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Effect of root damage due to simulated and real whitegrub attack on physio- biochemical characters of groundnut under various soil moisture levels

Neelam Yadav and Vijay K Yadav

Agricultural Research Station, Durgapura, Jaipur, 302018 India. Email neelam7561@yahoo.co.in

Abstract

The influence of water stress, simulated whitegrub and real whitegrub damage on water relation parameters and stress-induced biochemicals under various soil moisture levels in groundnut was studied. A novel method of simulating the white grub damage was developed by cutting the roots vertically by 25%, 50% and 75% at 40 days after sowing (DAS) in one set and by cutting the roots horizontally 30, 20 and 10 cm from the top soil surface at 30 and 60 DAS in other set to obtain an indication how the feeding activity of the insect affects growth of the plant. Cutting of roots reduced relative water content (RWC) significantly (4.18-8.53 %) to the levels almost equivalent to that in water-stressed plants (8.63 %). Proline content in plants with simulated white grub damage was elevated by 31.25-78.12 % and in plants under water stress by 56.25 % as compared to the control. When whitegrub larvae were introduced at 40 DAS, the plants showed markedly reduced relative water content, increased transpiration rate and proline level but no change in peroxidase level. RWC and peroxidase activity did not change while transpiration rate and proline content increased significantly in plants when whitegrub larvae were introduced at 60 DAS. In general, the plant physiological effects of both simulated white grub damage and the root damage caused by H. consanguinea larvae feeding appear to mirror closely the effects of water stress.

Key words

Biotic stress, Holotrichia consanguinea, Proline, Peroxidase, Relative water content, Transpiration

Introduction

The larvae of whitegrub (Holotrichia consanguinea) are subterranean and after hatching orient towards the roots to start feeding actively on the living roots of a wide variety of cultivated as well as uncultivated plants. This activity causes seedling death and stunts the growth of older plants (Whitman et al 1990). Consequent to feeding, the plants start experiencing physiological stress. The crops with tap root system suffer more as compared to those with adventitious root system. Almost all field crops grown during the rainy season (kharif) in India are damaged viz. groundnut, sugarcane, pearlmillet, sorghum, cowpea, pigeon pea, green gram, clusterbean, chilli. Plant water relation parameters like water potential, osmotic potential, turgor potential and transpiration are important indices for assessment of plant water status required for plant survival. Since whitegrub attacks the plant roots, it is likely that it causes water stress in the plant, thereby affecting water status of the plant. Thus the importance of these features was realized so as to generate more information on the relationship between whitegrub activity, water and crop losses. It is also not yet clear what stage of a plant is important from physiological and yield point of view vis--vis whitegrub damage. This experiment studied water status and related parameters like proline and peroxidase activity in response to stimulated and real whitegrub root damage. These parameters have been implicated as stress indicators.

Methods

Seeds of local peanut variety ‘chitra’ were sown on 4th June 2000 in PVC pipes (50 cm length x 10 cm dia) containing soil that was saturated with water to field capacity and excess water drained. There were five replicate pots per treatment. For simulation studies, pots were splitted vertically to 30 cm length from the top to cause 25%, 50% and 75 % root damage at 40 DAS and were grown under normal moisture level (10 0.5%). Control plants (without cut roots) were grown under normal soil moisture as well as under water stress (5 0.5% moisture). In another set, PVC pots were cut horizontally so as to cut the roots of the plants at 30, 20 and 10 cm from the top at 30 and 60 DAS. One set of these plants was grown under normal soil moisture and other set under water stress. The plants were irrigated after every three days with required amount of water. Sampling was done 10 days after simulating the whitegrub damage. In other experiment, second instar larvae of H. consanguinea were introduced (one larva/pot) at 40 DAS in one set and at 60 DAS in another set of plants. Calculated amount of insecticide chloropyriphos was applied in the pots 10 days after the introduction of whitegrub larvae. The pots were drenched thoroughly with water so that the insecticide reached the bottom and be in direct contact with the grubs. Just before applying the insecticide, samples were collected for estimation of water content, transpiration, proline and peroxidase at 50 and 70 DAS in independent sets. Remaining pots of both the sets were grown till harvest to record total dry weight of the plant and roots. Relative water content (RWC) was measured by taking fresh weight, turgid weight and dry weight of the leaves. The leaves were floated for three hours on water to bring them to full turgidity. A standard method was followed for estimation of proline (Bates et al. 1973) and peroxidase activity (Shannon 1971) in plant leaves.

Results

Effect of simulated whitegrub damage and water stress on RWC, rate of transpiration, proline and peroxidase activity is shown in Table 1. RWC decreased by 4.18-8.53 % due to simulated whitegrub damage created by cutting 25- 75% of the roots. The maximum decrease was obtained in plants with 75 % roots cut (8.53 %) which was almost equal to the decrease in plants under water stress (8.63 %). These physiological parameters were also studied in plants 10 days after introducing larvae at 40 and 60 days after sowing (Table 2). Fall in RWC was 20.13% and 23.15% of the control, in plants introduced with white grubs at 40 and 60 DAS respectively. Fall in RWC in plants introduced with grub at 40 and 60 DAS were greater than that under simulating whitegrub damage and under water stress. Similar observation was noticed in simulating whitegrub damage in groundnut by Wightman et al. (1994). Higher RWC under simulating whitegrub damage (93.23, 93.30, 89.00 respectively in 25%, 50% and 75% roots removed) than that under water stress at 40 DAS showed that plants with root cut absorbs more water than stressed plants.

Table 1. Physiological parameters at 10 days after simulating whitegrub damage at 40DAS.

Treatment

Relative Water Content (%)

Proline content
(μg/g fr wt)

Peroxidase activity
(Enzyme unit x 100)

Without root cut under normal moisture (control)

97.30

32.5

88.0

Without root cut under water stress (5%)

88.90

50.0

98.0

With 25 % root cut

93.23

42.5

90.2

With 50 % root cut

93.30

47.6

82.8

With 75 % root cut

89.00

57.5

64.0

S.Em.

1.09

0.81

1.09

CD (P=0.05)

3.23

2.40

3.23

Table 2. Physiological parameters at 10 days after introducing larvae at 40 and 60 DAS.

Treatments

Relative Water Content
(%)

Proline content
(μg/g fr wt)

Peroxidase activity (Enzyme unit x 100)

50 DAS

Without grub

98.68

30.00

98.00

 

With grub

78.81

46.00

104.00

70 DAS

Without grub

94.13

95.00

75.00

 

With grub

72..34

180.00

76.00

S.Em.

0.95

3.54

2.65

CD (P=0.05)

2.85

10.60

7.93

Proline content increased by 31.25%, 46.78% and 78.12% in plants having 25%, 50% and 75% roots removed, respectively. Water stress increased proline content by 56.25 % as compared to the control. Proline content thus, increased with increasing severity of simulated whitegrub damage. Proline content which was lower (30.0) at an early growth stage (50 DAS) than that at 70 DAS (95) increased due to root damage caused by feeding of whitegrub larvae. The increase in proline content was more due to whitegrub damage initiated at 60 DAS (89.47%) than at 40 DAS (53.33%). It is known that proline helps in stress tolerance either by rehydration of protoplasm or by providing energy for recovery of plants (Manjula et al. 2003). Tolerance to water stress with increased proline content was also observed in groundnut (Koti et al. 1994). There was no significant change in peroxidase activity due to removal of 25-50% roots. However the enzyme activity decreased by 27.27% in plants with 75% roots removed and increased by 11.36% due to water stress. Deterioration of physical as well as physiological conditions of the plant seems to result in decreased peroxidase activity due to removal of 75% roots. However, the increase in the enzyme activity was more due to attack of whitegrub larvae at 40 DAS (6.12%) than at 60 DAS (1.33%). Because of the grub attack, the percent increase in peroxidase activity was higher at 50 DAS while percent increase in proline accumulation was higher at 70 DAS.

It was observed that total plant dry weight and root dry weight reduced by 30.9% and 14.3% respectively due to damage caused by introducing grub larvae at 40 DAS . The plant in which larvae were introduced at 60 DAS did not survive till harvest indicating that late growth stage is more critical for whitegrub damage. This is substantiated by greater change in water content and proline content in plants introduced with whitegrubs at 60 DAS than plants introduced with the grubs at 40 DAS. Plant survival as well as yield reduced due to simulated whitegrub damage that was created at 30 and 60 DAS by cutting the roots horizontally at different depths both under normal moisture level and under water stress (Table 3). The maximum reduction in plant number and yield occurred due to cutting of roots at 10 cm depth from the top. The reduction in plant number was 60% while in yield it was 75% and 89% respectively under normal moisture level and water stress due to simulated whitegrub damage at 60 DAS. On the other hand plants cut at 10 cm at 30 DAS under water stress did not survive till harvest.

Table 3. Effect of simulated whitegrub damage on seed yield of groundnut.

Treatment

Sub-treatment (root cut at different length, cm)

Number of plants survived from sub-treatment at

Yield (g/plant) from the plants cut at

   

30 DAS

60 DAS

30 DAS

60 DAS

Non-stress

Control

15.00

15.00

0.85

0.94

 

30

9.00

12.00

0.79

0.86

 

20

6.00

12.00

0.39

0.70

 

10

3.00

6.00

0.35

0.23

Water-stress

Control

6.00

9.00

0.73

0.88

 

30

3.00

9.00

0.56

0.83

 

20

3.00

6.00

0.34

0.33

 

10

NIL

6.00

NIL

0.10

 

S Em

0.56

0.79

0.02

0.02

 

CD (P=0.05)

1.61

2.28

0.06

0.05

Conclusion

Simulated whitegrub damage at 30 DAS was more detrimental and resulted in up to 80% loss in plant number under normal moisture. Data also suggest that the growth stage of the plant at which whitegrub attacks was of immense importance as compared to percent and depth of root cutting. One of the reasons for higher reduction in RWC seems to be due to interruption in water absorption process by the grub larvae. It seems that some stimulant factor is secreted either by grub or plant that triggers physio-biochemical changes in the plant that indirectly interrupts the absorption of water by the plant. Over-production of proline under water stress, simulated whitegrub damage and damage caused by grub larvae as compared to their respective control shows that stress is manifested by proline accumulation. It was also noticed that peroxidase activity did not show any significant change due to feeding of roots by whitegrub larvae. Higher change in RWC in the plants growing under water stress (5% moisture) than that under simulated whitegrub damage indicates that moisture stress is more critical. This study thus shows that although effects of both simulated white grub damage and the root damage caused by feeding of H. consanguinea larvae appear to mirror closely the effects of water stress, their exists some differences among the three stresses. It is concluded from this study that actual percentage of root cut and depth of cutting (Yadav and Yadav 2000) had little influence on the yield. Although whitegrub is generally active in 10-20 cm root zone, variation in their depth of feeding is likely to have little effect on crop production. However, interference with crop root system can reduce the yield potential of the crop.

References:

Bate LS, Waldren R P and Tear, ID (1973). Rapid determination of free proline for water stress studies. Plant and Soil 39, 205-207

Koti RV, Chetti MB, Manjunath and Ameragowda, A (1994). Effect of water stress at different growth stages on biophysical characters and yield in groundnut (Arachis hypogaea L.) genotypes. Karnataka Journal of Agriculture Science 7, 158-162

Manjula K, Sarma PS, Ramesh T and Nageshwar Rao, T (2003). Evaluation of castor (Ricinus Communis L.) genotypes for moisture stress. Indian Journal of Plant Physiology 8, 319-322

Shannon L M (1971). Plant Physiology 47, 493-498.

Tanner CB and Sinclair TR (1983). Efficient water use in crop production: research or re-search. In‘Limitation to efficient water use in crop production’. (Eds. H. M. Taylor, W. R. Jorden and T. R.Sinclair) pp 1-28 (ASA-CSSA-SSSA, Madison ,USA).

Wightman JA, Dick KM, Ranga Rao GV, Shanower TG and Gold, CG (1990). Pests of groundnut in the semi-arid tropics. In ‘Insect pests of groundnut in the semi-arid tropics’. (Ed. S. R. Singh), John Wiley and sons Ltd

Wightman JA, Brier HB and Wright, GC (1994). The effect of root damage caused by simulated whitegrub attack on the growth, yield and water-use of groundnut plants. Plant and Soil 160, 267-27.

Yadav N and Yadav V K (2000). Effect of simulated whitegrub damage on transpiration and biochemical changes in groundnut, 3rd International Crop Science Congress, pp.104. Hamburg, Germany, 17-22 August 2000.

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