to control (Table 2). PR supplementation at all levels improved the GSH synthesis in As polluted soils. However, maximum improvement of 87.7 and 237.3% in sandy, 62.0 and 245.1% in loamy, and 54.9 and 248.7% in clayey soils was found with PR20 at As60 and As120, respectively compared to respective As treatment without PR. Glutathione concentration was positively correlated with achene yield (Fig. 6) in sandy soil (R2 = 0.7841), loamy soil (R2 = 0.4162) and clayey soil (R2 = 0.6306).
Table 2
Toxic effects of arsenic on physiological characteristics of sunflower (Helianthus annuus L.) in response to phosphorus in three texturally different soils
Treatments | H2O2 (µmol g− 1 FW) | MDA (µmol g− 1 FW) | GSH (mg g− 1 FW) |
As (mg kg− 1) | PR (g kg− 1) | S | L | C | S | L | C | S | L | C |
Control | PR-0 | 14.82e | 13.60ef | 11.96fg | 11.88e | 10.12ef | 8.97f | 29.12ab | 32.52a | 33.15a |
As-60 | PR-0 | 23.26bc | 19.52cd | 16.22de | 27.94bc | 25.57c | 20.88cd | 16.63d | 20.64cd | 22.98c |
| PR-5 | 19.64cd | 16.96de | 13.56ef | 24.22c | 19.77d | 17.12d | 20.65cd | 26.78b | 27.88b |
| PR-10 | 17.20de | 14.85e | 12.96f | 18.67d | 16.78de | 14.44de | 26.88b | 30.66ab | 31.44ab |
| PR-20 | 13.96ef | 12.44f | 10.38fg | 15.10de | 12.76e | 10.66ef | 31.22ab | 33.44a | 35.60a |
As-120 | PR-0 | 28.10a | 23.92b | 21.96c | 38.64a | 33.96ab | 31.24b | 8.10f | 9.88ef | 10.10ef |
| PR-5 | 25.42b | 20.62c | 18.60d | 27.96bc | 26.10c | 23.55cd | 14.22de | 18.79d | 19.76cd |
| PR-10 | 20.18cd | 17.77d | 15.95e | 24.60c | 21.92cd | 19.65d | 23.10c | 27.55b | 26.55bc |
| PR-20 | 17.45de | 15.20e | 14.10ef | 18.24d | 16.88de | 12.96e | 27.32c | 34.10a | 35.22a |
Mean values are of three replicates (n = 03). In each column, values with different letters differ significantly from each other at specific p ≤ 0.05 value following Duncan’s Multiple Range Test. (Control: no external As; As-60: 60 mg As kg− 1 soil; As-120: 120 mg As kg− 1 soil; PR0: no addition of phosphate rock; PR-5: 5 g phosphate rock kg− 1 soil; PR-10: 10 g phosphate rock kg− 1 soil; PR-20: 20 g phosphate rock kg− 1 soil. S: sandy soil; L: loamy soil; C: clayey soil). |
Plant arsenic concentration
The plant As concentration increased significantly (p ≤ 0.05) at both levels of As contamination and distributed in different plant parts in order of roots > shoots > achenes (Table 3). Root As concentration increased by 7.24, 6.02 and 5.17 folds at As60 while, 17.86, 13.96 and 11.68 folds at As120 in sandy, loamy and clayey soils, respectively compared with control. Shoot As increased by 5.66, 4.44 and 3.65 folds at As60 while, 8.83, 7.68 and 6.55 folds at As120 in sandy, loamy and clayey soils, respectively compared to control. Likewise, achene As increased by 2.92, 2.70 and 2.62 folds at As60 while, 5.30, 3.50 and 3.50 folds at As120 in sandy, loamy and clayey soils, respectively compared to control. PR supplementation to As contaminated soils reduced its accumulation by plants. Although, all PR supplementation levels effectively reduced As concentration in plant tissues but the effect was more profound with PR20 which decreased root As by 41.5, 45.1 and 43.7%, shoot As 46.0, 41.6 and 35.2%, and achene As 46.3, 46.6 and 50.0% in sandy, loamy and clayey soils, respectively at As120 compared to respective As treatment without PR. Achene As concentration was negatively correlated with achene yield (Fig. 7) in sandy soil (R2 = 0.9679), loamy soil (R2 = 0.5431) and clayey soil (R2 = 0.7027).
Table 3
Arsenic concentration in sunflower (Helianthus annuus L.) grown at different levels of arsenic and phosphorus in three texturally different soils
Treatments | Root As (mg kg− 1 DW) | Shoot As (mg kg− 1 DW) | Achene As (mg kg− 1 DW) |
As (mg kg− 1) | PR (g kg− 1) | S | L | C | S | L | C | S | L | C |
Control | PR-0 | 10.1g | 8.8gh | 7.3gh | 6.5fg | 4.7g | 3.8g | 0.13ef | 0.10f | 0.08f |
As-60 | PR-0 | 83.3de | 61.8ef | 45.1f | 43.3e | 25.6de | 17.7ef | 0.51c | 0.37d | 0.29de |
| PR-5 | 62.3ef | 50.2f | 40.1f | 34.7d | 22.1e | 14.5f | 0.40d | 0.32de | 0.27de |
| PR-10 | 57.1ef | 39.6f | 32.8fg | 28.6de | 19.6ef | 12.5f | 0.34d | 0.27de | 0.21e |
| PR-20 | 48.7f | 33.9fg | 25.4fg | 25.8de | 17.9ef | 10.7f | 0.26e | 0.22e | 0.16ef |
As-120 | PR-0 | 190.5a | 131.7c | 92.6de | 63.9a | 40.8c | 28.7de | 0.82a | 0.45cd | 0.36de |
| PR-5 | 125.8cd | 111.7d | 81.1e | 51.4b | 35.2d | 21.9e | 0.59bc | 0.38d | 0.29de |
| PR-10 | 105.5d | 93.6de | 74.9e | 42.3c | 28.5de | 18.8ef | 0.53c | 0.29de | 0.25e |
| PR-20 | 95.7d | 82.9de | 62.4ef | 34.5d | 23.8e | 17.6ef | 0.44cd | 0.24e | 0.18ef |
Mean values are of three replicates (n = 03). In each column, values with different letters differ significantly from each other at specific p ≤ 0.05 value following Duncan’s Multiple Range Test. (Control: no external As; As-60: 60 mg As kg− 1 soil; As-120: 120 mg As kg− 1 soil; PR0: no addition of phosphate rock; PR-5: 5 g phosphate rock kg− 1 soil; PR-10: 10 g phosphate rock kg− 1 soil; PR-20: 20 g phosphate rock kg− 1 soil. S: sandy soil; L: loamy soil; C: clayey soil). |
Bioaccumulation and translocation factors
PR supplementation to As contaminated soil decreased BF in all the three textural types. Results revealed that BF reduced by 24.6, 31.1 and 42.7% in sandy, 20.0, 36.2 and 46.6% in loamy, and 10.6, 30.6 and 46.6% in clayey soils at As60 with PR5, PR10 and PR20 compared to As60 treatment without PR. Likewise, BF reduced by 33.5, 44.3 and 49.3% in sandy, 14.7, 28.4 and 38.5% in loamy, and 14.5, 19.7 and 31.6% in clayey soils at As120 with PR5, PR10 and PR20, respectively compared to As120 treatment without PR. TF indicates the movement of metalloid from roots to shoots. A value of TF greater than one is the indicator of effective translocation of metalloid from roots to shoots. When comparing the textural types, highest TF was found in sandy followed by loamy and clayey soils, regardless of PR addition. In contrast to BF, there was no clear trend regarding the effect of PR on TF in all the three soil textural types at both levels of As contamination (Table 4).
Table 4
Bioaccumulation and translocation factors in sunflower (Helianthus annuus L.) grown at different levels of arsenic and phosphorus in three texturally different soils
Treatments | Bioaccumulation Factor | Translocation Factor |
As level (mg kg− 1) | PR (g kg− 1) | S | L | C | S | L | C |
As-60 | PR-0 | 1.38b | 1.05cd | 0.75e | 0.52a | 0.41c | 0.39c |
| PR-5 | 1.04d | 0.84de | 0.67ef | 0.54a | 0.44bc | 0.35d |
| PR-10 | 0.95d | 0.67ef | 0.52f | 0.49ab | 0.49ab | 0.38cd |
| PR-20 | 0.79e | 0.56f | 0.40g | 0.53a | 0.53a | 0.42c |
As-120 | PR-0 | 1.58a | 1.09cd | 0.76e | 0.34d | 0.31de | 0.32de |
| PR-5 | 1.05cd | 0.94d | 0.65ef | 0.42c | 0.31de | 0.28e |
| PR-10 | 0.88de | 0.78e | 0.61f | 0.40c | 0.30e | 0.25ef |
| PR-20 | 0.80e | 0.67ef | 0.52f | 0.36d | 0.28e | 0.27e |
Mean values are of three replicates (n = 03). In each column, values with different letters differ significantly from each other at specific p ≤ 0.05 value following Duncan’s Multiple Range Test. (Control: no external As; As-60: 60 mg As kg− 1 soil; As-120: 120 mg As kg− 1 soil; PR0: no addition of phosphate rock; PR-5: 5 g phosphate rock kg− 1 soil; PR-10: 10 g phosphate rock kg− 1 soil; PR-20: 20 g phosphate rock kg− 1 soil. S: sandy soil; L: loamy soil; C: clayey soil). |