Chapter 7
Improving Crop Productivity in India: Role of Allelochemicals Allelochemicals: Role in Agriculture and Forestry Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/10/15. For personal use only.
S. J. H. Rizvi and V. Rizvi Department of Botany and Plant Pathology, Rajendra Agricultural University, Pusa, Samastipur 848125, Bihar, India
The possible role of allelochemicals has been explored for pest control, crop rotation, and agroforestry. Various allelochemicals were screened for herbicibal and fungicidal activities against Amaranthus spinosus and Alternaria solani. Caffeine and geraniol completely inhibited germination of the test weed at 6.0 and 0.3 mM, respectively, and similarly inhibited mycelial growth of the test fungus at 10.3 and 2.5 mM. In a tobacco-maize rotation system, nicotine at 5 m M significantly increased the growth of maize. Leucaena leucocephala is widely recommended for agroforestry, so the effect of its most important allelochemical, mimosine, on several crops was studied. It appreciably inhibited growth of rice and wheat at 1 to 5 m M concentrations. These observations establish in principle that allelochemicals can be important in improving pest control, crop rotation, and agroforestry programs and thereby increase crop production.
B o t h the harmful and the beneficial effects o f plant-plant, plantmicroorganism, and plant-insect interaction must be considered aspects o f allelopathy, as advocated by R i c e (1). The role o f allelopathy i n natural and manipulated ecosystems i n poorly recognized. In the past decade we have been concerned mainly with exploring the frontiers of applied allelopathy that may lead to increased crop production. There are two ways o f this k i n d by which crop production may be improved: allelopathy can be exploited by developing new crop management systems and i m p r o v i n g existing ones, and p u r i f i e d allelochemicals may be used commercially as agrochemicals. Management Systems Different types o f management may be developed for weed, insect, and disease control, for crop rotation, and for agroforestry employing allelopathy. In the following paragraphs these are discussed separately. In weed management systems involving allelopathy, crop varieties may be screened or new varieties developed for their potential for controlling weeds. Such varieties may be left as residues i n the field, or be incorporated i n every rotation system, and/or used as a companion crop. Similarly, i f crop varieties allelopathic to pathogens can be found, their residues can be used similarly for disease control. Research groups of Putnam at M i c h i g a n State University and of Gliessman at the University o f California are two o f many that are involved i n research of this kind. 0097-6156/87/0330-0069$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society
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70
ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY
Crop rotation is a "cropping system i n which two or more crops are grown i n a fixed sequence" (2). A n y crop may release allelochemicals either directly i n the form o f exudate or vapors, or its residue/litter can produce them afterward. Irrespective o f their source, allelochemicals may affect the next crop i n the rotational sequence either positively or negatively. In India, many farmers grow two or three crops/year on the same field, and there are definite recommendations on the sequence o f crops. The rotational sequences are based on maintenance o f s o i l fertility, s o i l structure, and plant nutrients, etc., but they have been formulated with little or no consideration for allelopathy, although it is unlikely that when crops are continuously supplying the soil with allelochemicals, these would not affect the following crop. This thought persuaded us to test our views experimentally, with two locally practiced rotational sequences: tobacco and maize, and tobacco and rice. O n a very conservative estimate the leaf litter from a chewing tobacco crop adds about 133 kg/ha o f organic matter to the soil (2), and the average nicotine content i n leaves o f tobacco is aproximately 4%. Thus every harvested crop to tobacco adds about 5.32 k g nicotine/ha merely through its leaf litter. O f course, roots also contribute some nicotine. Therefore we studied the effect o f nicotine on germination and growth o f maize and rice i n vitro, using the technique o f R i z v i et al. (4). Nicotine significantly increased the radical and plumule length o f maize, but decreased all the parameters studied i n rice; the radical length was most affected at 5 m M concentration (Table I). These results demonstrated the differential effect o f nicotine, and suggest that careful consideration o f allelopathic interactions o f crops of a rotation management can improve its productivity, either by eliminating deterimental interactions or by exploiting beneficial ones. Agroforestry. Agroforestry may be defined simply as intercropping o f woody plants with food or forage crops i n order to maintain or increase total yields (5.). W h i l e agroforestry has a potential to increase yield, it has its own limitations: competition o f trees with food crops, damage to food crops during tree harvesting, etc. The literature remains silent on allelopathic interactions between trees recommended for agroforestry and food crops, and no suggestion has been made to consider such effects before recommending food/forage crops for agroforestry programs (É). Hence we worked with mimosine, and allelochemical produced by leguminous trees - Leucaena sp. The concentrations of mimosine i n air-dried leaves o f such species range from 2.35% to 6.37% (2). O u r studies showed that mimosine is toxic for rice and wheat. Seed germination and radicle and plumule length o f both rice and wheat were adversely affected by mimosine (Table II). Earlier reports of Smith and Fowden ( £ ) and K u o et al. (2) o n the injurious effect o f mimosine on mung bean and rice, respectively, support our results. Therefore, possible allelopathic interactions between trees and food/fiber crops should be studied to improve agroforestry management. Allelochemicals as Sources of Agrochemicals N o w we come to the other potential application o f allelochemicals: the development o f new, safer and effective agrochemicals as pesticides and growth promoters. However, we restrict the discussion o f pesticides to herbicides, fungicides, and broad-spectrum pesticides. It is obvious that synthetic pesticides are important i n controlling weeds and plant-pathogenic fungi, but i n judging the efficacy of a method, it is essential to assess its non target toxicity, including health and environmental hazards. The literature abounds with reports that synthetic pesticides affect nontarget plants, their consumers and the environment (2.). Thus, plant protection needs
7.
RIZVI A N D RIZVI
Improving
Crop Productivity
71
in India
Table L Change Caused by Nicotine i n Seed Germination, Radicle Length and Plumule Length over Control in T w o Crops Plants
Change
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Concentration (mM) Gennination
(%)
Radicle
Plumle
For Zea mays (var. Diarra composite): increase 2.5
2.73 ± 0.98
43.56* ± 1.10
25.82* ± 1.38
5.0
2.63 ±0.88
46.20* ± 0.98
33.11* ± 1 . 2 0
For Orvza sativa (var. Pusa 221): decrease 2.5
4.72 ± 0 . 2 1
14.99* ±0.98
3.10* ± 0 . 9 6
5.0
11.02 ± 0.82
57.64* ± 1.08
26.54* ± 1.01
* Significant at 5 % level.
Table II. Reduction of Seed Germination, Radicle Length and Plumule Length over Control i n T w o Crop Plants by M i m o s i n e
% Reduction
Concentration (mM)
Germination
1.0 2.0
2.44 ± 0.32 9.76 ± 0.91
Plumule
Radicle Oryza s a t i v a
( v a r . Saket)
5.48 ±0.18 11.39 ± 0 . 2 5
4.21 ± 0.68 15.72 ± 1.01
3.0
13.42* ± 1.01
19.40* ± 0 . 8 6
32.01* ± 1.20
4.0
21.96* ± 1.32
84.38* ± 1.20
83.15* ± 1.82
5.0
25.62* ± 1.42
100.00* ± 0.63
85.08* ± 1.82
Triticum vulgare (var. Sonalika) 2.27 ± 0 . 1 2
11.33 ± 1.10
14.84 ± 0.96
30.60* ± 0.98
20.20* ± 1.02
15.90* ±0.93
57.12*± 1.11
43.84* ± 0.98
4.0
19.88* ± 1.01
65.51* ± 1.01
70.44* ± 0.68
5.0
23.32* ± 1.32
80.93* ± 1.20
63.05* ± 1.20
1.0 2.0
12.72 ± 0.36
3.0
* Significant at 5 % level.
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ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY
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improvement. It is important to note that many allelochemicals are k n o w n to inhibit the germination and growth of weeds and fungi (1), which is an essential feature o f all herbicides and fungicides, respectively. Therefore, allelochemicals can be an alternative to synthetic pesticides, and, being natural products, are safer to use and easily biodegradable (9-11). Herbicides. The idea o f using allelopathy i n weed control was conceived i n the late seventies (12). and several workers have considered this possibility, as already mentioned (13.-16). However, characterization and possible use o f allelochemicals as selective herbicides received attention only recently, and our demonstrations o f the selective herbicidal activity o f caffeine are among the pioneer ones (17-21). T o explore the potential of allelochemicals as herbicides, we tested some monoterpenes and terpenoids for suppression o f germination and growth o f the test weed Amarathus spinosus. using the bioassay o f R i z v i et al. (4). A m o n g the monoterpenes tested, geraniol was most potent. It inhibited radicle growth by 100% and seed gennination by 9 0 % at a concentration o f 2 m M , and completely inhibited plumule growth at 3 m M concentration (Table ΠΙ). The phytoxicity o f geraniol indicates that i t and other allelochemicals may serve as alternatives to synthetic herbicides.. Fungicides. Since most commercial fungicides are synthetic products, almost all demerits o f synthetic pesticides i n general are associated with them. T o look for an alternative to such synthetics, some allelochemicals were tested i n vitro by a technique o f ours (17). Surprisingly, geraniol again proved the best. It inhibited mycelial growth o f Alternaria solani. the test fungus, by 9 2 % at a concentration of 2 m M , and proved fungicidal above 2 m M (Table IV), Thus geraniol i n particular and allelochemicals i n general may be o f use as fungicides. Multipurpose pesticides. Occurrence o f more than one pest-controlling property in a single allelochemical has persuaded us to propose using allelochemicals as multipurpose pesticides (IS). Such use would be beneficial i n several ways: (i) Reduction i n cost o f pesticides. Use o f a single compound for the control of several pests may reduce the total expenditure on crop protection. (ii) Reduction i n cost o f production and research. The cost o f production o f single multipurpose compound would probably be less than the total cost o f product o f several pesticides. (iii) Improved quality o f life. Being plant products, most allelochemicals should affect fewer nontarget organisms, and improve quality o f farm produce. Further, multipurpose pesticides can be very important i n developing integrated pest management systems (IPMS). A n y I P M S is designed to minimize losses o f crop y i e l d and quality due to pests, through integration o f various approaches to pest control so as to get m a x i m u m benefits w i t h m i n i m u m disadvantages. In spite o f their hazardous nature, synthetic pesticides are often among the dominating components of an I P M S . Moreover, an I P M S often requires simultaneous use of several synthetic pesticides, which, harmless singly, may become poisonous through interaction among themselves or their metabolites (22.23). Therefore, the number o f chemicals used i n any I P M S should be minimized. A viable approach to this may be to find multipurpose pesticides. A multipurpose allelochemical pesticide for I P M S would not only reduce the chances o f synergistic toxicity but also, being plant-derived, have all the merits of natural products. O u r studies with geraniol indicate its ability to control more than one agriculturally important pest, a weed and a pathogenic fungus. Moreover, up to
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RIZVI A N D RIZVI
Table ΠΙ.
Improving
Crop Productivity
in India
Amaranthus spinosus: Reduction of Seed Germination, Radicle Length, and Plumule Length over Control by Geraniol
Concentration (mM)
Germination
% Reduction Radicle
1.0
2.86 ± 0.64
9.16± 0.98
9.09 ± 1.6
2.0
90.90* ± 1.60
100.00* ± 1.10
35.00* ± 1.2
3.0
91.60* ± 0.98
100.00* ± 0.52
100.00* ± 1.4
* Significant at 5%
Table I V .
Plumule
level.
Alternaria solani: Inhibition of M y c e l i a l Growth over Control by Geraniol
Concentration (mM)
Inhibition of Mycelial Growth %
1.0
53.20* ±1.10
2.0
92.80* ±1.68
3.0
100.00* ± 0.09
* Significant at 5%
level.
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ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY
2.5 m M geraniol exerted no visible adverse effect o n a test crop, tomato (Lycopersicon esculentum). in which both the test pests are problems. Thus, the possibility of exploiting allelochemicals to develop multipurpose pesticides has promise.
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G r o w t h promoters. The beneficial effects o f allelochemicals have been recognized only recently (1). Our studies with nicotine (Table I) suggest that nicotine and other allelochemicals with such potential may be used as growth promoters. Conclusions A n y improvement or new development in crop management based on allelopathic studies could not only increase production, but reduce expenditures on farm labor and agrochemicals, and reduction in use of synthetic agrochemicals would lead to an improved quality of life. If allelochemicals can be developed as botanical pesticides, they would be better than synthetic ones owing to their smaller non target toxicity, easy biodegradability, and nonpollutive nature. It is even possible that allelochemical production can be induced by genetic manipulation into cultivars, to provide an inexpensive, safe and permanent means of biological pest control. Acknowledgments Financial assistance by the International Foundation for Science (Sweden) i n the form of a research grant N o . A/745-1/1984 is gratefully acknowledged. The first author is thankful to the authorities o f Rajendra Agricultural University for providing necessary facilities. W e acknowledge a gift of geraniol from George R. Waller.
Literature Cited 1. 2.
Rice, E. L. "Allelopathy"; Academic Press: New York, 1984. Lockerhart,J. Α.; Wiseman, A. J. "Introduction to Crop Husbandry"; Pergamon Press: Oxford, 1970. 3. Srivastava, R. P. Central Tobacco Research Station, Pusa, India, personal communication, 1985 4. Rizvi, S. J. H., Mukerji, D. J., Mathur, S. N. Indian J. Exp. Biol. 1980, 18, 777-8. 5. Vergara, Ν. T. "New Directions In Agroforestry: The Potential of Tropical Legume Trees" East West Centre: Hawaii, 1982. 6. Labelle, R. (International Council for Research in Agroforestry Nairobi, Kenya), private communication, 1984. 7. Kuo, Y.-L., Chou, C.-H, Hu, T-W. In "Allelochemicals and Pheromones". Waller, G. R.; Chou, C.-H., Eds. Institute of Botany, Academia Sinica: Taipei, R. O. C., 1982; pp. 107-119. 8. Smith, I. K.; Fowden, C. J. Exp. Bot. 1966, 17, 750-61. 9. Mathur, S. N.; Mukerji, D.; Rizvi, S. J. H.; Jaiswal, V. In "Current Trends in Life Sciences"; Chauhan et al., Eds.; Today's and Tomorrow's Publications: New Delhi, 1982; pp. 287-300. 10. Fawcett, C. H.; Spencer, D. M . Annu. Rev. Phvtopathol. 1970, 3, 403418. 11. Beye, F. Plant Res. Dev. 1978, 7, 13-31. 12. Putnam, A. R.; Duke, W. B. Science 1974, 185, 370-372.
7. RIZVI AND RIZVI Improving Crop Productivity in India 75
13. 14. 15. 16. 17.
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18. 19. 20. 21. 22. 23.
Robson, T. O. Aquatic Bot. 1977, 3, 125. Szezepanski, A. Hidrobiologia 1977, 12, 193-7. Lockerman, R. H.; Putnam, A. R., Weed Sci. 1979, 27, 54-7. Putnam, A. R., DeFrank, J. Proc. 9th Int. Cong. Plant Protection 1979, 580-2. Rizvi, S. H. J.; Jaiswal, V.; Mukerji, D.; Mathur, S. N . Naturwissenschaften 1980, 67, 459-460. Rizvi, S. H. J., Jaiswal, V., Mukerji, D., Mathur, S. N., Indian J. Mycol. Plant Pathol. 1980, 10, 72. Rizvi, S. J. H.; Mukerji, D.; Mathur, S. N. Agric. Biol. Chem. 1981, 54, 1255-1256. Rizvi, S. J. H.; Rizvi, V. Proc. 10th Internat. Cong. Plant Protection 1983, 1, 234. Rizvi, S. J. H.; Rizvi, V. Proc. 1st Trop. Weed Sci. Conf. 1984, 2, 393400. Samersov, V. F.; Prishchepa, I. A. Khim. Sel'sk. Khoz. 1978, 16, 75861; Chem. Abstr. 1978, 84, 141771. Ramakrishna, N.; Ramachandran, Β. V. Indian J. Biochem. Biophys. 1978, 15, 77.
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June 9, 1986
