Abstract

Media summary

Key words

Introduction

The allelopathic potential of sorghum was reported and well documented by several investigators (Panasiuk et al 1986 and Einhellig and Souza 1989). The toxicity of root exudates and residues of sorghum was reported to affect several weed species (Panasiuk et al 1986 and Alsaadawi et al 1986). Most of the researches have been focused on the allelopathic effects of sorghum residues against weeds where the residues mulched on surface of the field soil or left as a cover in no tillage cropping systems (Einhellig and Rassmussen 1989 and Weston et al 1989).

Recently, several hundreds of sorghum genotypes were introduced and cultivated in Iraq in order to select the most promising genotypes in terms of production, weeds competition and fitness to local environment. Ten genotypes were selected.

Field observations revealed that growth and population of companion weeds were varied among the stands of the selected genotypes. Also, differential growth and population variation were observed on weeds grown in the field after sorghum harvest. This suggests that allelopathy could be the mechanism responsible for the reduction of weeds growth and population and the differences among stands could be due to differences in the allelopathic potential of the test cultivars.

With this in mind the present study was conducted to screen the 10 sorghum genotypes for their allelopathic potential against weeds and determine if there is a relationship between the allelopathic activity of the test cultivars and their phytotoxins content.

Methods

Initial residues bioassay for material selection

Residues of each sorghum genotype were separately mixed with soil at a rate of 3 and 6 g per kg soil and placed in pots. For control, similar rates of peat moss were added to maintain consistency of soil organic matter (Alsaadawi and Rice 1982 ).Twenty-five seeds of Lolium tenulintum weed were planted in each pot. There were 5 pots for each treatment and all pots were placed in a green house in a complete randomized design. Two weeks after germination, the seedlings were thinned to 5 per pot, allowed to grow for 7 weeks, then harvested and oven- dried weight of whole plants was recorded. Based on the results of this experiment three cultivars (Giza 15, Giza 115 and Enkath ) showed stronger allelopathic potential while Rabeh was the least allelopathic cultivar. These cultivars were included in all subsequent experiments.

Field experiment

Plots measuring 1×1 m² were selected randomly from field infested heavily by weeds in March15, 2004. Sorghum straw was chopped in to pieces of 2-3 cm length and added to each plot at a rate of 3 and 6 g per kg soil. The plots were plowed by a spade to a depth of 30 cm and watered whenever necessary with water. Control plots were treated in the same manner except residues were replaced by peat moss to keep organic matter the same. Number of weeds and oven-dried weight of above weed biomass were taken two months after the beginning of the experiment. The experiment was conducted in a randomized complete block design with 5 replications.

Residues bioassay at different periods of decomposition

Straw and roots of sorghum cultivars Giza 15, Giza 115, Enkath and Rabeh were separately chopped in to pieces of approximately 2 cm length, mixed with soil at a rate of 25 and 50 g per kg soil.The pots were placed in a green house and just enough distilled water was added to each pot to keep soil moist throughout the decomposition period. Soil samples were taken from each pot weekly and 100 ml of water extract were prepared using a method described by Alsaadawi et al (1998). For the bioassay twenty-five seeds of Chenopodium album a weed which grows after sorghum harvest were placed in a Petri dish containing 75 g of pure quartz sand and 18 ml of appropriate test solution. The Petri dishes were sealed with Para film strips and placed in an incubator at 28 °C and 12 h light. Ten days after planting, seedlings length was recorded.

Root exudates bioassay

Root exudates of sorghum cultivars Giza 15, Giza 115, Enkath and Rabeh were tested against the companion Echinochloa colonum using stair case technique described by Alsaadawi and Rice (1982). Each treatment consisted of 5 pots. Plants were harvested two months after the beginning of the experiment and oven- dried weights of whole plants were recorded.

Phytotoxins identification and quantification

Water extract of the residues of Giza15, Giza 115, Enkath and Rabeh cultivars were prepared following procedure of Megh et al (1989). Residues of the test cultivars were soaked in distilled water (10 g :100 ml water) for 72 h under laboratory conditions, filtered and extracted with petroleum ether and diethyl ether to remove the lipids and glycosides respectively. The identification of phytotoxins was performed on a HPLC Schimatzo 290-A using procedure outlined by Hartley and Buchan (1979). The peaks appeared were detected by uv detector. Standards of suspected phytotoxins were run similarly for identification and quantification.

Results

Residues bioassay

Residues of all test genotypes significantly inhibited growth of Lolium tenulintum weed when compared to the control (Table1). The phytotoxicity of the residues differed among the test genotypes. Of the 10 genotypes tested 3 cultivars (Giza 15, Giza 115 and Enkath) reduced mean dry weights of the weed by 71-75%. Rabeh was the least allelopathic cultivar with growth reduction of 56%. Apparently the increase of residues in soil didn’t increase growth reduction significantly.

Table 1. Effects of residues of sorghum genotypes incorporated in soil on above biomass of Lolium tenulintum ( g × plant^-1 ).

Rate of residues (g. kg-1 soil )	Sorghum genotypes
	Con-trol	Kafeer	Enkath	Giza 115	Dew-ardoo	Rabeh	Giza 15	Arbel	Arge-nce	Rabeh x F4	F10-R-2002	Mean
3	2.217	0.56	0.806	0.702	0.886	1.123	0.685	0.916	0.835	0.835	1.034	0.887
6	2.617	0.734	0.577	0.698	0.861	0.982	0.525	1.100	0.803	0.803	0.819	0.825
Mean	2.410	0.799	0.691	0.700	0.873	1.053	0.605	1.008	0.819	0.819	0.927

LSD = 0.05 Genotypes = 0.224 Residues rate = 0.141 Genotypes × Residues rate = 0.365

The plots were heavily infested by several weed species namely Lolium rigidum , Lolium tenulintum , Malva pariflora, Carthumus oxycanthus, Silybum marianum, Melilotus indicus, Chenopodium album, Beta vulgaris, Polypogon monspeliensis, Trifolium repense and Plantago ovata. Above biomass and number of all weeds were reduced by the residues of the test sorghum cultivars. However, the response varied among the weed species (data not included).Residues of cultivars Giza 15, Giza 115 and Enkath provided 67, 59 and 63% reduction in average weed numbers and 58, 66 and 58% reduction in average weed biomass respectively (Table 2). Residues of Rabeh cultivars inhibited average weed numbers and average weed biomass by 41 and 52% respectively. The number of weeds was significantly decreased with increasing rate of residues in soil of the stronger allelopathic cultivars only.

Table 2. Effect of residues of sorghum cultivars incorporated in soil on total number and total above ground biomass of weeds.

Parameters	Rate of residues (g.kg-1 soil)	Sorghum genotypes
		Control	Giza 15		Giza 115	Enkath	Rabeh	Mean
Weed numbers / m2	3	76.3	27.0		36.3	34.6	48.0	44.4
	6	72.3	21.3		25.0	20.3	40.0	35.8
	Mean	74.3	24.2		30.7	27.5	44.0
LSD = 0.05 Genotypes: 15.3 Residues rate: 10.2 Genotypes x Residues rate: 11.2
Weed above dry biomass (g/m2)	3	421.1		196.8	178.8	172.2	198.5	233.5
	6	432.4		161.2	115.6	182.7	215.4	221.5
	Mean	426.8		179.0	147.2	177.5	206.9
LSD = 0.05 Genotypes: 100.8 Residues rate: 159.3 Genotypes × Residues rate: 133.3

Residues bioassay at different periods of decompositions

Aqueous extracts of residues of all test cultivars in soil significantly inhibited seedling length of Chenopodium album (data not presented). The phototoxicity started after one week of decomposition and persisted for 8 weeks at the low rate of residues and for 10 weeks at the higher rate of residues. The reduction increased with increasing rate of residues in soil during the first 6 weeks of decomposition. Giza 15, Giza 115 and Enkath showed a greater phytotoxicity than Rabeh cultivar at all decomposition periods.

Root exudates bioassay

Root exudates of all test cultivars significantly reduced growth of the companion weed Echinochloa colonum (Table 3). However, significant differences were observed among the test cultivars. Cultivars Giza 115 and Enkath were found to be more inhibitory than the others.

Phytotoxins identification

Chemical analyses revealed the presence of vanillic, syringic, ferulic, p-hydroxybenzoic, p-coumaric and gallic acids in the residues of Giza 15 and Enkath cultivars (Table 4). All these phytotoxins except gallic acid were found in the residues of Giza 115 while residues of Rabeh cultivar contained all phytotoxins except p-coumaric acid. Residues of Giza 115 and Giza 15 contained up to 5 times as much p-hydroxybenzoic acid as Rabeh cultivar whereas Enkath accumulated up to 3 times as much as Rabeh cultivar. Total isolated phytotoxins were found to be higher in Giza 115 and Giza 15 than in the others.

Table 3. Effect of root exudates of sorghum genotypes on growth of Echinochloa colonum weed.

Sorghum genotypes	Echinochloa colonum *
	Dry weight (mg)	Inhibition %
Control	177.0a	--
Giza 15	142.5b	19.5
Giza 115	127.3c	28.1
Enkath	122.5c	30.8
Rabeh	151.0b	14.7

* Numbers followed by the same letter are not significantly different at 0.05 level according to Duncan’s multiple range test.

Table 4. Phytotoxins isolated from residues of different sorghum cultivars.

Sorghum genotypes	Phytotoxins (microgram/g residues)*, **
	vanillic acid	syringic acid	ferulic acid	p-hydroxybenzoic acid	p-coumaric acid	gallic acid	Total
Control	--	--	--	--	--	--	--
Giza 15	1.42b	1.50a	5.10a	8.00a	1.24a	1.60a	18.84a
Giza 115	1.10b	0.94b	3.20b	8.14a	0.80b	--	14.80b
Enkath	1.00b	0.41c	2.53c	4.40b	0.90b	1.05b	10.29c
Rabeh	4.30a	1.04b	2.33c	1.66c	--	1.15b	10.48c

*Each value is an average of 3 replicates. **Numbers within each column followed by the same letter are not significantly different at 0.05 level according to Duncan’s multiple range test.

Discussion and conclusion

The differential phytotoxicity observed among the test cultivars indicates that allelopathy is an inherited trait. Similar observations were reported on several crops including sorghum (Alsaadawi et al 1986 and Leather 1982). Field experiment clearly revealed that the allelopathic effects of sorghum against weeds is not selective and suggested that allelopathy may be the causative factor responsible for the observed reduction in growth and population of weeds grown in the field after sorghum harvest. Subsequent experiments are further confirming this suggestion. The phototoxicity of the decaying residues in soil started one week after decomposition and persisted for at least 8 weeks which is quite enough to exert its effect on weeds grown after sorghum harvest. Chemical analysis indicated the presence of several phenolic acids and all these phenolic acids are found to be allelopathic agents in several crops (Rice 1984). Our results have shown that weed numbers and growth were more inhibited by sorghum cultivars with higher amount of p-hydroxybenzoic acid in their residues. However, the role of other phytotoxins in sorghum residues cannot be excluded. The significant reduction in growth of companion weed indicates that allelopathy may be the causative factor responsible for the reduction in growth and population of weeds observed in stands of the test sorghum genotypes with competition probably accentuating its effect. Our results lead to the conclusion that different sorghum cultivars have different allelopathic potential and that the exploitation of cultivars with higher allelopathic capacity would be of value for weed control particularly in no-tillage cropping stems. Selection of appropriate crop to rotate with sorghum should be taken into the consideration in order to alleviate the impacts of sorghum residues on the following crops.

References

Alsaadawi IS, Al-Uquaili JK, Al-Rubeaa AJ and Al-Hadithy SM(1986). Allelopathic suppression of weeds and nitrification by selected cultivars of Sorghum bicolor L ( Moench) Journal of Chemical Ecology 12, 209-219.

Alsaadawi IS and Rice EL (1982). Allelopathic effects of Polygonum aviculare L.1. Vegetational patterning. Journal of Chemical Ecology 8, 993-1009.

Alsaadawi IS, Zwain KHY and Shahata HA (1998). Allelopathic inhibition of growth of rice by wheat residues. Allelopathy Journal 5, 163-169.

Einhellig FA and Rassmussen JA (1989). Prior cropping with grain sorghum inhibit weeds. Journal of Chemical Ecology, 15, 951-960.

Einhellig FA and Souza IF (1989). Phytotoxicity of sorgoleone found in grain sorghum root exudates. Journal of Chemical Ecology 18, 1-11

Hartley RD and Buchan H (1979). High performance liquid chromatography of phenolic acids and aldehydes derived from plants or from decomposition of organic matter in soil. Journal of Chromatography 180, 139-143.

Leather GR (1982) Sunflowers (Helianthus annuus L.) are allelopathic to weeds. Weed science 31, 37-42.

Megh S, Rama VT and Herbert NN (1989).HPLC identification of allelopathic compounds from Lantana camara. Journal of Chemical Ecology.15, 81-89.

Panasiuk O, Bills DD and Leather GR (1986). Allelopathic influence of Sorghum bicolor on weed during germination and early seedlings. Journal of Chemical Ecology 2, 1533-1543.

Rice EL (1984) Allelopathy. 2nd ed. Academic Press, New York.

Weston LA, Harmon R and Mueller (1989). Allelopathic potential of sorghum-sudan grass hybrid (sudex). Journal of Chemical Ecology 15, 1855-1865.