Role of Allelopathy in Weed control-A review

Weeds are most serious problem in Agriculture. (Tull, 1731) used the term “weed” for unwanted plants in his book “Horse-hoeing Husbandry”. A plant that originated under a natural environment and in response to (human) imposed and natural environments evolved and continues to do so as an interfering associate with our desired plants(crops)and activities [1];Yaduraju, 2006) reported that weeds roughly account for 37%, insects for 29%, diseases for 22%, and other pests for 12% of the total annual loss of agricultural produce in India[2] from analysis of 10 years of data (1978-1988) reported annual losses to the tune of 9.3 million tonnes in cereals, 0.8 million tonne in pulses, 0.6 million tonnes in oilseeds and 7.2 million tonnes in fibers valued at Rs. 2799.00 crores occurring annually in India. Estimates also say that weeds generally reduce yield by 15-30% in wheat, 30-35% in Maize, sorghum,pulses and oilseeds. The term “allelopathy” is derived from two Greek words,“allelon or allele” means “ mutual or each other” and “ pathos or patho” means “ suffering or to suffer”. [3]coined the term allelopathy, which includes all stimulatory and inhibitory reciprocal biochemical interactions among plants including microorganisms. [4] reported that allelochemicals are secondary plant metabolites or byproducts of the primary metabolic process of plants. Importantsecondary metabolites identified as phenolics, alkaloids, flavonoids, terpenoids, momilactone, carbohydrates, Hydroxamic acids, brassinosteroid, jasmonates, salicylates, glucosinolates and amino acids [5] Allelochemicals affect numerous physiological and biochemical processes [6] and therefore play an important role in the basic metabolism of recipient plants.(Evenar,1949) stated that the presence of germination inhibitors are widespread phenomenon. Allelopathy may be allo-allelopathy and auto-allelopathy, true and functional allelopathy [1]and concurrent/direct and residual allelopathy. Action of these compounds is concentration-dependent [7]as they inhibit plant growth at high concentrations and promote that at low concentrations[8] These allelochemicals may thus be used as natural pesticides at high concentrations [9] Inhibitory role of allelochemicals is well explored and has been directly and indirectly used for weed management. A lot of research work has been done to explore the inhibitory potential of different allelopathic crops and trees for weed management [10]. It is pragmatic substitute of synthetic herbicides as allelochemicals do not have residual or toxic effects [11]. This inhibitory feature is attributed to the blockage or cessation of important physiological and metabolic processes of plant. On the other hand, allelochemicals promote growth and impart resistance against several abiotic stresses  at low concentrations [12] Only a few studies have been carried out to investigate the growth promotion by allelochemicals. Application of water extracts of allelopathic compounds at lower concentrations stimulates germination and growth of different crops [13]. Application of allelochemicals at low concentrations to crops can be a cost-effective and efficient way to promote growth and to enhance crop productivity[14]. The challenge associated with conventional weed control methods (e.g., hand weeding, mechanical control, herbicides,etc.) make it imperative to develop diversity in the current weed control methods. A variety of options would be available for site specific weed control if  diverse weed management methods are developed.The cost and ecological concerns can be firmly addressed by using diversified weed management options. Suppressing weeds by harnessing the allelopathic phenomenon is included among the important innovative weed control methods [15]).This review article discusses the practical application of allelopathy for weed control in agricultural systems.

2] Types of allelopathy:

2.1] Allo-allelopathy:It is an inter-specific chemical co-action. Allelochemicals are toxic to other species, other than which release it. Eg-Maize shows allelopathic effect on Chenopodium and Amaranthus.

2.2] Auto-allelopathy: It is an intra-specific chemical co-action. Allelochemicals are toxic to the same sp. From which they are released. Eg- Wheat, alfalfa, cowpea, rice, apple etc.

2.3] True allelopathy: It refers to the release into the environment of chemical compounds that are toxic the form they are produced by the plant.

2.4] Functional allelopathy: It refers to the release into the environment of chemical compounds that  are toxic after chemical modification by microorganisms.

2.5] Concurrent/direct allelopathy: It refers to the instantaneous direct effect of released toxin from the living plant to another growing in the vicinity. It is also called “living plant effect”. Ex- Sorghum supress many weeds in growing in the vicinity.

2.6] Residual allelopathy: It is the effect obtained on the plants growing in succession from the decaying residues, leaf litters, stem, and roots of the previous plants.  E.g. sorghum is allelopathic to wheat and  Phalaris minor and sweet potato (Ipomoea batatas) to cowpea.

3] Forms of allelopathic interaction:

                            Table1-Allelopathic effects of crops on weeds

S.no	Crops	Weed species	Source of inhibitors
1	Maize	Chenopodium album, Amaranthus retrofexus	Roots
2	Sorghum	Abutilon theophrasti, Amaranthus hybridus, Setaria viridis	Roots, Shoots, Foliage
3	Cassava	Amaranthus dubius, Amaranthus samguinalis	Leaves
4	Sweet potato	Cyperus rotandus, Cyperus esculentus	Shoots and Foliage

Table2-Allelopathic effects of weeds on crops

SL	Weeds	Crops	Source of inhibitors
1	Euphorbia sp	Flax	Root exudates
2	Commelina alyssum	Flax	Leaves
3	Cyperus rotandus	Sorghum, Soyabean	Tubers
4	Imperata cylendical	Several crops	Whole plant
5	Chenopodium album	Alfaalfa, cucumber ,oat,maize	Shoot , Foliage
6	Rumex sp	Maize, Sorghum	Shoot, Foliage
7	Parthenium histerophorus	Several crops	Shoot, Foliage
8	Abutilon theophrasti	Alfaalfa	Shoot, Foliage
9	Bidens pilosa	Several crops	Shoot, Foliage
10	Crisicumarvense	Several crops	Shoot, Foliage

Fig-1-Allelochemicals from Aerial part of Avena fatua L. And there allelopathic affect on wheat plant

Table.3-Weed on another weed

SL	Weed	weed	Source of inhibitors
1	Imperetacylendrica	Borreria hispada(Button weed)	Rhizomes
2	Sorghum helepense	Seteria faberi(Giant foxtail)	Rhizomes and leaves

Table.4- Allelopathicpotential of crop plants

Crop	Scientific name	Allelochemicals	References
Rice	Oryza sativa L.	Phenolic acids	[16]
Wheat	Triticum aestivum L.	Hydroxamic acid	[17]
Black mustard	Brassica nigra L.		[18]
Sorghum	Sorghum bicolor L.	Sorgelone	[19]
Oat	Avena sativa L.	Phenolic acids &Scopoletin	[18]
Sweet clover	Melilotus spp.	Phenolics	[19]

4] Mechanism of action of allelochemicals:[20] stated that the presence of germination inhibitors is widespread phenomenon. There are 15 chemical classes affecting 11 process or phenomenon in plants.

Table5- Mechanisms of action of allelochemicals

SN	Mechanism of action	Allelochemicals invoved
1	Cell division and cell elongation inhibition and production of ulterastructure of cells	Cinnamic acid and coumarin
2	Inhibition of gibberelic acid or indole acetic acid induced growth	Scopoletin, polyphenols
3	Inhibition pf porphyrin synthesis and photosynthesis	Polyphenols, scopoletin, quinones
4	Inhibition or stimulation of stomatal function	Polyphenols, quinones and scopoletin
5	Inhibition or stimulation of respiration.	Aldehydes, simple phenols and benzoic acids
6	Inhibition of protein synthesis and organic acid metabolism.	Cinamic acid and ferulic acid
7	Changes in membrane permeability to ions, water, certain biomolecules etc.	Salicylic acid, benzoic acid, and cinnamic acid
8	Inhibition of mineral nutrient uptake	All phenolic acid
9	Effect on corking and clogging of xylem element and internal water relations	Phenol, resorcinol, pyrogallol, salicylic and cinnamic acid
10	Inhibition or stimulation of specific enzymes	Polyphenol, simple phenols, benzoic acid
11	Inhibition of haemoglobin synthesis	Polyphenol, simple phenols, benzoic acid

Fig2-Mechanism of Allelopathy

5] Need of allelopathy in weed control:In agro-ecosystem, weed interfere or compete with the crop for nutrient, light, water, space etc. and finally they check or slows down the growth of the crop which thus effect the crop yield. So to solve that problem we need to solve this problem. .There are some physical, mechanical, and chemical methods there to control the weed population. These techniques attempt to achieve a balance between cost of control and crop yield loss. Among these control methods, some mechanical methods like hand weeding, digging, sickling are very time taken and labour-intensive methods. Now days, a chemical method for controlling weed takes a significant and effective role. Since their discovery in the 1950s, synthetic herbicides have developed as a major tool for weed management[20]. Chemical herbicides in one hand help to reduce the weed population effectively on the other hand they also contribute to increase the yield significantly. Indeed, without herbicides, labour would be a major cost of crop production in developed countries. However continuous application of those herbicides decreases the soil fertility, cause weed shift, and alsocreate herbicide resistant and also cause environmental and ecological problem. The negative impact of commercial herbicides makes it desirable to search for other alternative weed management options[21] and allelopathy seems to be one of the options[22] . Momilactone B inhibits the growth of typical rice weeds like Echinochloa crusgalli and E. colonum at concentrations greater than 1 µmol/L [23].

6] Crops with allelopathic potential in weed control:

6.1] Sorghum:[24] separated sorgoleone substances from root exudates of sorghum.Sorgoleone and also another three important benzoquinones constitute 90% of the root exudate of sorghum [25].[26] said that nine water soluble allelochemicals secreting from sorghum effectively retard the growth of Phalaris minor, Convolvulus arvensis, Rumexdentatus.He also observed that incorporation of sorghum root into the soil reduced the weed biomass up to 25-52% and also increased the yield upto 7-8%.[27] also observed that application of sorghum water extract reduced the weed biomass upto 20-40% and increased the wheat yield upto 14%. [28] checked the allelopathic effect on wheat in Punjab. These authors observed thar incorporation of sorghum shoots at 2, 4, 6 Mg/ha reduced the dry weight upto 42,48,56%, respectively. They also found the effect of concentration and frequency of sorghum water extract application. They also found that two or three Sorgaab (Sorghum water extract) at 1:10 gave the same result as three sprays at 1:20 ratio.

Table6-Application of allelopathy for suppression of weed in different crops

Weed Name	Allelopathic source	Weed control	Reference
Convolvulus arvensis	Allelopathic water extract sunflower (Helianthus annus L.)	Reduction in total weed density and dry weight ( 17.19% and 35.92% respectively)	[28]
Convolvulus arvensis	Allelopathic water extract sorghum (Sorghum bicolor L.)	Reduction in total weed density and dry weight (31.58% and 44.11-59.62% respectively)	[29][30]
Convovulus arvensis	Allelopathic crop rotation (Surface mulch)	Reduction in total weed dry weight (81%)	[31]
Convolvulus arvensis	Combine application of allelopathic water extract ( 10 L/hec)	Reduction in total dry weight (79.32%)	[32]
Setaria faberi	Maize-Spyabean and Soyabean-wheat-maize	Complete inhibit the weed growth	[33]
Capsellabursapastoris	Allelochemicals application of gramine/ hordenine	Inhibit the weed density and biomass	[22]
Major weeds ofO.sativa	Allelopathic intercropping with camelina with pea	Reduce the weed by 52-63%	[34]

Fig3- Overall process of allelopathy and factors affecting allelopathy. This figure the process of allelochemicals production in plant, i.e., root exudates, transpiration, stem flow, leaches from areal parts. In addition, it also reports that allelochemicals compounds mostly.

6.2] Rice (Oryza sativa L.):The allelopathic effect of rice has received a great attention. Several varietyof rice have the allelopathic potential that has been well observed both in the field and the laboratory experiment [35]. In a field experiment performed with 5000 rice cultivar to check allelopathic activity on HeterantheralimosaL. researchers found approximately 19 varieties with evident allelopathic activity [36].Another study examined that 45 of 1000 rice varietyshowed allelopathy against Monochorea vaginalis L. [37]. [38] observed that 749 rice varietiesand also japonica rice cultivars show allelopathy on Barnyard grass. Many allelopathic compounds like fatty acids, phenolic acids, terpenoids have been found in rice extract [4]. [39] observed incorporation of rice residues in soil reduced the growth of Echinochloa colona L. , Ammaniabaccifera L. and Phylllanyhusfraternus L . Rice residues incorporation in soil decreased the growth of Cyperus iria L. similar to the application of propanil and bentazon herbicides [37]. The incorporation of rice straw, rice flour, and hull can be directly used as a component of an integrated weed management program. Further research on allelochemical extraction from allelopathic rice cultivars and evaluating their mode of action will open up the chances of utilizing rice allelopathy for weed management.

6.3] Wheat (Triticum aestivum L.):[40] found that the allelopathic affect extract of roots, leaves, and stems of durum wheat varieties (Karim and Omrabii) on barley (Var Manel) and bread wheat(Var Ariana). [41] isolated p-coumaric acid from residues of wheat and other cereals. [42]found the production of hydroxamic acids in wheat (Triticum aestivum L.) which increase the plant until it was 40 days old, which proves that effect of allelopathy also varies with age of the plant.Allelopathic effect of wheat straw to corn (Zea mays L.) and cotton (Gossypium hirsutum L.) [43] was also reported by [44].[45] reported that the effect of allelopathy also depends on the extract, species and temperature. They also found that most affected species were ivy leaf, morning-glory (Ipomea hederacea L.) and barnyard grass (Echinochloa crusgalli L.)

6.4] MAIZE (Zea mays L.):[46] reported that aquous extract from Maize pollen grain retard the growth of Bidens pilosa, Rumex crispus, Cassia jalapensis. This indicates a great allelopathic potential of maize pollen grain to control weed as it is produced in abundance [2] Ethanol extract of pollen contains two chromatographic function 3A and 3B which inhibited the radicle growth of A. leucocarpusto 85 and 40 % respectively compared to control.The phytotoxicity of decomposed maize residues persisted for 22 weeks in soil [47] . 18 compounds ex- salicyladehyde, resorcinol, phenylacetic, 4-pheylbutyric, benzoic, p-hydroxybenzoic, vanillic caffeic acid were identified [48].These compounds were found phytotoxic to lettuce in bioassays but no information is available for weed species. Some of this compounds could be inhibitory to weed spices as well.

6.5] Eucalyptus: Eucalyptus leaf chelates and oil showed differential allelopathic effect on the growth of weeds. The leaf chelates of 20 percent concentration supressed the biomass production of Cynodone dactylon by about 50 percent, where as 1.0 percent oil cause 68 percent reduction. Application of 1.0 percent oil significantly shoot and root length, leaf chlorophyll and total production of cyperusrotandus[3]. Eucalyptus sp because of high allelopathiccontent, is expected to control weeds [49] .Aqueous extract of its bark, leaves and oil inhibited Parthenium hysterophorus.

7] Effect of allelopathy in crop growth:

Growth enhancing effect of allelopathy for the major crops are described in the literature [50][51]; however, there is a lack of information about allelopathy in promoting growth of O. sativa. Several studies were discussed, and it was found that the application of low concentrations of allelochemicals stimulates the germination process [52] and growth of major field crops. The application of aqueous crop extracts either applied exogenously (foliar spray), and seed treatment promotes the germination and growth crops [25]. Exogenous application of allelochemicals as a foliar spray could be approved as an effective method.

Table7-Comparison of different studies on allelopathic effect of agriculture land associated plants

CROP	PARTS	EFFECT	Reference
Sorghum (Sorghum halepense)	Sorghum water extract	Improvement of seedling growth, Chlorophyll content, carotenoid protein content of Maize	[53]
Chestnut ( Castanea henryi)	Extract	Seed germination, Increased in seedling development, Chlorophyll content, Antioxidant enzyme activity in Maize	[54]
Eucalyptus(Eucalyptuscamaldulensis)	Leaf, Bark, Pod	Inhibitory effect on seed germination, growth and yield in wheat.	[55]
Sweet clover (Melilotus indica)	Leaf, Stem, Root water extract	Inhibitory effect on seed growth and germination. Root extract exhibited the lowest inhibitory effect on seed germination and seeding growth of wheat. Germination rate and mean germination time decreased significantly by increasing in concentration of allelopathic extracts	[56]
Sunflower (Helianthus annus)	Leaf, Bark, Pod	Inhibit significant germination. Increased mean germination time. Reduced plumule and radicle growth in wheat.	[55]
Maize (Zea mays)		Defense against disesase, pest and stress	[56]

8.Limitation-Allelochemicals are the chemical substances that are made from different secondary metabolites following different pathways.Therefore, scientist must determine carefully whether plants have allelopathic potential or not and identify those chemicals by using organic solvents and water extract. The type of allelochemicals released into environment based on combined effect of plant itself and several environmental factors [57].Plants are from same family or same group do not necessarily show similar allelopathic affects [58]. The natural products represent a diverse class of chemical compounds. These natural secretedallelochemicals will have an impact on different species of plants. There are some important  limitations to the successful use of allelochemicals in weed management, some of these factors include: 1) the power and concentration of the compounds is very low 2) the half- life of the allelochemicals is usually very short 3) the selectivity towards weed is narrow5) cost of production is very high 6) Lack of knowledge.

Conclusion and Future prospects:Allelopathy is a novel, organic approach which offers multiple solution to control weed population. It is clear that there are some immense prospect of allelopathic mechanism as weed management purpose. Allelochemicals from several plants have been isolated and identified and there activity also been identified. There are some important points that makes allelochemicals asnatural herbicide tosuppresseweed -1) Firstly, along with laboratory experiments are needed to study its interaction with various physical, chemical, biological and physiochemical properties of soil.2) Secondly the movement of allelochemical, mode of actions, mode of mechanism, and there selectivity should be broadly studied.3) Finally the impact of use of allelochemicals from agronomic and environmental point of view needs special attention.

Acknowledgement:All the authors acknowledge and thank their respective Institutes and Universities.

Author’s contribution:All the authors contributed significantly.

Funding:This is a compilation written by its authors and required no substantial funding to be stated.

Disclosure statement:All authors declare that there exist no commercial or financial relationships that could, in any way, lead to a potential conflict of interest

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