Allelopathic potential of tea (Camellia sinensis (L.) Kuntze) on germination and growth of Amaranthus retroflexus L. and Setaria glauca (L.) P. Beauv.

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1 Journal of Plant Diseases and Protection Zeitschrift für Pflanzenkrankheiten und Pflanzenschutz Special Issue / Sonderheft XX, (2006), ISSN Eugen Ulmer KG, Stuttgart Allelopathic potential of tea (Camellia sinensis (L.) Kuntze) on germination and growth of Amaranthus retroflexus L. and Setaria glauca (L.) P. Beauv. A. REZAEINODEHI 1*, S. KHANGHOLI 1, M. AMINIDEHAGHI 2, H. KAZEMI 1 1 Department of Horticulture, University of Shahed, Tehran, Iran, rezaeinodehi@shahed.ac.ir 2 Department of Agronomy, University of Shahed, Tehran, Iran, amini@shahed.ac.ir * Corresponding author Summary Laboratory and greenhouse experiments were conducted to determine the effects of tea (Camellia sinensis (L.) Kuntze) extracts (leaf, flower and fruit) at different concentrations on germination and growth of garden cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), redroot pigweed (Amaranthus retroflexus L.) and golden foxtail (Setaria glauca (L.) P.Beauv.). Furthermore, the effects of dried residues of tea on leaf area and dry weight of redroot pigweed and golden foxtail were investigated. Concentrations ranged from 0, 2.5, 5 to 10 % for organ extracts and dried residues were tested in mixtures with perlite at rates of 0, 8, 16, 24 and 32 g kg -1. Polyethylene glycole (PEG) was used in order to distinguish between the inhibitory effect of possible allelopathic substances and effects caused by the osmotic potential of the extracts. Results showed that garden cress and redroot pigweed germination was reduced by all extracts of tea organs with the exception of the leaf extract at the lowest concentration. The same was observed for lettuce and golden foxtail where germination was inhibited with the exception of leaf and fruit extracts at a concentration of 2.5 %. Radicle growth was affected at all test plants used and at all extract concentrations. Lettuce hypocotyl growth was reduced by all extracts of tea organs with the exception of the flower extract at the 2.5 % concentration. Hypocotyl growth of the other test plant species was significantly reduced by all extracts of tea organs as compared with the untreated control. Radicle growth was more sensitive in comparison to hypocotyl growth. Leaf area and dry weight of root, shoot and leaf were significantly lower with tea organ residues incorporated in the growing medium especially at rates of 24 and 32 g kg -1. Low PEG concentrations (1.06 and 2.13 %) had no significant effects on garden cress and lettuce. Keywords: Allelopathy, Camellia sinensis, germination, growth, weeds Zusammenfassung Das allelopathische Potential von Tee (Camellia sinensis (L.) Kuntze) auf die Keimung und das Wachstum von Amaranthus retroflexus L. und Setaria glauca (L.) P. Beauv. Labor und Gewächshausexperimente wurden durchgeführt, um die Effekte wässriger Extrakte verschiedener Teeorgane (Blatt, Blüten und Früchte) auf die Keimung, das Wurzel- und Sprosswachstum von Gartenkresse (Lepidium sativum L.), Salat (Lactuca sativa L.), Zurückgekrümmter Amarant (Amaranthus retroflexus L.) und Gelbe Borstenhirse (Setaria glauca (L.) P.Beauv.) zu untersuchen. Darüber hinaus wurden Effekte getrockneter Pflanzenreste auf die Blattfläche und das Trockengewicht der Borstenhirse und des Amarant untersucht. Der getestete Konzentrationsbereich lag bei 0, 2.5, 5 und 10 % des Organrohextraktes. Die getrockneten Pflanzenreste wurden mit Perlite vermengt in Raten von 0, 8, 16, 24 und 32 g kg -1. Polyethylene glycole (PEG) wurde verwendet, um zwischen einem direkten, hemmenden Effekt durch Extraktbestandteile und Effekten des osmotischen Potentials der Extrakte zu unterscheiden. Die

2 448 REZAEINODEHI, KHANGHOLI, AMINIDEHAGHI, KAZEMI Ergebnisse zeigten, dass die Keimung von Gartenkresse und Amarant bei allen Extraktkonzentrationen der Teeorgane mit Ausnahme der Blattextrakte gehemmt wurde. Auch Salat und Borstenhirse zeigten ab einer Konzentration von 2,5 % Effekte auf die Keimung mit Ausnahme der Blatt- und Fruchtextrakte. Das Wurzelwachstum der Testpflanzen wurde durch alle Organextrakte beeinflusst unabhängig von der getesteten Konzentration. Das Sprosswachstum von Salat wurde durch alle Extraktkonzentrationen der Teeorgane mit Ausnahme der Blütenextrakt gehemmt. Das Sprosswachstum der restlichen Testarten wurde ebenfalls im Vergleich zur unbehandelten Kontrolle durch alle Organextrakte reduziert. Das Wurzelwachstum wurde im Vergleich zum Sprosswachstum durch die Wirkung der Extrakte stärker beeinträchtigt. Die Blattfläche und das Trockengewicht der Wurzel der Testpflanzen wurde bei Einarbeitung getrockneter Pflanzenreste von Tee in das Wachstumsmedium deutlich gehemmt, was vor allem bei 24 und 32 g kg -1 zu beobachten war. Niedrigere PEG-Konzentrationen (1,06 und 2,13 %) hatten keinen erheblichen Effekt auf Gartenkresse und Salat. Stichwörter: Allelopathie, Camellia sinensis, Keimung, Wachstum, Unkräuter Introduction Allelopathy is defined as direct or indirect harmful or beneficial effects of one plant on another through the release of chemical compounds into the environment (RICE 1984). These compounds, known as allelochemicals, are usually secondary plant products or waste products of the main metabolic pathways of plants and most of the compounds are products of the shikimic acid and acetate pathway (RICE 1984). Many allelochemicals and their derivatives are believed to serve as models for new herbicide or growth hormone development (PUTNAM 1988). Tea (Camellia sinensis (L.) Kuntze) is known to biosynthesize toxic alkaloids and to have potential allelopathic or autotoxic influences (WALLER et al. 1986, SUZUKI and WALLER 1987). Many allelochemicals have been identified in tea plants such as alkaloids (caffeine, teophiline and teobromine), tannins, polyphenols, catechins, and phenolic compounds (benzoic acid, caffeic acid, chlorogenic acid, ferulic acid, p-cumaric acid, vanillic acid) etc. (NARWAL and TAURO 1996, DUKE 2001). RIZVI and RIZVI (1992) proposed that since caffeine exerted a differential action on several plant species, it might be a useful selective herbicide. They found that a M concentration of caffeine reduced lengths of root and shoot of rice seedlings to 50 and 90 % after 6 days respectively. It was reported that reduction of germination by caffeine involved effects on amylase activity in germinating seeds of Amaranthus spinosus L. and the alkaloid also inhibited some noxious weeds at various concentrations (RIZVI and RIZVI 1992). KHOLDEBARIN and OERTLY (1992) found that tea seed powder containing phenolic compounds reduces nitrification. Three phenolic acids, p-coumaric, ferulic and vanillic acid, which are found in tea plants were reported to severely inhibit photosynthesis and protein synthesis of isolated leaf cells of velvetleaf (Abutilon theophrasti Medik.) (MERSIE and SINGH 1993). It was found that soil extracts of tea gardens reduced root growth of garden cress in comparison to the control (water) by 31 % (NARWAL and TAURO 1996). Furthermore, it was reported that catechin induced oxidation and cellular death in root cells of neighboring plants (BAIS et al. 2003). There is a strong feeling that allelopathic research can be applied to many current weed problems (PUTNAM and DEFRANK 1983). One approach to utilize this phenomenon implies screening of accessions of allelopathic crops for their ability to reduce weeds, and a few crops have been evaluated in this aspect (e.g. LEATHER 1982, EINHELLIG and LEATHER 1988). With respect to the presence of many allelochemicals in tea and the limited knowledge of its allelopathic potential, this research was conducted to assess potential allelopathic effects of tea plants (residues and extracts), naturally grown at large scale in the north of Iran, on several test species, including garden cress, lettuce, as well as redroot pigweed and golden foxtail, two typical weeds of tea gardens.

3 Allelopathic potential of tea 449 Materials and methods Seeds of redroot pigweed (Amaranthus retroflexus L.) and golden foxtail (Setaria glauca (L.) P.Beauv.) were collected at the campus of the faculty of agricultural sciences of University of Shahed in north of Iran (Ramsar city). Tea (Camellia sinensis var. sinensis) plant organs including leaves, flowers and fruits were collected from tea gardens in the north of Iran during autumn of Garden cress (Lepidium sativum L.) and lettuce (Lactuca sativa L. var. longifolia) seeds were obtained from a local shop. Preparation of extracts Organs of tea were separated and dried in an oven for 48 h at 60 C and ground in a Wiley mill through a 40-mesh screen. 20 g of each dried ground organ was soaked in 100 ml distilled water for 24 h at 24 C in a lighted room. The solutions were filtered through four layers of cheesecloth to remove debris and then centrifuged at 3000 rpm for 10 min. The supernatant was filtered through one layer of Whatman no. 42 filter paper. To prevent microorganism growth during the experiments, the solutions were subsequently filtered through a 0.2 mm Nalgene filter (Becton Dickinson Labware, Lincoln Park, NJ). Separate extracts were prepared with distilled water of each organ in concentrations of 2.5, 5 and 10 % (w/v) dry matter. The extracts were stored at 0.5 C until used to limit degradation of the allelochemicals. Effect of organ extracts Seeds of garden cress, lettuce, redroot pigweed and golden foxtail were surface-sterilized with a water: sodium hypochlorite solution (10:1) for 10 min. After sterilization, the seeds were thoroughly rinsed several times with sterile water and one hundred seeds of each species were placed on a Whatman no. 1 filter paper in sterilized 9 cm Petri dishes. Five ml of extract solution from each plant part was added to the Petri dishes. Distilled water was used as control. One or 2 ml of extract was added during the experiment as required to maintain seedling development for a period of 7 days. All Petri dishes were placed in a lighted room at 24 C. At the end of the test period (7 days), percentage of germination and radicle and hypocotyl length was measured. Radicle and hypocotyl lengths were determined by measuring 24 representative seedlings. Effect of tea organ residues on weeds Dried ground organs of tea plants were added and mixed thoroughly with horticultural perlite (d 2 mm) at rates of 8, 16, 24 and 32 g kg-1. The perlite/residue mixture was placed in pots of 22 cm diameter. Pots were initially equipped with 30 seeds of one weed species and when the seedlings had one true leaf, the number of plants per pot was thinned to 15 plants. Plants were grown in a greenhouse at constant temperature (26 C) with a 12/12h light/dark cycle and at a relative humidity of %. Pots filled with perlite without any residue added were considered as the control. The pots were irrigated every two days with 300 ml of Hoagland nutrient solution (Hoagland and Arnon 1950). After 40 days the leaf area was measured using a leaf area meter (LI-3000A Portable Area Meter) and plants were subsequently harvested to determine the dry weight of roots, stem and leaves. Effect of Polyethylene glycole (PEG) concentrations (osmotic stress) Under the same conditions and concurrent with the tea organ extract experiment, PEG (carbowax 6000) was used in order to distinguish between a possible inhibitory effect of allelochemicals and that of the osmotic potential of extracts. To test osmotic effects, different concentrations of PEG were tested [0, 1.06, 2.125, 4.25 and 8.5 % (w/v)]. Water potentials of the first four PEG solutions, tea leaf and fruit extract concentrations were the same, but water potentials of flower extract concentrations were in the range of the control and the highest PEG concentration. Values of water potential were measured using model C-51 with peltier type thermocouple psychrometer (Wescor, Logan, UT). At the end of the experiment period (7 days), percentage of germination as well as radicle and hypocotyl lengths of test plants were measured.

4 450 REZAEINODEHI, KHANGHOLI, AMINIDEHAGHI, KAZEMI Statistical analyses All experiments were conducted in a factorial design and treatments arranged in a completely randomized design with four replications. Pooled mean values were separated using least significant differences (LSD) at the 0.05 probability level following an analysis of variance (ANOVA). Results Effect of organ extracts Analysis of variance showed that the average radicle and hypocotyl length as well as germination percentage differed among the test plants, tea organs and concentrations (P < 0.01). Flowers and fruits of tea had stronger effects compared with leaf extracts on all measured parameters. As concentration of extracts increased, all parameters significantly decreased. Lettuce germination was significantly reduced by all extracts of tea organs with the exception of leaf and fruit extracts at 2.5 % compared with the untreated control. In contrast, garden cress germination was initially significantly stimulated in response to leaf extracts and then decreased (Tab. 1). Tab. 1: Effect of different tea organ aqueous extracts on germination and initial growth of garden cress, lettuce, redroot pigweed and golden foxtail in germination assays. Tab. 1: Effekt wässriger Teeextrakte auf die Keimung und das Wachstum von Gartenkresse, Salat, Amarant und Borstenhirse im Keimtest. Germination (%) Radicle length (mm) Hypocotyl length (mm) Organ Extract concentration (%) Garden Cress Leaf 92.2a 94.2a 83.3b 58.4c 73.0a 15.6b 11.6c 6.1d 35.9a 21.9b 19.5bc 6.5c Flower 91.1a 64.4b 63.3b 59.9b 69.0a 2.7b 1.3b 1.1b 34.6a 6.6b 4.8bc 2.6b Fruit 93.3a 82.2b 74.4c 62.2d 77.0a 7.3b 2.3c 1.1c 37.5a 12.9b 4.6bc 2.8b Lettuce Leaf 76.7b 89.9a 63.3c 45.5d 37.0a 10.5b 5.8c 1.7d 44.9ab 49.0a 34.7bc 12.2c Flower 81.1a 61.1b 56.6b 42.3c 40.3a 5.4b 3.5bc 0.7c 47.1a 23.2b 18.5bc 5.4c Fruit 71.1bc 82.2a 76.6ab 68.8c 35.6a 6.3b 3.5bc 1.7c 42.7a 35.5a 21.2bc 13.1b Pigweed Leaf 78.7a 80.3a 66.6b 45.3c 22.0a 12.3b 7.3c 3.3d 21.2a 15.7b 10.3cc 5.7d Flower 76.3a 68.7b 58.7c 50.3d 22.2a 5.7b 3.3c 2.7c 21.7a 11.3b 6.7cc 1.7d Fruit 76.3a 71.8b 60.3c 42.7d 21.7a 7.3b 4.7c 2.3d 22.2a 13.3b 8.3cc 4.7d Fox tail Leaf 72.0b 76.6a 64.6c 35.7d 25.0a 9.8b 4.7c 2.7d 16.2a 10.7b 6.3cc 3.3d Flower 71.5a 62.7b 54.3c 47.7d 24.2a 8.3b 3.4c 1.7d 17.2a 9.8b 4.7cv 2.3d Fruit 69.7b 72.7a 52.3c 38.3d 21.1a 6.3b 3.3c 1.3d 15.7a 11.7b 5.3cc 1.6d Means within raws for each organ that are followed by the same letter are not significantly different (LSD test, P 0.05) All tea organs significantly also affected germination percentage of the two weedy species (P < 0.01). Redroot pigweed germination was reduced by all extracts of tea organs with the exception of leaf extract at 2.5 % and its radicle and hypocotyl growth was significantly reduced as well. Golden foxtail germination was initially stimulated with leaf and fruit extracts at 2.5 % and then reduced with increasing extract concentrations. Radicle growth of the other test plants was affected by all organ extracts. Garden cress hypocotyl growth was inhibited by all organ extracts, but lettuce hypocotyl growth first increased in response to leaf and flower extracts at 2.5 % and then decreased at higher concentrations (Tab. 1). On average organ extracts were more effective in suppressing radicle growth as compare with hypocotyl growth. Radicle and hypocotyl growth of the two weedy species were significantly affected by all organs extracts as well.

5 Allelopathic potential of tea 451 Effect of tea organ residues on weed species Golden foxtail leaf area was significantly lower with tea residues incorporation in the growing medium at rates of 24 and 32 g kg -1 for leaf residues, at a rate of 32 g kg -1 for flower residues and at rates of 16, 24 and 32 g kg -1 for fruit residues compared to the control. Tea organ residues also significantly reduced redroot pigweed leaf area and golden foxtail root dry weight at rates of 16, 24 and 32 g kg -1. Redroot pigweed root dry weight was significantly reduced by tea leaf and fruit residues at nearly all rates, but with flower residue only at higher rates (24 and 32 g kg -1 ). Shoot dry weight was significantly lower with tea leaf and fruit residues incorporation at rates of 16, 24 and 32 g kg -1 as compared with control. Flower residues affected shoot growth of redroot pigweed only at higher rates (24 and 32 g kg -1 ) and in golden foxtail only at the highest rate. Leaf and flower residues significantly decreased leaf dry weight in both species only at higher rates (24 and 32 g kg -1 ), and the same was true for fruit residues inhibiting golden foxtail, while inhibitory effects from fruit residues appeared already at 16 g kg -1 in redroot pigweed (Tab. 2). Tab. 2: Effect of incorporated tea organ residues on leaf area and dry weight of root, shoot and leaf of redroot pigweed and golden foxtail. Tab. 2: Einfluss eingearbeiteter Pflanzenreste von Tee auf die Blattfläche und das Trockengewicht von Wurzeln, Spross und Blättern bei Amarant und Borstenhirse. LA (cm 2 ) RDW (g) SDW (g) LDW (g) R Leaf Flower Fruit Leaf Flower Fruit Leaf Flower Fruit Leaf Flower Fruit Golden Foxtail a 123.3a 119.3a 0.27a 0.30a 0.31a 0.24a 0.22a 0.21a 0.20a 0.21a 0.19a ab 122.6a 114.8a 0.24a 0.28ab 0.29a 0.25a 0.21a 0.19ab 0.18ab 0.20ab 0.18a ab 118.0a 103.7b 0.19b 0.25bc 0.24b 0.18b 0.21a 0.15bc 0.18ab 0.17abc 0.15ab b 117.3a 96.0c 0.18b 0.23c 0.18c 0.15bc 0.18ab 0.13cd 0.15bc 0.16bc 0.12bc c 110.2b 90.5c 0.12c 0.18d 0.14c 0.12c 0.15b 0.10d 0.12c 0.14c 0.09c Redroot pigweed a 229.3a 226.0a 0.37a 0.36a 0.36a 0.38a 0.40a 0.38a 0.51a 0.50a 0.51a a 225.7ab 221.0a 0.36a 0.34a 0.33ab 0.37a 0.39a 0.35ab 0.52a 0.50a 0.47ab b 221.3bc 211.2b 0.29b 0.32ab 0.29b 0.31b 0.36a 0.33b 0.48a 0.47ab 0.43b b 216.3c 206.0b 0.25bc 0.28bc 0.24c 0.26c 0.29b 0.27c 0.43b 0.45bc 0.38c c 210.0d 190.3c 0.21c 0.26c 0.12d 0.21d 0.26b 0.23c 0.40b 0.42c 0.33d Means within a column for each organ that are followed by the same letter are not significantly different (LSD test, P 0.05), LA(leaf area), RDW(root dry weight), SDW(shoot dry weight), LDW(leaf dry weight) and R(residue, g kg -1 ) Effect of PEG concentrations (osmotic stress) Germination of garden cress was unaffected by PEG concentrations of 1.06 and %. Germination of lettuce was unaffected up to 4.25 % PEG. Although radicle length of garden cress was decreased at all PEG concentrations as compared with the control, but concentrations effects were not significant. Radicle length of lettuce was unaffected by PEG concentration of 1.06 %, but decreased at higher PEG concentrations. Hypocotyl length of garden cress was unaffected by PEG concentrations of 1.06 and % but affected by higher concentrations. In case of lettuce the hypocotyl growth was only affected at the highest PEG concentration tested (8.5 %) (Tab. 3). It is mentionable that the parameters were significantly reduced by most of the extracts concentrations of tea organs with same water potentials (Tab. 1).

6 452 REZAEINODEHI, KHANGHOLI, AMINIDEHAGHI, KAZEMI Tab. 3: Effect of PEG concentrations on germination and initial growth of lettuce (L) and garden cress (C) in germination assays. Tab. 3: Einfluss unterschiedlicher PEG-Konzentrationen auf die Keimung und das Wachstum von Salat (L) und Gartenkresse (C) im Keimtest. Germination (%) Radicle length (mm) Hypocotyl length (mm) PEG concentration (%) C 87a 85a 87a 81b 80b 55a 47b 47b 49b 48b 25a 25a 24a 21b 20b L 98a 95ab 96ab 98a 94b 94a 95a 88b 75c 75c 31a 30a 28ab 28ab 26b Means within a raw for each test plant that are followed by the same letter are not significantly different (LSD test, P 0.05). Discussion Aqueous extracts of tea organs significantly affected germination, radicle and hypocotyl growth of different test plants, including two weeds. This indicates that reduction in these parameters might have been the result of water soluble allelochemicals in the extracts and their inhibitory effects or phytotoxicity on the measured parameters. Results showed that tea plant organs varied in their allelopathic activity against garden cress and lettuce. Extracts of flowers and fruits of tea showed a higher allelopathic potential on the test plants as compared to leaves. These results are in accordance with previous studies reporting that allelopathy may vary among plant parts (TURK and TAWAHA 2002, PENG et al. 2004). For all extracts, allelopathic activity increased with increase in extract concentration. This is in agreement with the finding that under certain conditions, the rate of an elementary reaction is positively related to the reactant s concentration. Previous studies have shown that the phytotoxicity of extracts was significantly increased as their concentration increased (RICE 1984, PUTNAM 1994, SINKKONEN 2001, PENG et al. 2004). Garden cress germination was reduced by all extracts of tea organs as compared with the untreated control. Lettuce germination was first stimulated by leaf and fruit extracts at the lower extract concentrations and then reduced. The same was observed with golden foxtail germination which was also initially stimulated by leaf and fruit extracts at the lowest extract concentration. This confirms previous reports that biological responses of receiver plants to allelochemicals are concentration dependent with a response threshold. Responses are stimulated at low concentrations of allelochemicals (and some herbicides) and inhibited as the concentration increases (RICE 1984, RICE 1986, SANG-UK et al. 2000, PENG et al. 2004). Thus, it is essential to identify concentrations at which each specific response occurs if allelopathic interactions are to be used in weed management programs. Radicle length was much more sensitive to the extracts than hypocotyl length. This is in accordance with previous reports showing that root growth is more sensitive to extracts than shoot growth (CHUNG and MILLER 1995, TURK and TAWAHA 2002). This may be attributable to the fact that roots have direct contact and are the first to come in contact with allelochemicals. Redroot pigweed and golden foxtail leaf area, root, shoot and leaf dry weight 40 days after planting were significantly lower with tea organs residues incorporated in the growing medium especially at rates of 24 and 32 g kg -1, suggesting the presence of allelopathic effects. The observed inhibition of the two weeds could be attributable to a contribution of allelochemicals released from the incorporated tea residues, as the allelochemicals are water-soluble and can accumulate upon release in the rhizosphere in bioactive concentrations. There are several reports form literature showing that addition or incorporation of plant residues into the growth environment of another plant can result in growth inhibition (PATTERSON 1981, QASEM 1994, CHUNG and MILLER 1995, AL-KHATIB et al. 1997). We observed leaf chlorosis on golden foxtail plants at high rates of tea organ residues incorporated. This may be due to tea phenolic allelochemicals inhibitory effect on Mg-chelatase activity and accumulation of chlorophyll and porphyrin contents in the leaves (YANG et al. 2002). Some authors reported that adding of leachates and plant debris to the growing medium may deplete nitrogen (HARPER 1977) and, thus, the observed inhibitory effects may be partly caused by nitrogen depletion. Redroot pigweed did not show chlorosis at any rate of residues, indicating that allelopathic compounds may be selective or plant tolerance to allelopathic agents may vary.

7 Allelopathic potential of tea 453 Effects of higher PEG concentrations (4.25 and 8.50 %) on garden cress and lettuce suggest that reduction in measured parameters may have been the result of either or both, the osmotic potential of the extracts or the presence of allelochemicals in the extracts. Besides possible allelochemicals, higher chemical concentrations in the extracts might possibly cause an osmotic stress during seed germination and seedling growth. The measured parameters of test plants were unaffected by low PEG concentrations. But the parameters were significantly reduced by most of extracts concentrations of tea organs with same water potentials. This indicates that any reduction observed using extracts of tea organs must have been the result of allelochemicals in the extracts. Many allelochemicals have been identified in tea plants such as caffeine, teophiline, teobromine, tannins, polyphenols, catechines, ferulic acid, benzoic acid, chlorogenic acid, p-cumaric acid, vanilic acid, caffeic acid, etc. (NARWAL and TAURO 1996, DUKE 2001). Allelopathic potential of some of these compounds has been studied. For example, it is reported that catechin induces oxidation and cellular death in root cells of neighboring plants. Catechin is a potent phytotoxin that causes plants to self-destruct by producing free radicals and by triggering genes that kill the cells (BAIS et al. 2003). Caffeine is mainly produced in young leaves and immature fruits and continues to accumulate gradually during the maturation of these organs (ASHIHARA et al. 1996). This compound has been shown to have a strong negative allelopathic potential on seed germination of other plants, growth of rice seedlings and some weeds and inhibitory effects on amylase activity in germinating seeds of Amaranthus spinosa (RIZVI and RIZVI 1992). Multiple physiological effects have commonly been observed from treatments with many phenolics. These effects include reduction in plant growth, absorption of water and mineral nutrients, ion uptake, leaf water potential, shoot turgor pressure, and osmotic potential (PATTERSON 1981, GERALD et al. 1992). Further studies are needed now to determine what constituents of tea plants account for its allelopathic activity. Allelopathic effects of tea plant on weeds under field conditions also need further research. Acknowledgement The authors gratefully acknowledge the financial support of the University of Shahed and scientific assistance of Dr. H. Abbasipour. References AL-KHATIB, K., C. LIBBEY, R. BOYDSTON: Weed suppression with Brassica green manure crops in green pea. Weed Science 45, , ASHIHARA, H, A.M. MONTEIRO, T. MORITZ, F.M. GILLIES, A. CROZIER: Catabolism of caffeine and related purine alkaloids in leaves of Coffea arabica L. Planta 198, , BAIS, H.P., R. VEPACHEDU, S. GILROY, R.M. CALLAWAY, J.M. VIVANCO: Allelopathy and Exotic Plant Invasion: From Molecules and Genes to Species Interactions. Science 301, , CHUNG, I.M., D.A. MILLER: Natural herbicide potential of alfalfa residues on selected weed species. Agronomy Journal 87, , DUKE, J.A: Handbook of phytochemical constituents of GRAS herbs and other economic plants. CRC Press, London, EINHELLIG, F.A., G.R. LEATHER: Potential for exploiting allelopathy to enhance crop production. Journal of Chemical Ecology 14, , GERALD, F., L. BOOKER, U. BLUM, E.L. FISCUS: Short term effects of ferulic acid on ion uptake and water relations in cucumber seedlings. Journal of Experimental Botany 43, , HARPER, J.L: Population Biology of Plants. Academic Press, London, HOAGLAND, D.R., D.I. ARNON: The water-culture method for growing plants without soil. California Agriculture Experimental Station Circular 347, 32-50, KHOLDEBARIN, B., J.J. OERTLY: Allelopathic effects of plant seeds on nitrification: effects on ammonium oxidizers. Soil Biology and Biochemistry 24, 59-64, LEATHER, G.R.: Sunflowers are allelopathic to weeds. Weed Science 31, 37-42, 1982.

8 454 REZAEINODEHI, KHANGHOLI, AMINIDEHAGHI, KAZEMI MERSIE, W., M. SINGH: Phenolic acids affect photosynthesis and protein synthesis by isolated leaf cells of velvet- leaf. Journal of Chemical Ecology 19, , NARWAL, S.S, P. TAURO: Allelopathy in pests management for sustainable agriculture. Scientific publishers, Jodhpour, India, PATTERSON, D.T.: Effects of allelopathic chemicals on growth and physiological responses of soybean (Glycine max). Weed Science 29, 53-59, PENG, S.-L., J. WEN, Q.-F. GUO: Mechanism and active variety of allelochemicals. Acta Botanica Sinica 46, , PUTNAM, A.R., J. DEFRANK: Use of phytotoxic plant residues for selective weed control. Crop Protection 2, , PUTNAM, A.R.: Allelochemicals from plants as herbicides. Weed Technology 2, , PUTNAM, A.R.: Phytotoxicity of plant residues. In: Unger, P.W. (ed): Managing agricultural residues, Lewis Publishers, Boca Raton, , QASEM, J.R.: Allelopathic effect of white top (Lepidium draba) on wheat and barley. Allelopathy Journal 1, 29-40, RICE, E.L.: Allelopathy. Academic Press, New York, RICE E.L.: Allelopathic growth stimulation. In: Putnam, A.R., Tang, C.S. (eds): The science of allelopathy. John Wiley and Sons, New York, 23-42, RIZVI, S.J.H., V. RIZVI: Exploitation of allelochemicals in improving crop productivity. In: Rizvi, S.J.H., Rizvi, V. (eds): Allelopathy: Basic and applied aspects. Chapman and hall, London, , SANG-UK, C., H.C. JOHN, C.J. NELSON: Effects of light, growth media, and seedling orientation on bioassays of alfalfa autotoxicity. Agronomy Journal 92, , SINKKONEN, A.: Density-dependent chemical interference, an extension of the biological response model. Journal of Chemical Ecology 27, , SUZUKI, T., G.R. WALLER: Allelopathy due to purine alkaloids in tea seeds during germination. Plant and Soil 98, , TURK, M.A., A.M. TAWAHA: Inhibitory effects of aqueous extracts of black mustard on germination and growth of lentil. Pakistan Journal of Agronomy 1, 28-30, WALLER, G.R., D. KUMARI, J. FREEDMAN, N. FREEDMAN, C. H. CHOU: Caffeine autotoxity in Coffea arabica. In: Putnam, A.R., Tang C.S. (eds): The science of allelopathy. John Willey and Sons, New York, , YANG, C.-M., C.-N. LEE, C.-H. CHOU: Effects of three allelopathic phenolics on chlorophyll accumulation of rice (Oryza sativa) seedlings: I. Inhibition of supply-orientation. Botanical Bulletin of Academia Sinica 43, , 2002.