Proc. Indian Acad. Sci. (Plant Sci.), Vol. 97, No. 6, December 1987, pp. 457--460. 9 Printed in India.

Effect of benthiocarb on epicuticular wax and silica content in rice and barnyard grass T R A J E N D R A P R A K A S H and P M SWAMY Department of Botany, Sri Venkateswara University,Tirupati 517 502, India MS received 12 January 1987; revised 9 September 1987 Abstraet. The effect of benthiocarb [S-(4-chlorobenzyl)-N, N-diethylthiocarbamate] at 1500 ppm on the levels of epicuticular waxes and silica content was studied to understand the differential sensitivity of rice and barnyard grass. The herbicide significantly inhibited the synthesis of epicuticular wax in barnyard grass. A high silica content was observed in the rice. The differencesto the extent of synthesis of epicuticular wax and the levels of sitica might be the causative factors for the differential sensitivity of rice and barnyard grass to benthiocaro.

Keywords. Benthiocarb;epicuticular wax; silica; rice; barnyard grass. 1.

lntroduction

The herbicide, benthiocarb [S-(4-chlorobenzyl)-N, N-diethylthiocarbamate] is one of the recent thiocarbamate formulations which has high inter-genus selectivity between rice (Oryza sativa L) a C 3 crop plant and barnyard grass (Echinochloa crusgalli (L) Baevu), a weed exhibiting krantz syndrome. De D a t t a (1972) reported the highest weed control rating and lowest visible toxicity rating of benthiocarb on rice. The thiocarbamate and dithiocarbamate herbicides are usually applied to soil and primarily inhibit growth of emerging shoots of grasses. Dawson (1963) observed kinking of the first internode of barnyard grass seedlings into a zigzag pattern by Sethyl dipropyl thiocarbamate (EPTC). Most of the workers have come to the conclusion that the primary site of action of thiocarbamates is located either in the shoot or in the leaf tissue (Yamaguchi 1961; Dawson 1963; Appleby et al 1965; Banting 1967; Prendeville et al 1968). Differences in the stomatal behaviour between C 3 and C4 plants towards the thiocarbamate herbicide was considered as a basis of selectivity of weed and crop (Das and Santakumari 1975). The present study is therefore intended to examine whether the deposition of silica and leaf epicuticular wax would form the basis of selectivity between rice and barnyard grass towards benthiocarb. 2.

Materials and methods

Rice and barnyard grass seeds were soaked in water for 3 days. The germinated seeds were then sown in 45 cm (diameter) earthenware seed pans containing puddled black soil supplemented with farmyard manure. The seedlings were maintained under natural photoperiod (about 2000/~E m -2 s - t ) , with a light regime of approximately 11 h at 34~176 followed by 13 h dark at 16~176 In pots the water was allowed to stand 4-6 cm high above the soil mimicking the anaerobic conditions of a natural paddy field. Benthiocarb of 1500 ppm concentration was given a s a foliar spray with 457

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T Rajendra Prakash and P M Swamy

a pneumatic sprayer to the drip point. The treatment was given at 2-3 leaf stage of the weed at which stage the chemical is found to be more effective. Leaves of rice and barnyard grass were excised at intervals of 12, 24, 48 and 72 h after treatment for analysis of the leal epicuticular wax and silica content. 2.1

Total leaf epicuticular wax content

The extraction and quantitative analysis of leaf epicuticular wax was carried out as per the method of Ebercon et al (1977). Epicuticular waxes were removed by stirring the known area of leal discs of rice and barnyard grass in 15 mi of chloroform for 15 s. The wax extract was ¡ and evaporated on a boiling water bath. To each sample 5 ml of the acidic dichromate reagent was added. The reagent was prepared by mixing 40 mi of distilled water with 20 g of potassium dichromate. The resulting slurry was mixed vigorously with 1 liter of conc. H2SO4, and heated below boiling point until a clear solution was obtai'ned and the samples were placed on a boiling water bath for 30 min. The samples were removed from water bath, cooled and added 12 mi of deionized water and the samples were allowed to stand for 15 min. The developed colour intensity was scanned for absorbance at 590 nm in Shimadzu spectrophotometer (UV-240}. 2.2

Silica content

Silica content of leaf material of both rice and barnyard grass was determined by classical gravimetric technique. Tissue samples were ashed in Platinum crucibles at about 500~ The ash was treated repeatedly with 6N HC! to remove other mineral impurities. The solution was filtered through ashless filter paper and ignited. The silicofl dioxide content was determined as differences in weights before and after treatment with the acid. From silicon dioxide, the silica content was calculated. 3.

Results and discussion

The treatment of benthiocarb caused yellowing of leaves in barnyard grass followed by necrosis and scorching immediately after 6-8 h. Although, thd foliar application of benthiocarb did not show any significant visual morphological changes in the leaves of rice until two days, bleached spots appeared on the lcaves. The fresh leaves of rice which emerged after application of benthiocarb looked healthy with dark green in colour. The present observations revealed that the primary site of action could be on noncellular, lipoidal layer of epicuticular wax, which may interfere, with the permeability of foliarly applied chemical. Treatment with benthiocarb resulted in inhibition of synthesis of epicuticular wax in the barnyard grass leaves, while in the rice treatment did not affect the synthesis of total epicuticular wax (figure 1). However the synthesis was to a lower extent in treated teaves when compared to control. Silica content on the lcaves of both rice and barnyard grass was less affected by bcnthiocarb treatment. However, the treatment decreased the deposition of silica in barnyard grass over 72 h, i.e. 14'9'Yo over control (table 1). Initially a high amount of silica is associated with the leaves of rice when compared to that of barnyard grass.

459

Benthiocarb effect on epicuticular wax and silica lE "o7. 4

Bornyord gross

Rice

O"~

~6"6

w

Control

Control~r~ ~

~5.8 -~ 5.0 "5

._u

~ 4.2

I

I

12 @4

I

I

I

I

I

48 72 12 24 48 Hours ofter treatment

I

72

Figure 1. Influenceof foliar spray of benthiocarb on the levels of leaf epicuticular waxes of rice and barnyard grass. Table 1. Effectof benthiocarb on leaf silica content of rice and barnyard grass (mg g- 1 dry woight). Hours after treatment Treatment Rice Control Treatment

Barnyard grass Control Treatment

12

24

48

72

13-2+2.1 12.54- 1.2

13.94-2-4 13.54-2.5

16'54-3.2 14.24-3.2

17.3-4-2.1 15.9+4-0

6.4+0.9 5.2+0.9

7.44-0.7 5.94-0-8

8'24-0'4 6.3+ 1.1

8.94-0-5 7.5+0.8

Values are mean of 4 replications 4-SE.

At all stages of growth, the levels of silica on the leaves of rice and barnyard grass were in the ratio of 2 : 1. The penetrat]on of foliar herbicides is always influenced by the leaf surface characteristics. The amount of epicuticular wax has a significant effect on herbicide penetration. Bayer and Lumb (1973) have shown that the amount of epicuticular wax has a significant effect on herbicide penetration. Gentner (1986) reported that EPTC, a thiocarbamate type of herbicide decreased epicuticular wax content and increased spray retention in Brassica oleracea var capitata. It was observed that the uptake of thiocarbamates varied widely between the species, leaves of different ages and even the surfaces of the leaves (Hunt and Baker 1982). Wilkinson and Hardcastle (1970), Harwood and Stumpf (1971) and Hunt and Baker (1982) have reported that the application of E P T C would stimulate the total fatty acid synthesis and reduce alkane synthesis and ultimately leading to reduction in the deposition of epicuticular waxes. Therefore in the present study the decreased levels of wax in the susceptible E. crusgalli may be due to accumulation of fatty acids and decreased alkane synthesis leading to decreased wax content consequently causing death to the plant. This assumption is further supported by studies of Raja Reddy et al (1979, 1982) in s ome semi arid scrub species.

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T Rajendra Prakash and P M Swamy

However in rice, the wax content was not significantly affected, which may basically restrict the entry of herbicide into the leaves. The difference in the sensitivities of rice and barnyard grass to the benthiocarb may also be attributed to the remarkable difference in the levels of silica deposition on the leaves.

References Appleby A P, Furtick W R and Rang S C 1965 Soil placement studies with EPTC on Avena sativa; Weed Res. 5 115-122 Banting J D 1967 Factors affecting the activity of diallate and triallate; Weed Res. 7 302-315 Bayer D E and Lumb J M 1973 Penetration and translocation ofherbicides (New York: Marcet Dekker Inc.) pp 387-440 De Data S K 1972 Chemical weed control in Tropical Rice; PANS 18 433-440 Das V S R and Santa Kumafi 1975 Stomatal behaviour towards four classes of herbicides as a basis of selectivity of certain weeds and crop plants; Proc. lndian Acad. Sci. 82 108-116 Dawson J H 1963 Development of barnyard grass seedlings and their response to EITC; Weeds 11 60-66 Ebercon A, Bhen A and Jordon W R 1977 A rapid colorimetric method for epicuticular wax content of Sorghum leaves; Crop Sci. 17 179-180 Gentner W A 1986 The influence of EPTC on external foliage wax deposition; Weeds 14 27-31 Hunt G M and Baker F A 1982 Developmental and environmental variations in plant epicuticular waxes: some effects on the penetration of Napthalence acetic acid; in The plant cuticles (eds) D F Cutler, K L Alvin and C E Price (New York: Academic Press) pp 279 Harwood J L and Stumpf P K 1971 Fat metabolism in higher plants XLIII. Control of fatty acid synthesis in germinating seeds; Arch. Biochem. Biphys. 142 281-291 Prendeville G N, Otiver L R and Schreiban M M 1968 Species differences in site of shoot uptake and tolerance to EPTC; Weed Sci. 16 538-540 Raja Reddy K, Rao J V S and Rama Das V S 1979 Inhibition of epicuticular wax deposition by Ansar 529 and EPTC in some semiarid shrubs; lndian J. Exp. Biol. 17 813-815 Raja Reddy K, Rao J V S and Das V S R 1982 Epicuticular wax composition and herbicide induced changes in the rates of transpiration in some semi arid scrub species; Indian J. Plant Physiol. 25 55-64 Yamaguchi S 1961 Absorption and distribution of S-EPTC; Weeds 9 374-380 Wilkinson R E and Hardcastle W S 1970 EPTC effects on total leaflet fatty acids and hydrocarbons; Weed Sci. 18 125-128