Allelopathic Effects of Aqueous Extracts of Ricinus communis L. on the Germination of Six Cultivated Species

Aims: The effect of aqueous extracts of castor bean (Ricinus communis L.) On the germination of six cultivated crops was investigated. Study Design: Samples of R. communis were harvested from the region of Gabès (south of Tunisia) and tested at laboratory on crops seeds. Place and Duration of Study: Regional station of GabèsNational Institute of Research in Rural Engineering, Waters and Forests – Tunisia, between October and December 2014. Methodology: Extracts were obtained after an incubation of 9 g of dry plant material (powder) in 100 ml distilled water during 48 hours at 60°C. The extraction was made from five parts of R. communis (root, twig, leaf, seed and pericarp) and used in germination tests. Results and Conclusion: Results showed two types of behavior: the aqueous extracts of root, Short Research Article Saadaoui et al.; IJPSS, 7(4): 220-227, 2015; Article no.IJPSS.2015.148 221 twig, and seed had a low effect of inhibition for all studied species. The aqueous extracts of leaf and pericarp had a high effect of inhibition. C. olitorius is the most tolerant species; its germination rate is 99, 99, 97, 96, 69.5 and 53.5% for aqueous extracts of root, twig, seed, leaf and pericarp respectively. L. culinaris is the most sensitive species; its rate of germination is 97, 68.5, 44, 3 and 0% for aqueous extracts of root, twig, seed, leaf and pericarp respectively. Leaf and pericarp aqueous extracts are the most inhibitive of germination resulting in germination rates lower than 16.5% for all studied species, except for C. olitorius, its germination rate is 69.5 and 53.5% with pericarp and leaf extract respectively. The aqueous extracts of R. communis inhibited root length in all studied species, essentially with the aqueous extracts of leaf and pericarp. These two parts of R. communis showed high allelopathy effect in percentage of germination and root length for all studied species.


INTRODUCTION
Some plants may inhibit germination, emergence and subsequent growth of other plants by exuding toxic substances. These substances are called allelopathic chemicals or allelochemicals and the process is called allelopathy. This phenomenon can be understood as the ability of different plant parts to inhibit or stimulate growth of other plants in the environment by exuding chemicals [1]. Natural products identified as allelochemicals represent a myriad of compounds and their chemical nature and groups have been already reported in many instances. However, phenolic compounds are the major group repeatedly reported and successfully detected and isolated from plants or soil [2,3]. Free phenolic compounds may accumulate in rhizosphere soils, especially in soils flooded with vegetable waste waters, thereby influencing the accumulation and availability of soil nutrients and rates of nutrient cycling, which both ultimately affect plant growth [2]. The analysis of effect of the water extracts of a species on the germination of other species is a method to estimate allelopathy in the laboratory.
Ricinus communis L. (Euphorbiaceae) is originally native to northeastern Africa and the Middle East. It has escaped cultivation and become naturalized as a weed almost everywhere in the world under tropical or subtropical climate [4]. R. communis is one of the oldest drugs known to man. The first mention of it as a laxative can be found in 3500 year-old Ancient Egyptian papyrus scrolls [5]. Ranging from cool temperate moist to wet through tropical desert to wet forest life zones, castor bean is reported to tolerate annual precipitation of 2.0 to 42.9 dm annual temperature of 7.0 to 27.8°C and pH of 4.5 to 8.3. It grows best where temperatures are rather high throughout the season, but seed may fail to set above 38°C for an extended period. Plant requires 140-180 day growing season and is readily killed by frost [4]. Castor bean is a hardy crop, easy to establish on the field, resistant to drought, tolerate different types of soil even marginal soil [6]. R. communis was naturalized in Tunisia; it exists in spontaneous state, its distribution is from the North to the South, it occupies marginal soils, urban areas and agricultural land. R. communis (2n = 20) is characterized by low morphological diversity; the diversity analyze of 12 Iranian accessions showed low genetic diversity of this species [7]. The low observed heterozygosity values suggest the predominance of autogamy in this species, which is known to have a mixed mating system, being both self-and crosspollinated by wind [8].
R. communis is an oilseed crop; it is primarily of economic interest as a source of castor oil, used for the production of high-quality lubricants because of its high proportion of the unusual fatty acid ricinoleic acid [9]. The seeds of castor bean contain more than 45% oil with high content in biodiesel (86 to 91%) [10,11,12]. Yields comprised between 2242 and 3363 kg seeds per hectare and 350 -900 kg oil per hectare were obtained [6,12]. R. communis contain chemical compounds like Ricin A, B and C which have antitumor action, as well as alkaloids (ricinine) and glycosides which may be useful for various anti-inflammatory, analgesic, antipyretic, cardiotonic and antiasthmatic formulations [13]. The seeds contain 2.8-3% toxic substances, in amount sufficient to produce lethal effects with 2.5-20 seeds. The principal toxin is the albumin, ricin; whose presence poses a problem for castor bean as a widely cultivated oilseed crop. However, the plant produces antigenic or immunizing activity, producing in small doses an antitoxin analogous to that produced against bacteria [4,9]. The plant possess beneficial effects such as antioxidant, antihistaminic, anti-asthmatic, antiulcer, immunomodulatory, anti-inflammatory, insecticidal, larvicidal and antiviral properties [14,15,16,17,18]. Essential oils of R. communis contain βthujone (31.71%), 1,8-cinole (30.98%), α-pinene (16.88%), camphor (12.98%) and camphene (7.48%) and high antioxidant activity of the essential oil was observed [19]. Also, antimicrobial and anticarcinogenic properties of the essential oil have been showed [20]. Castor bean leaves are used in preparing long acting biocide compounds which are safe and stable and can completely control pests such as mosquitoes, flies, cockroaches, ants, fleas and lice in 24 hours. The leaves have insecticidal properties [4]. R. communis had high allelopathic effect on the germination and the growth of other species [3,21,22,23,24]. At concentration of 300mg DW equivalent extract/mL, R. communis water extract completely inhibited the germination of cress and barnyard grass, and the seedling growth of cress, lettuce, Italian ryegrass, and barnyard grass [25].
In this study we investigated the allelopathic effect of root, twig, leaf, seed and pericarp water extract of R. communis on the germination and root growth of six cultivated species: Corchorus olitorius, H. vulgare, C. arietinum, M. sativa, T. foenum-graecum and L. culinaris

MATERIALS AND METHODS
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Sample Collection and Extraction
Leaves, twigs, roots, seeds and pericarps of castor bean (Ricinus communis) were collected from spontaneous plants in Gabes (South of Tunisia; 33°53' N and 10°07' E). The aqueous extracts were prepared from dried plant material ground in powder form. Nine grams of powered material were added to 100 ml distilled water and incubated at 60° for 48 h. The extracts were filtered with filter paper. Aqueous solutions of 9% were obtained for each part of the plant. The methodology of extraction is similar to other authors [26,27,28].

Germination Test and Root Length
The seeds were thoroughly washed with distilled water and surface sterilized with sodium hypochlorite (12%) for 2-3 minutes. Fifty seeds were placed in each sterilized Petri dish of 90 mm diameter and 15 mm height, and irrigated with 2 ml of water extracts (leaves, twigs, roots, seeds and pericarps) at 28°C. The control was treated with 2 ml distilled water. Each treatment had four replicates. Analyzed seeds belong to six cultivated species: barley (Hordeum vulgare), alfalfa (Medicago sativa), jute (Corchorus olitorius), fenugreek (Trigonella foenumgraecum), lentil (Lens culinaris) and chickpea (Cicer arietinum). Every day, seed germination was investigated. Three days after germination, root length was measured. Data are given as rate of germination (%) and root length (mm).

Statistical Analysis
Analysis of variance (ANOVA) and the differences between the parameters were evaluated by Student-Newman-Keuls test. Two way ANOVA (UNIANOVA) was conducted to see whether there is an interaction between species and origin of extracts (plant organ) in the parameters tested. Differences were considered when P = .05. Software used is XLSAT(2012) and SPSS 21.

Rate of Germination
The effect of aqueous extract of R. communis varies according plant parts. In general, high inhibitory effect on germination of all studied species was observed with leaf and pericarp water extract and low inhibitory effect was reported with root, seed and twig aqueous extract (Figs. 1-6). Nevertheless, the response was different depending on the species (P < .05 in the UNIANOVA test). The lowest germination percentage was registered for five species treated with leaf water extract: H. vulgare, C. arietinum, M. sativa, T. foenum-graecum and L. culinaris; its germination rate varied between 3 and 16.25% (Figs. [1][2][3][4][5]. Highest tolerance to allelopathic effect of leaf water extract was observed only with C. olitorius, its germination percentage is 53.5% (Fig. 6). A similar result was obtained with pericarp aqueous extract: low tolerance was observed for five species, with germination percentages ranging between 0% for L. culinaris and 15.5% for H. vulgare (Figs. [1][2][3][4][5]. High tolerance was observed for C. olitorius treated with pericarp water extract; its germination percentage is 69.5% (Fig. 6). Twig water extract had a significant inhibitory effect on the germination of three species: T. foenumgraecum, H. vulgare, M. sativa, and L. culinaris, with germination percentages of 81, 64, 62.5 and 44% respectively (Figs. 1, 2, 3 and 5). Finally, seed and root water extract had a significant inhibition only for H. vulgare and L. culinaris (Figs. 2 and 3). The two last species had low tolerance to allelopathic effect of all R. communis organs.
This result was observed with others species; inhibitory effect of leaf extract of castor bean on seed germination of lentil was registered [23]. Seyyedi et al. [22] showed high inhibitory effect of aqueous extracts of root, stem, leaf and whole plant of R. communis on germination of dodder (Cuscuta compestris). Also, aqueous extract of R. communis (5%) has higher allelopathic inhibition on the germination of Convolvulus arvensis and Amaranthus retroflexus than aqueous extract of Crocus sativus and Sorghum vulgar [21,24]. C. olitorius is the most tolerant species for aqueous extracts of all R. communis parts; this tolerance was observed with aqueous extracts of other species: Eucalyptus occidentalis, Acacia ampliceps and Prosopis juliflora [28].

Root Lenght
Significant difference was registered between treatments as well as for the interaction between treatment and species. The reduction percentage was between 48% for C. olitorius and 95.9% for T. foenum-graecum (Table 1). For all treated species, the lower inhibition was observed with aqueous extract of root (Table 1). Leaf and pericarp water extracts had the highest inhibitory effect on the growth of roots of the six studied species (the percentage of reduction was over 89.5%).
Water extracts had inhibitory effect on the root growth of the six studied species; this effect was registered by other authors. Leaf extract of castor bean inhibited growth of lentil roots [23]. Also, water extracts of Eucalyptus occidentalis, Acacia ampliceps and Prosopis juliflora inhibit root growth of H. vulgare, M. sativa and C. olitorius [28]. For C. arietinum, inhibitory effect of aqueous leaf extracts of Ficus infectoria, Emblica officinalis and Acacia leucophloea was much pronounced in root development than seed germination [29]. The presence of phenolic compounds could be the cause of reduction in germination and growth of seedlings [26]. Indeed, R. communis is rich in phenol [16,30]; the monoterpenoids (1,8-cinole, camphor and α-pinene) and sesquiterpenoids (βcaryophyllene), gallic acid, quercetin, gentisic acid, rutin, epicatechin and ellagic acid are the major phenolic compounds isolated from leaves. Indole-3-acetic acid has been extracted from the  [17,31]. Phenolic allelochemicals can lead to increased cell membrane permeability inhibiting plants from absorbing nutrients from the surroundings and affecting the normal growth of plants [2]. Also, the phenolic allelochemicals could inhibit cell division and alter the ultrastructure of the cells [32].

CONCLUSION
Our results show a very high allelopathic effect of the aqueous extracts from Ricinus communis, an spontaneous and frequent species in agricultural land in Tunisia, on germination and root growth of six crops (H. vulgare, M. sativa, C. olitorius, T. foenum-graecum, L. culinaris and C. arietinum). This inhibitory effect is increasingly high for two organs of the plant: the sheet and the pericarp, and depends on the species considered.