INTRODUCTION

Allelopathy as a mechanism of plant interference in agro-ecosystems (Kimber, 1973) offers an opportunity to manage weeds in a crop sequence (Aldrich and Kremer, 1997) that could lead to reduced labor costs and increased efficiency (Chung et al., 2003). Previous studies have shown that sorghum (Sorghum bicolor L.) vegetation possess a variety of potent inhibitors such as dhurrin, acyanogenic glycoside (Chung et al., 2003) and phenolics (Inderjit, 2006) which are potentially allelopathic to weeds (Alsaadawi et al., 1986; John and Nelson, 1998) with a maximum of inhibitory activity at harvest (Bogatek et al., 2004). This was not the case for all grasses, some exhibited higher toxicity to wheat seedling growth when their residues were still green (Hedge and Miller, 1990). Straw extracts of Rice have the highest inhibition rate on Barnyardgrass (Chung et al., 2003). The inhibition of barnyardgrass germination and seedling growth by rice hull extracts may reflect the allelopathic potential of individual rice culedxtivars.

Bioassays of germination, radicle growth and coleoptile growth are used to test the allelopathic potential of a crop species (Moncef et al., 2001). The allelopathic potential can be observed in the form of autotoxicity as in the case of alfalfa (Medicago sativa L.) (Haskins et al., 1984) or heterotoxicity as in the case of tall fescue (Festuca arundinacea L.) (Joung and Chung, 2000).

Since the allelopathy of small grain cereals has been little studied, the present work aimed to test the heterotoxicity of barley on wild-oat, study changes in allelopathic potential over four growth stages on wild-oat identify the most allelopathic plant part in each stages.

MATERIALS AND METHODS

Five barley (Hordeum vulgare) varieties namely Jonob, Kavir, Karoon, Tropy and Eizeh were grown at Research Institute of Forests and Rangelands, Ahwaz, Iran, in 2005 to 2007. From soil preparation to crop harvest, standard cultural practices of the semiarid zone were applied. Plants under experiment were irrigated whenever severe wilting of plants were observed. Destructive sampling of barley plants were made at its four growth stages (Kimber, 1973):

At stage 4, whole plant of barley was used to prepare water extracts. Whereas at stage 8 and stage 10 roots, stem and leaves of barley plants were used to prepare water extracts and at stage 11 roots, stem, leaves and penicles were used to prepare water extracts. Water extracts of barley plants were prepared by following the methods described by Moncef et al. (2001). All of the water extracts prepared at different growth stages of barley were used to determine the allelopathic effect on seed germination, root length (cm) and shoot length (cm) of wild oat (Avena ludoviciana).

To determine the allelopathic effect of barley extracts wild-oat seeds were collected in October 2005, cleaned and stored at -35°C. Before the start of experiments for the determination of allelopathic effect, the wild oat seeds were surface sterilized in a 1:10 (v/v) dilution of commercial hypochlorite bleach for 10 min and rinsed several times with distilled water. These sterilized seeds were placed on a paper towel for about 2 h. Then wild-oat seeds were placed on a filter paper in sterilized 9 cm diameter petri dishes. The experiment was designed under Completely Randomied Design (CRD) with four replications. Ten milliliter of the barley plant extract was added to each Petri dish and distilled water was used as a control. All the Petri dishes were placed in a lighted growth chamber at 24°C. Germination percentage, seedling growth, root and shoot length were regularly recorded. The data was analyzed by using Analysis of Variance method and means were compared by using Duncan`s Multiple Range Test (Steel and Torrie, 1980).

RESULTS

Extracts of whole plant (stage 4) of all barley varieties significantly affected the germination percentage, root and shoot length of wild oat plants (Table 1). As compared to control, maximum seed germination percentage was recorded in extracts of barley variety Karoon, very closely followed by Jonob variety, indicating very less allelopathic effect on seed germination of wild oat (Table 1). Minimum seed germination percentage recorded was noted in extracts of barley variety Eizeh showing a considerable allelopathic effect on seed germination. Maximum root length of wild oat plants was retarded by the extracts of barley variety Jonob. In case of shoot length of wild oat there were non significant differences were recorded among the extracts of barley varieties, however maximum allelopathic effect was recorded in the extracts of barley variety Eizeh.

Table 1:	Germination percentage, root length and shoot length wild-oat with water extract of barley varieties at stage 4
*Significantly different from control (p<0.05) assessed by Duncan`s multiple range test. Values are the means of four replications. Variants possessing the same letter(s) are not statistically significant at p<0.05 level, according to Duncan`s multiple range test

Table 2:	Germination percentage, root length and shoot length of wild-oat with water extract of different plant parts of barley varieties at stage 8
*Significantly different from control (p<0.05) assessed by Duncan`s multiple range test, Values are the means of four replications. Variants possessing the same letter(s) are not statistically significant at p<0.05 level, according to Duncan`s multiple range test

Table 3:	Germination percentage, root length and shoot length wild-oat with water extract of different plant parts of barley varieties at stage 10
NSNo significantly different.*Significantly different from control (p<0.05) assessed by Duncan`s multiple range test, Values are the means of four replications. Variants possessing the same letter(s) are not statistically significant at p<0.05 level, according to Duncan`s multiple range test

Table 4:	Germination percentage, root length and shoot length wild-oat with water extract of different plant parts of barley varieties at stage 11
*Significantly different from control (p<0.05) assessed by Duncan`s multiple range test. Values are the means of four replications. Variants possessing the same letter(s) are not statistically significant at p<0.05 level, according to Duncan`s multiple range test

At stage 8, the extracts of barley varieties significantly inhibited the seed germination of wild oat (Table 2). Maximum seed germination was reduced by the leaf extracts of barley variety Eizeh (Table 2). Root length of wild oat was strongly affected by the leaf extracts of Karoon variety, root extract of Jonob and root extract of Kavir variety. The allelopathic effect of leaf extract of Eizeh and Jonob barley varieties on root length of wild oat was also considerable. With regard to shoot length significant allelopathic effect of barley was recorded in leaf extract of Karoon variety, root extract of Kavir variety, stem extract of Karoon variety, leaf extract of Eizeh and root extract of Jonob variety.

Allelopathic effect of different plant parts extracts of barley varieties at stage 10 showed non-significant differences on germination percentage while significant differences with regard to root length and shoot length of the plant (Table 3). Maximum root inhibitory effect was noted in the stem extract of Karoon variety, very closely followed by the stem extract of Tropy, leaf extract of Karoon, root extract of Jonob and leaf extract of Eizeh variety. Shoot length of wild oat was inhibited by stem extract of Karoon variety leaf extract of Tropy Leaf extract of Karoon root extract of Jonob root extract of Kavir and stem extract of Tropy variety of Barley.

Water extracts of different plant parts of barley varieties at stage 11 showed significant inhibitory effects in seed germination, root length and shoot length of the plant (Table 4). The compare of mean values showed that maximum seed germination was inhibited by panicle extract of Tropy variety (Table 4). With regard to root length of plant maximum inhibitory effect was showed by the leaf extract of Eizeh variety, closely followed by root extract of Tropy and leaf extract of Karoon varieties. Maximum shoot length inhibitory effect was showed by leaf extract of Eizeh variety, very closely followed by root extract of Tropy variety.

DISCUSSION

Results of present study suggested that the response by wild-oat (Avena ludoviciana) varied depending on the source of allelochemicals (plant part) and the growth stage of the barley plant and kind of the variety. Germination bioassays of barley at four different phenological stages were sensitive enough to detect the heterotoxicity potential of any plant component of barley. However, seedling growth bioassays were sensitive to allelopathic effects with the radicle being relatively more sensitive than the coleoptile (Table 1). Results of both types of bioassay are in agreement with the findings reported by Hedge and Miller et al. (1990), Kimber (1973), Panasiuk et al. (1986) and Weston et al. (1989).

The allelopathic potential of a barley plant on wild oat varied according to the source of extracts as was found with sorghum and white mustard by Bogatek et al. (2004), Guenzi et al. (1967), Inderjit (2006) and Sebile and Karaman (2007). In addition, the allelopathic potential of barley was unstable over the life cycle of the barley plant. This potential was at maximum near physiological maturity as was for sorghum plant (Kimber, 1973). Seedling growth bioassays demonstrated that the wild-oat responded differently to the allelopathic potential of barley. For wild-oat radicle growth and coleoptile growth were more depressed than germination, though.

During all the growth stages leaves and stems were appeared as the most phytotoxic part of barley plant for seed germination, root and shoot length of oat, these results are in agreement with the findings of Moncef et al. (2001) and Yansen (2007).

These results support the use of seedling bioassays as a tool to screen for tolerance or sensitivity of a crop species to the allelopathic potential of wild oat and other crop species. This study suggests that the allelopathic compounds may serve as a potential natural herbicide by inhibiting seed germination and growth of oat. These varieties could be re-used to contribute to the control of wild oat in the barley fields and they may also be used as genetic markers to identify allelopathic varieties.

Overall the Eizeh barley variety showed allelopathic effects at its growth stages 4, 8, 10 and 11 by retarding the seed germination, root length and shoot length. Karoon and Jonob varieties showed toxicity only to root and shoot length of wild oat at their growth stages 8 and 10. Whereas, variety Kavir appeared as toxic to root length of wild oat at its growth stage 8, root and shoot length at its growth stages 10 and 11. These findings are also in conformity with the results of Moncef et al. (2001).

The results showed that Eizeh barley variety was the only whose growth stage 4 and 8 retarded the seed germination of oat whereas the other varieties retarded the root and shoot length of wild oat. These results were leading to conclude that Eizeh variety of barley is good to grow as it has good check on seed germination of wild oat plants as well as it also retarded the growth of root and shoot length of oat.