the metabolism of chlortoluron, diuron, and cga 43 057 in tolerant and susceptible plants

PESTICIDE BIOCHEMISTRY AND PHYSIOLOGY 16, 213-221 (1981)

The Metabolism of Chlortoiuron, Diuron, and CGA 43 057 in Tolerant and Susceptible plants

P. J. RYAN,* D. GROSS,~ W. J. OWEN,* AND T. L. LAANIO?

*Department of Biochemistry, Royul Hollowuy College, University of London, Englefield Green, TW20 OEX, England; urld tAgricultura1 Division, Ciba-Geigy Limited,

CH-4002 Basel, Switzerland

Received May I 1, 1981; accepted September 24, 1981

The metabolism of chlortoluron, I-(3-chloro-4-methylphenyl)-3,3-dimethylurea, diuron l-(3,4- dichlorophenyl)-3.3-dimethylurea, and CGA 43,057, I-(3-methyl-4-chlorophenyl)-3,3-dimethylurea was investigated in several plant species. Chlortoluron was degraded via both N-demethylation and ring-methyl oxidation reactions. The latter pathway was the major degradative route in wheat and barley. Since the products of ring-methyl oxidation are nonphytotoxic, the existence of such a detoxification mechanism in these cereals may account for their enhanced resistance to chlortoluron. The major metabolites in the susceptible cereal weeds, Avenafutuu. Alopecurus myosuroides, and Lolium perenne. and in resistant cotton were products of N-demethylation. This reaction in the cereal weeds proceeded only very slightly beyond the stage of mono-N-demethylated metabolites some of which retain considerable phytotoxicity. In cotton, however, N-demethylation was more efftcient, giving rise to substantial amounts of N-didemethylated metabolites which are nonphytotoxic. With the exception of cotton, all species studied metabolized CGA 43 057 much more slowly than chlortoluron. In particular, the 3-methylphenyl group of CGA 43 057 was far less susceptible to oxidation than the 4-methylphenyl group of chlortoluron. Cotton, however, extensively N-didemethylated both CGA 43 057 and diuron to give nonphytotoxic products, an observation consistent with the greater resistance of cotton to these phenylureas. N-Demethylation of diuron in the cereals and cereal weeds was similar to that of CGA 43 057 in that the reaction did not proceed to any major extent beyond mono-N-demethylated metabolites.

INTRODUCTION

Since their discovery in the early 1950s the substituted phenylureas have grown into one of the most prominent and diver- sified groups of herbicides. The first important phenylureas, exemplified by monuron and diuron, were chlorine-substituted dimethyl derivatives with a high order of phytotoxicity. Though these compounds were initially suggested for use as industrial weed killers, in subsequent years their po- tential for selective application in agricul- ture and horticulture was recognized (1, 2).

The compounds developed in more recent years have commonly contained alkyl or alkoxy ring substitutents either in addition to or instead of halogen groups with the result that considerable improvements in selectivity have been achieved (3). Of particular importance in this respect has been the development of chlortoluron, l-(3-

chloro-4-methylphenyl)-3,3-dimethylurea, for selective weed control in winter wheat (4-7). In addition to controlling a range of broadleaf weeds, the herbicide is also effective against blackgrass (Afopecurus myosuvoides) and wild oat (Avena futua), species which constitute a major threat to the successful cultivation of winter-sown cereals (8).

The selective properties of the substituted phenylureas have been comprehen- sively reviewed recently (9) and several ex- cellent accounts of their metabolic fate in plants are available elsewhere (lo- 12). Stepwise N-demethylation, often followed by conjugation of the intermediates as 0-glucosides, has generally been considered the major degradation pathway for di- methylphenylureas in plants. The N- monomethyl metabolites are usually less phytotoxic than their dialkylated parent

213 0048.3575/81/060213-09$02.00/O Copyright @j 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

214 RYAN ET AL.

compounds whereas phytotoxicity disap- pears completely concomitant with the sec- ond demethylation (9). However, a recent study of the metabolic fate of chlortoluron in wheat (13) clearly demonstrated that N-demethylation did not represent the major pathway of degradation and that oxidation of the 4-methylphenyl group to give benzyl alcohol and benzoic acid derivatives (Fig. I), a proportion of which may become conjugated, was quantitatively more important. A similar metabolic fate for chlortoluron has also been recorded previously in the rat ( 14) and Japanese quail (15). Since an appropriate balance between lipophilic and hydrophilic properties is considered to represent an important criterion for the ef- fectiveness of the phenylurea herbicides as inhibitors of photosynthesis (16, 17), the occurrence of such a pathway in plants could lead to the production of nonphytotoxic metabolites.

In the present investigation, a detailed study of the qualitative and quantitative metabolism of chlortoluron was carried out

in both tolerant and sensitive plant species, particularly with a view to elucidating the basis for the selective action of the herbicide in winter cereals. Diuron was included in the study for purposes of comparison, as was the experimental herbicide CGA 43 057, 1-(3-methyl-4-chlorophenyl)- 3,3-dimethylurea, a chlortoluron analog.

MATERIALS AND METHODS

Chemicals. The labeled herbicides and nonlabeled reference compounds used in the present study were synthesized using conventional procedures, i.e., by reaction of dimethylamine, methylamine, and am- monia with the corresponding phenyliso- cyanates. Their structures, confirmed by elementary analysis and/or spectroscopic means, are listed in Table 1. [carhonyl-“‘Cl- Chlortoluron (sp act, 30.6 /Ki/mg), [cur- horzyl-14C]CGA 43 057 (sp act, 34.9 $LYmg), and [carhotryl-‘lC]diuron (sp act, 34. 4 &II mg) all had a radiochemical purity greater than 9%. P-Glucosidase isolated from al- mond was purchased from Fluka A.G.,

=,.I3 ;-:-NHcH3 - ‘CH3

c’ Chlortoluron

Hov

Cl

I

Cl

I

HOOC

FIG. 1. Pathways of chlortoluron degradation in sheaf. I, III, etc., code numbers used in text and Table 1.

METABOLISM OF CHLORTOLURON, DIURON, AND CGA 43 057 IN PLANTS 215

TABLE 1 Structures and Code Numbers of Compounds Referred to in the Text

Code number RI % %

Chlortoluron CH, Cl -NH-CO-N(CH,), CGA 16 339 (1) CH, Cl -NH-CO-NHCH, CGA 16 340 (11) CH, Cl -NH-CO-NH, CGA 18 029 (III) HOCH, - Cl -NH-CO-N(CH,), CGA 16 342 (Iv) HOCH, - Cl -NH-CO-NHCH, CGA 25 932 (VII) HOCH, - Cl -NH-CO-NH, CGA 15 140 HOOC- Cl -NH-CO-N(CH,), CGA 15 139 g, HOOC - Cl -NH-CO-NHCH, CGA 14 119 (XII) HOOC- Cl -NH-CO-NH,

CGA 43 057 Cl CH,- -NH-CO-N(CH,), CGA 101 562 (VIII) Cl CH,- -NH-CO-NHCH, CGA 101 563 (IX) Cl CH,- -NH-CO-NH, CGA 98 168 Cl HOCH*- -NH-CO-N(CH& CGA 98 169 Cl HOCH, - -NH-CO-NHCH, CGA 98 167 Cl HOOC- -NH-CO-N(CH,), CGA 98 166 Cl HOOC- -NH-CO-NHCH,

Diuron Cl Cl -NH-CO-N(CH,), C 2770 m Cl Cl -NH-CO-NHCH, CGA 21091 WI) Cl Cl -NH-CO-NH*

Buchs, Switzerland. Thin-layer chromatography (TLC) plates were supplied by Merck A.G., Darmstadt, West Germany, as were the solvents used throughout.

Plunts und their treatment. The plant species included in the study were: wheat (Triticum uestivum var. “Atou”), barley (Hordeum vulgare var. “Astrix”), cotton (Gossypium hirsutum Delta Pine var. 61), wild oat (Avena fatua type “USA”), blackgrass (Alopecurus myosuroides), and perennial rye grass (Lofium perenne L). Seedlings were cultivated in lo-cm plastic pots filled with soil (clay loam, pH 7.65: organic matter content, 5.3%; water- holding capacity, 56%) to about 2 cm below the lip. The seeds were covered with a 2-cm layer of soil already treated with either 1.2 (for cereals and cotton) or 0.6 mg (for weed species) of (carbonyf-‘“C)-labeled herbicide. These treatments corresponded to field applications of 1.5 and 0.75 kg ailha, respectively. None of the herbicides caused

phytotoxicity to the plants at the rates used, so that the metabolic activity of treated plants was expected to be normal. The pots were placed in a controlled-environment cabinet maintaining a 16-hr-day regime (light intensity 10,000 lx) and day/night temperatures of 20 and 14”C, respectively, for cereals and cereal weeds and 28 and 22°C for cotton. Relative humidity was maintained at 60% in the case of cereals and cereal weeds and at 80 (day) and 60% (night) for cotton. When necessary, the pots were watered from above. After 10 days cultivation all plants were harvested and weighed prior to analysis of the radioactive metabolites.

Extruction und anulysis of metabolites. Parent herbicides and their metabolites were exhaustively extracted from the plant tissue by homogenizing with methanol/ water, 8:2 (v/v). The radioactivity in the extracts was measured directly in a liquid scintillation counter (Packard Tri-Carb

216 RYAN ET AL.

Model 3375). Nonextractable radioactivity was also determined by the liquid scintillation technique following combustion of the residual plant material in a sample oxidizer (Packard Tri-Carb Model 306). For meth- anolic extracts a dioxane-based scintillation mixture was employed, whereas lib- erated 14C0, was counted in the proprie- tary toluene-based mixture, Permafluor (Packard) after trapping in Carbo-sorb. Quenching was corrected by the AES channels ratio or the internal standard method.

The parent herbicides and their different metabolites present in the extracts were separated by two-dimensional thin-layer chromatography on precoated 0.25mm silica gel 6OF,,, plates (Merck, Darmstadt). For chlortoluron and CGA 43 057 a two- dimensional solvent system, consisting of (i) chloroform/ethanol/acetone, 9:2: 1 (v/v) and (ii) chloroform/ethanol/acetic acid, 9:l:l (v/v), was routinely employed, whereas for diuron adequate separation was obtained after a one-dimensional development with either benzene/acetone, 2: 1 (v/v) or chloroform/ethyl acetate, 1: 1 (v/v).

Radioactive zones were located using a &radiochromatogram camera (LKB, Mod- el 2105) prior to their removal from the plates for quantitation by liquid scintillation counting in a scintillation mixture com- prised of Instagel (Packard). Metabolites were characterized by TLC analysis, their Rf values being compared with those of authentic reference compounds in solvent systems described previously (13).

Conjugated metabolites were collected by preparative TLC and incubated with al- mond P-glucosidase at 37°C for 4 hr. The radioactivity was extracted from the aque- ous incubation media with ether using a Kutscher- Steudel apparatus and analyzed by the TLC and scintillation counting procedures described previously.

RESULTS AND DISCUSSION

Determination of the total radioactivity in plants following application of [“Clchlor- toluron, diuron, or CGA 43 057 revealed

only very slight species differences, so that no correlation could be shown to exist between susceptibility and herbicide uptake. These observations were supported by radioautography of treated plants which also indicated that the distribution of the Y-labeled herbicides and their metabolites within the various plant species studied was the same for each compound.

The amounts of unchanged chlortoluron and its metabolites, expressed as a percentage of total radioactivity recovered, extracted from lo-day-old plants of the various species are shown in Table 2. These data show that in general the overall degradation of chlortoluron correlated well with the susceptibility of the plant, the herbicide being more efficiently degraded in the tolerant wheat, barley, and cotton than in sensitive Alopecurus, L&urn, and Avena. Chlortoluron was shown to be degraded by both N-demethylation and ring-methyl oxidation pathways in that a variety of ring- methyl oxidation derivatives with various degrees of N-demethylation could be de- tected (metabolites III-VII). In agreement with previous observations (13), ring- methyl oxidation derivatives predominated in the nonpolar metabolite fraction of wheat and barley, I-(3-chloro-4-hydroxymethyl- phenyl)-3,3-dimethylurea (III) being the major metabolite. In contrast to wheat and barley, however, in Alopecurus, Loliutn, Avrtza, and cotton N-demethylation activity clearly predominated over ring-methyl oxidation. In addition, varying proportions of most of the metabolites were also extracted as polar conjugates. The metabolites associated with these fractions were characterized by thin-layer chromatography after treatment with &glucosidase. as described under Materials and Methods. Of particular significance is the high percentage of metabolite III released from the conjugate fraction extracted from wheat and barley. With the exception of cotton, the amount of un- characterized radioactive material extracted from treated plants was generally small.

The importance of the positioning of the

‘TAB

LE

2 Pa

ttern

of

Extr

acta

ble

Rudio

uctiv

e M

etab

olite

s in

Pl

unts

Tr

eate

d wi

th

[‘“C]

Chlor

tolur

o,la

Meta

bolite

fra

ction

Conju

gates

of

Spec

ies

I II

III

IV

V VI

VI

I Un

know

n No

n- Ch

lor-

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

CGA

meta

bo-

extra

ct-

to1uro

n 16

339

1634

0 18

029

1634

2 15

140

1513

9 16

339

1634

0 18

029

1634

2 25

932

1514

0 15

139

lites

able

Whe

at (var.

Atou

) 22

.2 2.1

ND

0 3.6

0.6

1.5

2.1

0.8

ND

50

.7 11

.8 ND

0.7

0.8

ND

3.1

Ba

rley (va

r. As

trix)

29.3

4.5

ND

6.0

1.5

1.2

1.5

1.6

ND

33.3

11.5

ND

5.8

ND

ND

3.7

Loliu

m

peren

ne

47.5

5.9

ND

1.8

0.6

0.2

0.1

11.6

ND

17.7

5.4

ND

0.06

0.2

1.1

7.7

AVtV

lll fa

tun

38.7

22.6

1.9

2.8

2.1

1.7

ND

2.1

3.4

4.2

6.7

1.1

ND

ND

6.1

6.6

Alop

ecur

us

myo

suroi

des

39.9

5.8

ND

1.9

1.3

0.4

0.6

13.1

ND

20.3

5.3

ND

ND

0.4

1.2

9.8

cotto

n (Delt

a Pin

e va

r. 61

) 26

.5 7.6

7.6

0.4

0.4

0.5

0.5

5.3

5.5

12

.8 6.5

ND

7.1

ND

16

.5 2.6

n Ex

press

ed

as

perce

ntage

of

total

radio

activ

ity

found

in

whole

pla

nts

after

10

day

s of

treat

men

t. b

Nond

etecta

ble.

218 RYAN ETAL.

ring-methyl group of chlortoluron with respect to susceptibility to metabolic attack was indicated by the results (Table 3) of a parallel study made using CGA 43 057, 1-(3-methyl-4-chlorophenyl)-3,3-dimethylurea, an analog of chlortoluron in which the positions of the ring chlorine and methyl substituents are reversed. These data indicated that the degradation of CGA 43 057 proceeded more slowly than that of chlortoluron in all grass species studied. In such plants, CGA 43 057 degradation was characterized by the presence of similar or slightly higher amounts of nonpolar (un- conjugated) metabolites but much lower amounts of polar degradation products than with chlortoluron. The nonpolar metabolite fraction consisted primarily of N-demethylated substances (metabolites VIII and IX) of which 1-(3-methyl-4-chlorophenyl)-3-methylurea (VIII) predominated. It is also apparent from the data presented in Table 3 that the ring-methyl group of CGA 43 057 is considerably more resistant to oxidation than that of chlortoluron in all species studied. These data are in agreement with previous observations that CGA 43 057 was of little value as a selective herbicide in cereal crops (Ciba-Geigy, unpublished results). In contrast, however, degradation of CGA 43 057 was much more extensive in cotton which possessed a more effective mechanism for N-demethylation than the cereals and cereal weeds, as indicated by the presence in the extracts of a considerable amount (some 33% of total extracted radioactivity) of the N-didemethylated metabolite (IX), most of which was associated with the nonpolar fraction.

For comparison with the results obtained for chlortoluron, a similar metabolic study was carried out using the related substituted phenylurea, diuron, which, although structurally very similar to chlortoluron, does not posses a ring-methyl substituent (see Table 1) and is nonselective in cereals. The results obtained (Table 4) indicate that with the exception of cotton, which is resistant to diuron, all species degraded this


TABLE 4 Degradation Products of [‘4C]Diuron in Treated Plants”

Metabolite fraction

Species Diuron

Wheat (var. Atou) 55.4 Barley (var. Astrix) 42.5 Lolium perenne 69.6 Avena fatua 43.2 Alopecurus tnyosuroides 44.8 Cotton (Delta Pine var. 61) 23.3

X XI

C CGA 2770 27091

Conjugates of

C CGA Unknown 2770 2709 1 metabolites Nonextractable

16.8 3.5 4.4 6.9 8.8 4.1 24.0 9.6 5.7 10.0 5.1 3.1 9.4 0.7 2.0 0.3 14.4 3.6

20.1 3.2 3.0 4.0 10.5 16.0 13.8 13.6 7.3 6.3 7.8 6.4 11.4 33.2 1.0 7.8 16.8 6.5

u Expressed as percentage of total radioactivity found in whole plants after 10 days of treatment.

herbicide more slowly than chlortoluron, this being particularly evident in the case of Lolium which retained some 70% of unchanged parent herbicide. The only metabolites characterized were products of N-demethylation (X and XI) and, with the exception of Lalium, rates of N-demethylation of diuron were similar in all species, an observation consistent with the lack of selectivity of diuron in cereals. In cotton, however, not only was diuron more

extensively degraded than in the other plants studied, as was the case for CGA 43 057, but also N-demethylation appeared considerably more effective in this species so that the greatest percentage of the metabolites (some 40% of the total) was con- tributed by N-didemethylated diuron (XI).

The results obtained for all three phenylureas are conveniently summarized in histogram form in Fig, 2. Presented in this way, the relative contributions of N-

Wheat Barley Lolium AVMM AlOpeCWUS COttOll %

Chlottoluron ;g

20

CGA 43057

80

60 Diuron

40

unchanged herbicide mono-N-demethylated

cl

ring methyl oxydation metabolites metaboliter

N-didemethylated metabolites iB!i

others

FIG. 2. Degradation of chlortoluron. diuron. and CGA 43 057 in tolerant and susceptible plants.

220 RYAN ET AL.

demethylation and ring-methyl oxidation pathways to the overall metabolism of each of the three herbicides studied can be read- ily compared. Reference to Fig. 2 clearly shows that in the case of chlortoluron a definite correlation exists between the oxidation of the ring-methyl substituent and the degree of susceptibility of the cereals and cereal weeds, extracts of resistant species such as wheat and barley containing a much higher percentage of ring-methyl oxidation metabolites than those of sensitive Alopecurus, Lolium, and Avenu. As this oxidation simultaneously results in detoxification of the herbicide, as indicated by the absence of an effect of any of the oxidation products on photosynthesis (Ryan and Owen, unpublished observations), a biochemical basis for the selectivity of chlortoluron is clearly seen to exist. In contrast to wheat and barley, in Alnpecurus, Lolium, and Avena the contri- bution of N-demethylation to the overall degradation of chlortoluron was propor- tionally greater. With the exception of Avenrr, in which very small amounts of N-didemethylated metabolites were detect- able, this reaction did not proceed beyond the stage of monodemethylated derivatives some of which still retain considerable phytotoxicity (Ryan and Owen, unpublished observations).

In agreement with earlier reports (18-20) metabolism of diuron was entirely depen- dent on N-demethylation in all species examined, as was primarily the case for CGA 43 057. For these herbicides also, this reaction in the grass species did not proceed to any major extent beyond the stage of monodemethylation, a fact which probably accounts for the higher sensitivity of wheat and barley to diuron and CGA 43 057 and for the nonselectivity of these herbicides in cereals.

In cotton, though ring-methyl oxidation of chlortoluron was low compared to wheat and barley, N-demethylation of all three herbicides is more complete and results in the formation of N-didemethylated derivatives in which no significant phytotoxicity

is retained. This latter observation may well have a direct bearing not only on the existence of a tolerance to chlortoluron in cotton comparable to that of the wheat and barley varieties examined but also on the enhanced resistance of cotton to CGA 43 057 and diuron.

From the results obtained in this study we therefore conclude that an enhanced ability of cereals to oxidize the 4-methylphenyl group of chlortoluron provides a sound basis for the selective action of the herbicide in such crops. The results clearly illustrate the drastic effect small variations in structure, in this case the sub- stitution of the 4-chloro group of diuron by a methyl group to give chlortoluron, may have on susceptibility to metabolic attack. In addition, the location of the methyl group in the para position of the phenyl ring of chlortoluron is of major importance with respect to rate and extent of oxidation. Fi- nally, the resistance of cotton to the substituted phenylureas studied is attributed to a greater capacity for N-didemethylation. a reaction which like ring-methyl oxidation also gives rise to nonphytotoxic products.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of the Science Research Council in the form of a CASE Studentship (P.J.R.) and of the University of London Central Research Fund for the provision of environ- mental growth chamber facilities (W.J.O.). Thanks are also due to Dr. H. R. Gerber for many helpful discus- sions during the work.

REFERENCES

1. A. S. Crafts, “The Chemistry and Mode of Action of Herbicides,” p. 269, Wiley-Interscience, New York, 1961.

2. H. Geissbiihler. The substituted ureas, in “Deg- radation of Herbicides” (P. C. Kearney and D. D. Kaufman, Eds.), p. 79, Dekker, New York, 1969.

3. F. M. Ashton and A. S. Crafts, “Mode of Action of Herbicides,” p. 367, Wiley-Interscience, New York, 1973.

4. F. J. H. van Hiele, A. Hommes, and G. J. Ver- velde, Cultivar differences in herbicide tolerance and their exploitation, Proc. 10th Bit. Weed Control Cmf., 111 (1970).

5. R. H. Tysoe. The control ofAvenafutua in winter


sown cereals with chlortoluron, Proc. 12th Brit. Weed Control Cot&, 41 (1974).

6. K. R. Hubbard and D. B. Livingston, Chemical control of Alopecurus myosuroides in winter wheat, Proc. 12th Brit. Weed Control Cotzf. 67 (1974).

7. R. T. Hewson, Isoproturon, a new selective herbicide for control of Alopecurus myosuroides in winter cereals, Proc. 12th Brit. Weed Control Conf., 75 (1974).

8. P. Wormell, The battle against wild oats and blackgrass, Big Farm Manag. 5, 35 (1972).

9. R. Emami-Saravi, “Studies on the Varietal Sus- ceptibility in Winter Wheat to Substituted Phenylurea Herbicides,” Ph.D. thesis, Univer- sity of London, 1979.

10. D. S. Frear, H. R. Swanson, and F. S. Tanaka, Herbicide metabolism in plants, in “Recent Ad- vances in Phytochemistry” (V. C. Runeckles and T. C. Tso, Eds.), Vol. 5, p. 225, Academic Press, New York, 1972.

11. H. Geissbtihler, H. Martin, and G. Voss, in “Her- bicides: Chemistry, Degradation and Mode of Action” (P. C. Kearney and D. D. Kaufman, Eds.). Vol. 1, p. 209, Dekker, New York, 1975.

12. A. W. Naylor, Herbicide metabolism in plants. in “Herbicides: Physiology, Biochemistry and Ecology” (L. J. Audus, Ed.), Vol. 1, p. 397, Academic Press, New York/London, 1976.

13. D. Gross, T. Laanio, G. Dupuis, and H. 0. Esser, The metabolic behavior of chlortoluron in wheat and soil, Pestic. Biochem. Physiol. lo,49 (1979).

14. W. Mticke, R. E. Menzer, K. 0. Ah, W. Richter, and H. 0. Esser, The metabolism of chlortoluron in the rat, Pestic. Biochem. Physiol. 6, 430 (1976).

15. R. K. Hinderer and R. E. Menzer, Comparative enzyme activities and cytochrome P-450 levels of some Japanese quail tissues with respect to their metabolism of several pesticides, Pestic. Biochem. Physiol. 6, 161 (1976).

16. N. E. Good, Inhibitors of the Hill reaction, PIant Physiof. 36, 788 (1961).

17. D. E. Moreland and K. L. Hill, Inhibition of photochemical activity of isolated chloroplasts by polycyclic ureas, Weeds 11, 284 (1963).

18. J. W. Smith and T. J. Sheets, Uptake, distribution and metabolism of monuron and diuron by several plants, J. Agr. Food Chem. 15, 577 (1967).

19. J. H. Onley, G. Yip, and M. H. Aldridge, A metabolic study of 3 - (3,4 - dichlorophenyl) - 1.1 - dimethylurea (Diuron) applied to corn seedlings, J. Agr. Food Chem. 16,426 (1968).

20. C. R. Swanson and H. R. Swanson, Metabolic fate of monuron and diuron in isolated leaf discs, Weed Sci. 16, 137 (1968).

the metabolism of chlortoluron, diuron, and cga 43 057 in tolerant and susceptible plants

Documents