Effect of selected insecticides on plant growth and mycorrhizal development in sorghum

Agriculture, Ecosystems and Environment, 43 ( 1993 ) 337-343

337

Elsevier Science Publishers B.V., Amsterdam

Short communication

Effect of selected insecticides on plant growth and mycorrhizal development in sorghum J. Veeraswamy, T. Padmavathi and K. Venkateswarlu Department of Microbiology, Sri Krishnadevaraya University, Anantapur 515 003, India (Accepted 20 July 1992)

ABSTRACT

Veeraswamy, J., Padmavathi, T. and Venkateswarlu, K., 1993. Effect of selected insecticides on plant growth and mycorrhizal development in sorghum. Agric. Ecosystems Environ., 43: 337-343. Three insecticides viz., carbofuran, endosulfan and quinalphos were assessed for their toxic effects on plant growth and vesicular-arbuscularmycorrhizal (VAM) fungi development in sorghum. Samples of a black alluvial soil, contained in earthen pots, with indigenous VAM populations were drenched with graded concentrations of the insecticides ranging from 0 to 10 kg ha-1 after 30 days (single application), or 30 and 60 days (two repeated applications), of plant growth. Single or two repeated applications of carbofuran at levels of 0.5-2.5 kg ha- l were non-toxic to plant growth and mycorrhizal development. However, 5 and 10 kg ha-1 levels of carbofuran were phytotoxic and significantly inhibited colonization by VAM fungi. Plant growth and VAM formation were not affected by 0.5 kg ha- I levels of both endosulfan and quinalphos. Single or two repeated applications of endosulfan, and quinalphos, at concentrations of 5 and 10 kg ha-1, exerted toxicity to the plant growth and VAM fungal colonization.

INTRODUCTION

The vesicular-arbuscular mycorrhizal (VAM) fungi with their ubiquity ( Gerdemann, 1968; Hayman, 1982; Harley and Smith, 1983 ) are known to form extensive mycorrhizae on sorghum (Veeraswamy et al., 1988) under field conditions, and are implicated in enhancing phosphorus uptake and growth of sorghum (Raju et al., 1990 ). Introduction of high-yielding varieties of sorghum (Sorghum bicolor (L.) Moench) such as CSH5, CSH6 and CSH9 has led to the outbreak of major insect pests. According to the Department of Agriculture, Andhra Pradesh, India, insecticides viz., carbofuran, endosulfan and quinalphos have been extensively used in cultivation of sorghum (AnonCorrespondence to: Professor K. Venkateswarlu, Department of Microbiology, Sri Krishnadevaraya University, Anantapur 515 003, India.

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ymous, 1988). These insecticides are generally applied after 30 days of plant growth for an effective control of major insect pests such as shoot fly (Atherigona variasoccatta), stemborer (Chilopastellus) earhead bug (Calocoris angustatus) and jowar midge (Contarinia sorghicola). Carbofuran granules (3G) are either furrow applied, sprayed, or applied to whorls of the plant at levels of 0.45-2.5 kg a.i. ha-1. Endosulfan is applied as a dust formulation or sprayed at levels of 0.75-2 kg a.i. ha-1, while quinalphos is sprayed at levels of 0.5-1.75 kg a.i. ha -1. Many pesticides tested adversely affect the mycorrhizal symbiosis (Menge, 1982; Trappe et al., 1984), while others do not appear to damage mycorrhizal fungi (Ocampo and Hayman, 1980). Under certain conditions, some pesticides may even increase mycorrhizal colonization and development (Bird et al., 1974; Ocampo and Hayman, 1980). Pesticides could be more deleterious in less-fertilized soils by inhibiting a larger indigenous mycorrhizal population in a situation where its stimulatory effects on plant nutrient uptake is more important to the crop (Ocampo and Hayman, 1980). Although insecticides are widely used on sorghum, no information is available on VAM development in this host under semi-arid tropical conditions. Keeping this in view, an attempt has been made to determine the impact of promising insecticides, carbofuran, endosulfan and quinalphos, on the development of mycorrhizae by indigenous VAM fungi and plant growth response in sorghum. MATERIALS AND METHODS

Samples (5 kg) of black alluvial soil, collected from a fallow sorghum field, were placed in earthen pots (9" diameter). The soil used as the potting medium was phosphorus-deficient (Olsen P, 6.88/tg g-l soil) with a pH of 8.13 and contained a mixed indigenous population of 440 resting spores of VAM fungi per 100 g soil. Five clear surface-sterilized (0.1% aqueous HgC12 for 30 min) seeds of sorghum (cultivar CSH5 ) were sown in each pot. The pots were watered regularly once every 3 days. After 20 days of sowing, the seedlings were thinned out to one per pot. Aqueous solutions of the insecticides, Furadan 3G (3% carbofuran--2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate), Thiodan 35 EC (35% endosulfan--6,7,8,9,10-hexach1oro- 1,5,5a, 6,9,9 a-hexahydro-6,9 -methano-2,4,3-benzodioxathiepin-3-oxide) and Ekalux 25 EC (25% quinalphos--O,O-diethyl-O-quinoxalin-2-ylphosphorothioate) were used to drench the potting medium 30 days after sowing. Each insecticide was used at concentrations of 0, 0.5, 2.5, 5 and 10 kg ha-l and all conversions from parts per million (/tg g-1 soil) to kilograms active ingredient per hectare in the case of soil application of the insecticides have been based upon 108 g of soil ha- t to a depth of 1 cm (Anderson, 1978 ). The potted plants receiving the above treatments were arranged following a randomized block design in open shade with a temperature range of 27-39 ° C.

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After the insecticide treatment, all plants were fed once a week with 60 ml of Hoagland's nutrient solution minus the phosphate ion, and were also watered as required. A set of five replicates from each treatment was harvested after 60 days of insecticide treatment. In a separate experiment, potted plants received the insecticides twice (as repeated applications ) after 30 and 60 days of sowing. Five replicates were harvested at the end of 30 days after the second application. At harvest, plant height and fresh weights of roots and shoots were taken. A 0.2 g sample of root, excised from different parts of the root system of each plant, was withdrawn for assessing VAM fungal colonization and estimating total root length following a gridline intersect method (Giovannetti and Mosse, 1980) after clearing the roots with KOH and staining with trypan blue (Phillips and Hayman, 1970). The remaining root samples and shoots were oven-dried at 80°C for 48 h and weighed. Specific root length was estimated by dividing root length (m) by fresh weight of root (g). Colonized root length (m) was calculated by multiplying percent colonization by root length. The data were subjected to analysis of variance, and the means were compared using a Duncan's new multiple range test at the 5% level. The data related to percent root colonization and root length or dry weights of plants were analyzed for correlation coefficients. RESULTS AND DISCUSSION

Application of carbofuran at levels above 2.5 kg ha- ~ effected an initial significant phytotoxicity resulting in complete scorching of leaves and reduction in the height of plants. However, the plants subjected to higher concentrations of carbofuran had recovered to some extent by 90 days (Table 1). The biomass (dry weights of shoot and root) production is not in direct relation to the height of the plant. For example, application of the insecticide at a level of 2.5 kg ha-m caused no change in dry matter of the shoot after 90 days of plant growth. The 0.5 kg ha-~ level effected a significant increase in growth response throughout. No change in the total root and specific root length of plants treated with 2.5 kg ha-1 of carbofura'n was observed after 90 days. Again, colonization by indigenous VAM fungi and root length colonized were not affected at this concentration. However, higher levels of 5 and 10 kg ha- 1adversely affected root length as well as colonization by VAM fungi. Similarly, a significant reduction in root infection and development of VAM fungi in groundnut was reported by Backman and Clark (1977). Furthermore, 25 p.p.m, of carbofuran was shown to enhance VAM development in Rhodes grass (Chloris gayana) (Sreenivasa and Bagyaraj, 1988). However, Parvathi et al. (1985) reported no toxicity towards development of a VAM fungus, Glomus mosseae, in groundnut treated with carbaryl, a closely related methyl-carbamate insec-

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TABLE 1 Plant growth and mycorrhizal development in sorghum 90 days after sowing as influenced by application of carbofuran, endosulfan and quinalphos Insecticide/ concentration (kg h a - ~)

Plant height ( cm )

Dry weight (g)

Total root length (m)

Specific root length (m/g)

Root colonization (%)

Total root colonized (m)

Shoot

Root

3.89c 4.41d 4.18cd 3.52c 2.70b

2.82c 3.65d 3.49d 2.52b 1.83a

470d 494d 481 d 409c 309b

43.76c 45.1c 44.6c 40.2b 35.3a

81c 89d 87d 75b 54a

381e 440f 420f 308d 167b

4.92d 3.80c 2.62b 1.20a

4.03d 3.01c 2.15b 1.11a

676e 483d 358b 193a

48.6d 44.7cd 38.9b 33.7a

91 d 85cd 63b 56a

615g 41 le 226c 108a

3.89d 3.74d 3.60d 2.67c 1.97b

2.82e 2.62e 2.58e 1.44c 1.00b

470d 453d 450d 266c 14lab

43.3c 42.2c 40.8c 35.7b 30.0ab

81e 80e 79e 57d 50c

381e 362e 356e 152d 70b

3.27d 2.44c 1.42b 0.84a

2.68e 1.88d 1.24bc 0.79a

448d 268c 163b 89a

40.3c 34.9b 32.6ab 27.9a

76e 49c 38b 30a

340e 131c 62b 27a

3.89d 3.80d 3.65d 2.88c 2.62c

2.82d 2.80d 2.82d !.88c 1.64c

470f 458f 446f 321d 311d

43.3d 42.0d 41.1d 37.4bc 35.4b

81c 80c 78c 63b 58b

381f 367f 348f 202d 180c

3.27d 2.20c 1.46b 0.90a

2.76d 1.85c 1.36b 0.73a

396e 283c 245b 104a

38.8c 37.8bc 34.0b 30.5a

73c 62b 57b 43a

289f 175c 140b 45a

Carbofuran Single application1 0 57bc 0.5 67d 2.5 60c 5.0 54bc 10.0 50ab Two applications 2 0.5 67d 2.5 55bc 5.0 50ab 10.0 47a

Endosu~an Single application ~ 0 57d 0.5 56cd 2.5 55cd 5.0 51bc 10.0 48b Two applications 2 0.5 56cd 2.5 54cd 5.0 44b 10.0 37a

Quina~hos Single application ~ 0 57d 0.5 55c 2.5 54c 5.0 53bc 10.0 49b Two applications z 0.5 55c 2.5 51bc 5.0 46b 10.0 40a

qnsecticide was applied 30 days after sowing. 21nsecticide was applied 30 and 60 days after sowing. Mean values in each column for an insecticide followed by the same letter are not significantly ( P < 0.05 ) different from each other according to Duncan's multiple range test.

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341

ticide, at field application doses. The inhibitory effect noticed with the higher levels of carbofuran in the present study may be ascribed to the prolonged persistence of carbofuran residues under non-flooded soil conditions (Williams et al., 1976; Venkateswarlu et al., 1977 ). Two successive applications of carbofuran at a level of 0.5 kg ha-~ significantly increased plant growth, in terms of plant height, biomass production and total root length. Concentration of 2.5 kg ha-~ carbofuran effected no change in growth response while levels of 5 and 10 kg ha- ~ significantly inhibited plant growth parameters and mycorrhizal colonization. The present results thus clearly indicate that carbofuran when applied singly or twice successively at field application doses would have no inhibitory effects either on plant growth or VAM development in sorghum. Endosulfan application at all concentrations significantly reduced plant height by 60 days (not shown ). However, the phytotoxicity caused by 0.5 and 2.5 kg ha- 1 was alleviated by the end of 90 days. Only 5 and l0 kg ha- ~levels were inhibitory to root infection after 90 days. Two successive applications of endosulfan at a level of 0.5 kg ha-~ were not phytotoxic to sorghum while concentrations ranging from 2.5 to l0 kg h a - t significantly affected plant growth and VAM colonization. However, single or two repeated applications of endosulfan to soil was found to initially delay the colonization of G l o m u s m o s s e a e in groundnut (Parvathi et al., 1985 ). These findings suggest that a single application of endosulfan at recommended rates does not inhibit plant growth and VAM development while two repeated applications at these concentrations adversely affects all the parameters except plant height. Despite the reasonably long persistence values of endosulfan in arable soils (Martens, 1977 ) the insecticide is not effective in reducing the VAM fungal infection if sorghum receives a single application of endosulfan at field doses. A single application ofquinalphos at a level of 2.5 kg ha- 1seemed to reduce plant growth and percent root colonization initially, but was innocuous by 90 days after sowing. The higher concentrations of 5 and 10 kg ha- t exhibited both phytotoxicity and fungitoxicity during this period. However, two repeated applications of quinalphos at a level of 0.5 kg ha-~ were non-toxic while the 2.5 kg ha-1 level exerted toxicity towards plant growth and VAM development. Evidently, 5 and 10 kg ha-~ levels proved to be toxic to plant growth and VAM fungi. Most striking reduction in G. m o s s e a e infection was reported with parathion, an organophosphorus insecticide (Parvathi et al., 1985 ). Further, analysis for correlation coefficients clearly indicated that there exists a strong positive (r> 0.91 ) relationship between percent colonization and plant dry weight or total root length of sorghum under the influence of selected insecticides. To sum up, in the soil types employed in the present study, no stimulation in development of mycorrhiza was observed under the impact of endosulfan and quinalphos, and a single or two repeated applica-

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tions of carbofuran up to a level of 2.5 kg ha- t enhanced the mycorrhizal status of sorghum. ACKNOWLEDGEMENT

The authors are grateful to the Council of Scientific and Industrial Research, New Delhi, for financial support.

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