Effect of DPC dosage on protective enzyme activities in cotton during the flowering and boll-forming period.
In this experiment, the soluble protein content and the POD and CAT activities in cotton for each DPC concentration treatment were higher than those of the control. At 20 days after treatment, the soluble protein content, CAT activity, and POD activity of cotton plants sprayed with 0.25 g/L DPC were significantly higher than in the control by 2.60-, 2.33-, and 2.78-fold (p <0.05) (Figures 1A, 1C, and 1D). When the DPC concentration was less than or equal to 0.5 g/L, the SOD activity decreased as the DPC concentration increased, after which it increased. The SOD activity in the 0.5 g/L DPC treatment was the lowest, and it was significantly lower than the control by 13.13% (p <0.05) (Figure 1B).
However, at 20 days after DPC treatment, the SOD and CAT activities in cotton showed the opposite trend compared with the early stage of the experiment (SOD activity increased and CAT activity decreased), which may be due to the gradual decrease in DPC efficacy over time (Figures 1B and 1D). Therefore, we conclude that spraying with DPC at a concentration of 0.25 g/L during the flowering and boll-forming period can most enhance stress resistance in cotton, and DPC can be applied again 20 days later to regulate cotton growth.
Effect of DPC treatment on detoxifying enzyme activities in cotton aphids.
The specific activity of GST in cotton aphids was increased by spraying the plants with DPC in the field, and GST activity was positively correlated with DPC concentration. With increasing treatment duration, GST activity in cotton aphids decreased, which may be due to enhanced adaptation of cotton aphids to DPC (Figure 2A). However, the activities of carboxylesterase and acetylcholinesterase decreased with increasing DPC concentration (Figures 2B and 2C).
The leaf dipping test showed that when the DPC concentration was ≤0.25 g/L, the GST activity in cotton aphids was significantly higher than in the control and the 0.05 g/L and 0.1 g/L DPC treatments. When the DPC concentration was >0.25 g/L, the specific activity of the enzyme decreased as the DPC concentration increased, although it remained significantly higher than in the control. Therefore, it appears that DPC concentration ≤0.25 g/L has a certain toxicity to cotton aphids, and that the sensitivity of cotton aphids to DPC decreased at the two higher concentrations used in the experiment (Figure 3A). When the DPC concentration was ≤0.5 g/L, the specific activity of carboxylesterase decreased with increasing DPC concentration (Figure 3B). The specific activity of acetylcholinesterase decreased significantly as the DPC concentration increased from 0.05 g/L to 1 g/L (Figure 3C).
DPC mainly affects the GST activity in cotton aphids, but not the other detoxification enzymes assayed in this study. Therefore, even though DPC acts as a plant growth regulator, when the DPC concentration < 0.5 g/L, the growth of cotton aphids will also have a certain impact, and treatment with higher DPC concentrations will reduce the sensitivity of cotton aphids.
Effects of DPC on protective enzyme activities in cotton under aphid stress during the flowering and boll-forming period.
After 5 days of DPC treatment, there was no significant difference in cotton protein content among different treatments. Under low population density stress, the cotton protein content of the 0.25 g/L DPC treatment was the lowest, significantly lower (5.57%) than that of controls (P<0.05). Under high population density stress, the protein content of cotton treated with 0.1 g/L DPC was the highest, significantly higher (3.93%) than that of controls (P<0.05). Under the same concentration of DPC, the protein content of cotton without aphids and under low population density feeding stress first decreased then increased. The protein content of 0.1 g/L and 0.25 g/L DPC treatments reached minimum values at the same time, which were 722.68 mg/g and 666.28 mg/g, respectively. The protein content of cotton under high population density stress displayed the opposite trend, and reached a maximum of 773.08 mg/g when treated with 0.1 g/L DPC (Table 1).
Table 1
Effects of DPC treatment on protein content in cotton leaves under aphid feeding stress. Unit, mg/g. Black lower-case letters for the same time and the same population density indicate significant differences between the different concentration DPC treatments. Red letters in the same column indicate the significant differences between treatments with different aphid population densities at the same time and the same DPC concentration. There were no significant differences for treatments with the same letter (p >0.05), and there were significant differences between treatments with different letters (p <0.05). Low density, 100-300 aphids; High density, >500 aphids.
DPC treatment (days after spraying)
|
Aphid density
|
DPC concentration (g/L)
|
0
|
0.05
|
0.1
|
0.25
|
0.5
|
1
|
5 d
|
0
|
749.61±3.14 a a
|
761.88±1.74 a a
|
722.68±16.35 b b
|
762.55±5.43 a a
|
758.28±6.12 a a
|
737.35±0.35 ab a
|
Low density
|
705.61±2.89 d b
|
754.95±2.89 ab a
|
729.88±2.08 c b
|
666.28±1.67 e b
|
763.88±4.41 a a
|
746.81±7.59 b a
|
High density
|
743.88±3.59 b a
|
728.28±9.71 bc b
|
773.08±5.66 a a
|
769.88±2.43 a a
|
723.75±3.93 cd b
|
711.08±2.66 d b
|
10 d
|
0
|
315.75±2.31 d b
|
427.88±21.25 b b
|
358.82±1.5 cd c
|
395.48±39.04 bc b
|
758.55±7.39 a b
|
758.68±2.89 a a
|
Low density
|
18.28±1.29 e b
|
414.15±2.22 d b
|
766.01±3.24 b a
|
723.08±18.24 c a
|
811.08±16.19 a a
|
692.01±17.55 c b
|
High density
|
623.48±25.30 d a
|
721.75±2.08 bc a
|
739.35±6.35 b b
|
702.95±1.96 c a
|
754.15±2.89 ab b
|
778.28±2.54 a a
|
15 d
|
0
|
327.88±2.20 e c
|
527.08±2.23 c b
|
649.75±8.20 b b
|
445.48±25.77 d c
|
686.95±1.04 a a
|
692.68±0.46 a a
|
Low density
|
535.48±4.54 c b
|
527.08±32.79 c b
|
708.68±6.25 a a
|
702.41±4.75 a a
|
646.68±16.75 b a
|
670.28±3.49 ab b
|
High density
|
690.95±3.61 a a
|
689.88±0.92 a a
|
706.41±2.15 a a
|
500.81±5.20 b b
|
694.95±24.94 a a
|
703.08±4.41 a a
|
20 d
|
0
|
149.75±2.45 e c
|
458.41±6.58 c a
|
493.75±3.81 b b
|
538.81±4.33 a a
|
503.35±1.09 b a
|
442.15±5.54 d c
|
Low density
|
374.15±4.88 d a
|
446.81±13.66 c a
|
537.48±3.21 a a
|
479.88±1.29 b b
|
451.88±6.71 c b
|
528.95±1.50 a a
|
High density
|
178.68±1.62 c b
|
399.88±1.85 b b
|
393.21±0.74 b c
|
407.88±8.81 b c
|
498.95±7.05 a a
|
488.28±8.58 a b
|
After 10 days of DPC treatment, without aphids and under low population density stress, differences in cotton protein content between different DPC treatments were significant (P<0.05), but the content of different DPC treatments did not change significantly under high population density stress. Under low population density feeding stress, the protein content of the 0 g/L DPC treatment was only 18.28 mg/g, significantly lower than that of cotton treated with DPC. The protein content of the 0.5 g/L DPC treatment reached the maximum value (811.08 mg/g), significantly higher than that of cotton without aphid feeding but treated with DPC (P>0.05; Table 1)
After 15 days of DPC treatment, the protein content of cotton in 0, 0.5 g/L and 1 g/L DPC treatments showed no significant differences, while the protein content in other treatments was significantly higher than that in controls. The protein content in the low aphid density and ≥0.1 g/L DPC treatments was significantly higher than that of controls, while the protein content in the 0.05 g/L DPC treatment showed no significant differences (P>0.05). The protein content of cotton without aphid feeding was only 327.88 mg/g. The protein content of cotton with low aphid density and 0.1 g/L DPC treatment reached the maximum value, significantly higher (32.3 %) than that of controls. At the same concentration of DPC, the effect of feeding stress on cotton protein content was not significant at 0.5 g/L DPC, while the effect of other DPC concentrations on protein content was significant (P<0.05; Table 1)
After 20 days of DPC treatment, the protein content of cotton was significantly affected by different population densities at the same concentration of DPC. The protein content of cotton treated without aphids first increased then decreased with increasing DPC concentration, and the protein content without DPC treatment was the lowest (149.75 mg/g), while the protein content of 0.25 g/L DPC treatment was the highest, 2.60-fold higher than that of controls. Under low aphid density feeding stress, there were significant differences among different concentrations of DPC, and the protein content at 0.1 g/L DPC was significantly higher than that without DPC treatment (537.48 mg/g). Under high insect density feeding stress, the protein content of cotton increased gradually with increasing DPC concentration. There was no significant difference in protein content between 0.05, 0.1, and 0.25 g/L treatments, but it was significantly higher than that of cotton without DPC treatment (Table 1).
After 5 days of DPC treatment, there was no significant difference in SOD activity among different treatments. The SOD activity without aphids and 0.1 g/L DPC treatment was the highest, significantly higher (6.79%) than that of controls (P<0.05). Under high population density stress, the SOD activity of cotton treated with 0.1 g/L DPC was the lowest, significantly lower (5.76%) than that of controls (P<0.05). At the same concentration of DPC, there was a significant difference in SOD activity between treatment without aphids and a high population density feeding stress, and there was no significant difference in SOD activity between 0.05 g/L and 0.5 g/L DPC (P>0.05; Table 2).
Table 2
Effects of DPC treatment on SOD activity in cotton leaves under aphid feeding stress. Unit, U·g−1FW. Black letters for the same time and the same population density indicate significant differences between the different DPC treatments. The red letters in the same column indicate the significant differences between the treatments with different aphid population densities at the same time and the same DPC concentration. There were no significant differences for treatments with the same letter (p >0.05), and there were significant differences between treatments with different letters (p <0.05). Low density, 100-300 aphids; High density, >500 aphids.
DPC treatment (days after spraying)
|
Aphid density
|
DPC concentration (g/L)
|
0
|
0.05
|
0.1
|
0.25
|
0.5
|
1
|
5 d
|
0
|
12.66±0.04 b b
|
12.86±0.08 b ab
|
13.52±0.10 a a
|
12.86±0.09 b a
|
12.71±0.10 b b
|
12.92±0.06 b a
|
Low density
|
12.96±0.05 bc a
|
13.08±0.09 b a
|
13.35±0.08 a a
|
12.52±0.08 d b
|
13.05±0.07 bc a
|
12.84±0.05 c a
|
High density
|
13.01±0.12 a a
|
12.76±0.07 ab b
|
12.26±0.53 b b
|
12.51±0.12 ab b
|
12.76±0.08 ab ab
|
12.43±0.07 b b
|
10 d
|
0
|
12.35±0.06 b b
|
12.46±0.09 b ab
|
13.23±0.11 a a
|
12.48±0.10 b a
|
12.30±0.12 b b
|
12.54±0.07 b a
|
Low density
|
12.58±0.05 bc a
|
12.71±0.11 b a
|
13.02±0.08 a a
|
12.09±0.06 d b
|
12.69±0.08 b a
|
12.45±0.05 c a
|
High density
|
12.64±0.08 a a
|
12.37±0.08 ab b
|
10.83±0.60 c b
|
12.09±0.14 ab b
|
12.36±0.09 ab b
|
12.00±0.07 b b
|
15 d
|
0
|
11.28±0.06 a a
|
11.12±0.22 a b
|
11.01±0.14 a a
|
11.26±0.08 a a
|
10.93±0.12 a ab
|
11.10±0.06 a a
|
Low density
|
11.12±0.16 b a
|
11.79±0.09 a a
|
11.14±0.05 b a
|
10.66±0.06 c b
|
10.44±0.14 c b
|
10.82±0.21 bc a
|
High density
|
11.16±0.02 b a
|
10.96±0.19 a b
|
10.98±0.04 b a
|
11.10±0.08 c a
|
11.04±0.18 c a
|
11.09±0.10 bc a
|
20 d
|
0
|
12.95±0.13 b a
|
13.56±0.12 a a
|
13.07±0.21 b a
|
11.78±0.17 d b
|
11.25±0.11 e b
|
12.32±0.15 c b
|
Low density
|
12.23±0.19 d b
|
12.71±0.07 bc c
|
13.28±0.07 a a
|
12.93±0.14 ab a
|
12.44±0.21 cd a
|
12.92±0.05 ab a
|
High density
|
12.77±0.14 b a
|
13.14±0.08 a b
|
13.16±0.04 a a
|
12.81±0.12 b a
|
12.85±0.06 ab a
|
12.79±0.13 b a
|
After 10 days of DPC treatment, the SOD activity without aphids and 0.05 g/L or 0.1 g/L DPC treatments was significantly different from that of the same period, and the activity of 0.1 g/L DPC treatment was the highest (13.23 U·g−1FW). Under 0.1 g/L DPC treatment, the SOD activity of cotton treated with low and high population densities was significantly different from that of controls. The SOD activity of cotton treated with low population density was significantly higher (3.50%) than that of controls, while the SOD activity of cotton treated with high insect density was the lowest, significantly lower (14.32%) than that of controls. At the same DPC concentration, SOD activity was highest under low population density feeding stress, and the SOD activity of cotton treated with 0.25 g/L DPC was significantly lower than that of cotton not exposed to aphids (P<0.05; Table 2).
After 15 days of DPC treatment, the SOD activity of cotton was generally lower than that of other treatment durations, and under low aphid density feeding stress, the SOD activity of cotton treated with 0.05 g/L DPC was the highest (11.79 U·g−1FW), but there was no significant difference compared with controls at the same time (P>0.05). The SOD activity of 0.5 g/L DPC-treated cotton was only 10.44 U·g−1 FW, which was not significantly different from that of controls (P>0.05). There was no significant difference in SOD activity of cotton treated without aphids and high population density stress with different DPC concentrations, while the SOD activity of cotton treated with low population density stress first increased then decreased with increasing DPC concentration, and the SOD activity was highest at 0.05 g/L DPC, significantly higher than that of cotton without aphid feeding (Table 2).
After 20 days of DPC treatment, the SOD activity of cotton without aphids was significantly different among different concentrations of DPC, and the SOD activity of 0.05 g/L DPC was the highest, significantly higher (4.71%) than that of controls (P<0.05), while the SOD activity of 0.5 g/L DPC was lowest value, significantly lower (13.13%) than that of controls (P<0.05). The SOD activity of cotton under low population density feeding stress changed markedly with increasing concentration of DPC, first increasing then decreasing, and reaching a maximum at 0.05 g/L DPC, significantly higher than that of cotton without DPC treatment. However, the SOD activity difference at this DPC concentration was the most significant, and the SOD activity under low population density feeding stress was significantly lower (6.27%) than that of cotton without aphid feeding (P>0.05). The SOD activity of cotton under high population density feeding stress changed gently with increasing DPC concentration, and there was no significant difference between treatments. The SOD activity was significantly higher than that of controls only when the concentration of DPC was 0.05 g/L or 0.1 g/L (P<0.05; Table 2).
After 5 days of DPC treatment, the POD activity of cotton without aphids and with low population density stress was significantly different with changing DPC concentration (P<0.05). At low population density stress and 0.1 g/L DPC treatment, POD activity was the lowest (6400 OD470·min−1·g−1 FW). Meanwhile, under high density feeding stress, the POD activity of cotton was not significantly affected by different concentrations of DPC, and the POD activity reached the highest value at 0.25 g/L DPC, significantly higher (62.54%) than for untreated controls. Under 1 g/L DPC treatment, the POD activity of cotton did not change significantly under different population densities (P>0.05). With DPC concentrations <1 g/L, the change in enzyme activity in different population densities was significant (P<0.05), and the POD activity under high population density was the highest under the same concentration of DPC. (Table 3).
Table 3
Effects of DPC treatment on POD activity in cotton leaves under aphid feeding stress. Unit, OD470·min−1·g−1FW. Black letters for the same time and the same population density indicate significant differences between the different DPC treatments. The red letters in the same column indicate significant differences between the treatments for the different aphid population densities at the same time and the same DPC concentration. There were no significant differences for treatments with the same letter (p >0.05), and there were significant differences between treatments with different letters (p <0.05). Low density, 100-300 aphids; High density, >500 aphids.
DPC treatment (days after spraying)
|
Aphid density
|
DPC concentration (g/L)
|
0
|
0.05
|
0.1
|
0.25
|
0.5
|
1
|
5 d
|
0
|
9863.33±287.52 e b
|
11143.33±479.55 d b
|
8736.67±278.51 f b
|
15438.33±251.60 a b
|
12110.00±37.86 c c
|
13920.00±64.49 b a
|
Low density
|
7591.67±15.90 d c
|
10545.00±37.38 c b
|
6400.00±244.34 e c
|
11758.33±290.12 b c
|
13653.33±129.82 a b
|
11563.33±79.07 b b
|
High density
|
15900.00±162.10 b a
|
14960.00±129.23 ab a
|
15121.67±48.07 ab a
|
16031.67±59.25 a a
|
14338.33±137.15 ab a
|
13911.67±192.04 b a
|
10 d
|
0
|
7230.00±91.70 f b
|
10283.33±116.92 c b
|
8158.33±62.27 e c
|
8671.67±4.41 d c
|
15630.00±18.93 a a
|
14615.00±49.24 b b
|
Low density
|
6136.67±19.65 f c
|
10571.67±8.82 d b
|
14626.67±28.33 a a
|
12270.00±5.77 b b
|
11411.67±103.13 c b
|
8021.67±34.68 e c
|
High density
|
14471.67±168.98 b a
|
13481.67±108.06 c a
|
10368.33±54.19 e b
|
12336.67±9.28 d a
|
5718.33±134.79 f c
|
15561.67±12.02 a a
|
15 d
|
0
|
2415.00±2.89 e c
|
9103.33±104.50 b a
|
6495.00±45.37 c a
|
1813.33±8.82 f c
|
13385.00±100.04 a a
|
4085.00±58.38 d c
|
Low density
|
3816.67±156.96 c a
|
1330.00±68.25 d c
|
3890.00±86.07 c c
|
3826.67±114.18 c b
|
4560.00±24.66 b c
|
5428.33±58.40 a a
|
High density
|
3420.00±57.74 e b
|
4421.67±90.48 d b
|
5038.33±17.64 c b
|
8145.00±47.52 a a
|
6003.33±100.84 b b
|
5061.67±68.46 c b
|
20 d
|
0
|
2651.67±41.87 f c
|
6851.67±95.50 e b
|
7456.67±106.31 d b
|
10038.33±159.17 c b
|
15598.33±21.67 b b
|
16243.33±128.85 a a
|
Low density
|
3550.00±64.49 d b
|
12491.67±34.80 b a
|
10060.00±38.84 c a
|
12711.67±69.36 b a
|
17148.33±194.60 a a
|
12690.00±77.51 b b
|
High density
|
6011.67±60.09 c a
|
5676.67±20.88 d c
|
2673.33±114.65 e c
|
5886.67±136.94 cd c
|
9436.67±92.62 b c
|
10005.00±67.88 a c
|
After 15 days of DPC treatment, the POD activity without aphids was significantly different with different concentrations of DPC, and the POD activity reached the maximum value at 0.5 g/L DPC, 4.54-fold higher than that of controls at the same period. Under low density feeding stress, POD activity of cotton increased with increasing DPC concentration, and POD activity was lowest under 0.05 g/L DPC treatment, significantly lower (65.14%) than that of controls. Under with high insect population density feeding stress, the POD activity of cotton first increased then decreased with increasing DPC concentration. POD activity was highest under treatment with 0.25 g/L DPC, 1.38-fold higher than without DPC treatment Under the same DPC concentration, the effects of different population densities on POD activity of cotton were significant (P<0.05; Table 3).
After 20 days of DPC treatment, the POD activity of cotton under low population density feeding stress was significantly different with different DPC concentrations, and the POD activity reached the maximum value under 0.5 g/L DPC treatment, 3.83-fold higher than that without DPC treatment. At DPC concentrations <0.25 g/L, the POD activity of cotton decreased with increasing DPC concentration under high insect density feeding stress, and the POD activity was lowest at 0.1 g/L DPC (2673.33 OD470·min−1·g−1FW), significantly lower (55.53%) than that of cotton without DPC treatment. The POD activity of cotton increased with increasing DPC concentration when the concentration of DPC was ≥0.25g/L. The POD activity of cotton with low population density stress was significantly higher than that of cotton with high population density stress (P<0.05; Table 3).
After 5 days of DPC treatment, when the concentration of DPC was ≤0.25 g/L, the CAT activity of cotton without aphids gradually increased with increasing DPC concentration, and the CAT activity reached the maximum value at 0.25 g/L DPC, significantly higher (71.96%) than that of untreated controls. When the concentration of DPC was >0.25 g/L, the CAT activity of cotton without aphid treatment decreased significantly with increasing DPC concentration (P<0.05). The CAT activity under low population density stress first decreased then increased with increasing DPC concentration, and the CAT activity reached the minimum value at the concentration of 0.25 g/L, significantly lower (50%) than that of cotton without DPC treatment. At concentrations of DPC <0.25 g/L, there was a significant difference in CAT activity with population density without the application of DPC, and the CAT activity of cotton at a high population density was significantly lower (35.93%) than that at a high population density (P<0.05; Table 4).
Table 4
Effects of DPC treatment on CAT activity in cotton leaves under aphid feeding stress. Unit, OD240·min−1·g−1FW. Black letters for the same time and the same population density indicate significant differences between the different DPC treatments. Red letters in the same column indicate the significant differences between the treatments for different aphid population densities at the same time and the DPC same concentration. There were no significant differences for treatments with the same letter (p >0.05), and there were significant differences between treatments with different letters (p <0.05). Low density, 100-300 aphids; High density, >500 aphids.
DPC treatment (days after spraying)
|
Aphid density
|
DPC concentration (g/L)
|
0
|
0.05
|
0.1
|
0.25
|
0.5
|
1
|
5 d
|
0
|
796.67±18.56 d b
|
946.67±20.28 cd b
|
1040.00±26.46 bc a
|
1370.00±127.02 a a
|
1180.00±55.08 b a
|
953.33±38.44 cd a
|
Low density
|
1260.00±45.09 ab a
|
1326.67±49.78 a a
|
1103.33±50.44 bc a
|
630.00±32.15 d b
|
956.67±20.28 c b
|
1153.33±90.62 b a
|
High density
|
510.00±56.86 d c
|
1310.00±75.06 a a
|
983.33±26.67 c a
|
1093.33±94.04 bc a
|
1200.00±75.72 ab a
|
1030.00±20.82 bc a
|
10 d
|
0
|
1275.00±31.22 b b
|
1338.33±39.83 b b
|
1680.00±55.08 a a
|
1751.67±45.31 a a
|
1761.67±31.93 a a
|
1340.00±52.92 b b
|
Low density
|
1875.00±36.17 a a
|
1858.33±35.63 a a
|
1588.33±24.55 b a
|
956.67±26.67 d c
|
1443.33±58.40 c b
|
1541.67±51.99 bc ab
|
High density
|
870.00±34.64 c c
|
1713.33±82.93 a a
|
1408.33±55.25 b b
|
1506.67±80.07 ab b
|
1553.33±41.77 ab b
|
1640.00±70.95 a a
|
15 d
|
0
|
2116.67±43.33 bc b
|
2030.00±15.28 cd b
|
2236.67±18.56 ab a
|
2346.67±26.67 a a
|
1973.33±62.27 d b
|
2230.00±40.41 ab a
|
Low density
|
2996.67±29.06 a a
|
2513.33±23.33 b a
|
1666.67±46.67 d b
|
2566.67±143.80 cd a
|
2496.67±27.28 b a
|
2223.33±28.48 bc a
|
High density
|
2010.00±51.32 c b
|
2026.67±18.56 c b
|
2350.00±70.24 b a
|
2506.67±13.33 a a
|
1593.33±18.56 d c
|
1600.00±26.46 d b
|
20 d
|
0
|
550.00±26.46 d b
|
1360.00±20.82 c b
|
1330.00±20.82 c b
|
1833.33±31.80 a a
|
1326.67±18.56 c b
|
1696.67±33.83 b a
|
Low density
|
3113.33±124.41 a a
|
1223.33±21.86 d c
|
1796.67±39.30 b a
|
1566.67±86.86 c b
|
1823.33±80.90 b a
|
1546.67±35.28 c a
|
High density
|
683.33±31.80 c b
|
1630.00±20.82 a a
|
1406.67±18.56 b b
|
1656.67±48.07 a ab
|
1426.67±24.04 b b
|
1556.67±69.84 a a
|
After 10 days of DPC treatment, the CAT activity of cotton without aphid treatment increased gradually with increasing concentration of DPC at DPC ≤0.5 g/L, and the CAT activity reached the maximum value at 0.5 g/L, significantly higher (38.1 %) that that without any treatment. At a DPC concentration of 1 g/L, the CAT activity of cotton treated without aphids decreased significantly (P<0.05). CAT activity under low insect density stress showed the opposite trend, first decreasing then increasing with increasing DPC concentration, and CAT activity reached a minimum value at 0.25 g/L DPC, significantly lower (49.0 %) than that of cotton not treated with DPC (P<0.05). The CAT activity of cotton under high density feeding stress was significantly different (P<0.05; Table 4).
After 15 days of DPC treatment, the CAT activity without aphids and under low population density stress was significantly different with increasing DPC concentration. Under high population density stress, at DPC ≤0.25 g/L the CAT activity of cotton increased gradually with increasing DPC concentration, and the CAT activity reached the maximum value under 0.25 g/L DPC, significantly higher than that of controls. With increasing DPC concentration, the CAT activity decreased gradually, and it reached the minimum value at 0.5 g/L DPC, significantly lower (20.7 %) than that of cotton without DPC treatment. Under the same concentration of DPC, there was no significant change in CAT activity among different aphid density treatments (P>0.05; Table 4).
After 20 days of DPC treatment, the CAT activity of cotton under different population densities was significantly different with different concentrations of DPC. Without aphids, there was no significant difference in CAT activity between 0.05, 0.1, and 0.5 g/L DPC treatments, and the CAT activity without DPC treatment was the lowest (550 OD470·min−1·g−1FW). Under high population density stress, the CAT activity of cotton not treated with DPC was 24.18% higher than that of cotton without any treatment, but the difference was not significant (P>0.05). The CAT activity of cotton under low population density stress was the highest, 4.66-fold higher than that of cotton without any treatment. When treated with 1 g/L DPC, the difference in CAT activity with population density was not significant (P> 0.05; Table 4).