Weeds harm crops, competing them for water, nutrients and light. Unless protected, while the diseases cause 16.4% and pests 11.2% of the yield to be loss, the losses caused by weeds can be as high as 24%. Many different sort of chemicals are being used to decrease these losses. However, these application of chemicals bring about some undesired side effects. Today, many researches have been conducted to develop environmentally sound plant protection methods.
Weed can affect the crops by allelopathic effect as well as they compete them for water, nutrients and light. When these two effects occur concomitantly, the harm caused becomes even greater.
Allelopathy is one plants directly affecting another plants growth either positively or negatively, exuding chemical substances. Many phytotoxic chemical substances are known to be exuded by plants to suppress emergence or growth of the other plants. Some over 10 thousand chemicals are estimated to be produced by the plants to protect themselves against, deceases, pests and other plants, especially weeds. As the knowledge on these substances advances, these substances may be used as herbicide, which will be very beneficial for environment. The weeds have allelopathic superiority over crops besides their competition superiority. In allelopathy, relations between weeds and crops, between weeds and weeds and between crops and crops are been examined and the means to benefit from these relations have been studied[2-4]. Chon et al. found that extracts of Lactuca sativa L., Xanthium occidentale L. and Cirsium japonicum DC inhibited root growth of Medicago sativa L. In another study, water extracts of Eucalyptus cameldulensis Dehn., Juglandis regio, L., Melia azederanch L., Nerium oleander L., Raphanus sativus L. and Thymus sp. showed no adverse effect on corn growth under field conditions. Especially, residue of N. oleander and M. azederach increased corn yield and decreased weed courage area.
Amaranthus retroflexus L., Abutilion theophrastii Medik. are weeds abundant in Turkey, especially in the areas where summer crops are grown. A. retroflexus is characterized among allelopathic effective-weeds. The plant extracts of M. sativa inhibit emergence of weeds such as Chenopodium album L., A. retroflexus, A. theophrastii and Setaria faberi L.. However, Turk et al. found that plant extracts of Brassica nigra L. affected the germination and growth of M. sativa, indicating that some plants known allelopathic-effective can be affected negatively by other plants.
Just as in the World, Lolium multiflorium L., Descurania sophia
L., Webb. Ex. Prant, Avena sterilis L. and Trifolium repens L.
are predominant weed species in Turkey occurring in winter time in small grain
production areas[11-13]. These weeds are presumed to antagonize growth
of crops, by their competitive and allelopathic effects. To know their natural
enemies may help decrease their harm and better control these weeds.
In this study, the effect of the extracts of 22 plants, mostly weeds, on emergence of seeds of nine weeds was studied under in vitro conditions and potential use of allelopathy to decrease the harmful effects of these weeds was investigated.
MATERIALS AND METHODS
Sampling of plant materials and preparation of extracts: Weed species were sampled from different parts of Tokat during flowering or fruit development stages between May and August, 2002 (Table 1). All the weed materials were dried, ground and stored in dark condition at room temperature in the glass jars until use. To obtain extracts, 100 g ground materials from each weed species were placed in a 1000 mL flask containing 600 mL absolute methanol and shacked for 24 h on an orbital shaker (120 rpm) at room temperature. The extract was filtered through a four-layer cheesecloth to separate solid materials.
The methanol was evaporated at 32°C by use of ratory evaporator and then solid phase was dissolved in 10% acetone to obtain 10% concentration of each extract. Single concentration (10%) of each extract was used for the germination tests.
Germination tests: Germinations tests were carried out with each of the weed extracts. Seeds of 9 weeds (Table 2) were surface sterilized with 10:1 water/bleach solution for 2 min before soaking into 10% extracts and then seeds were dried on laboratory benches before being evenly placed on steril watman No:10 filter papers in 9 cm petri dishes. Acetone (10%) was used as the control. Based on seed size, 50 seeds were placed per petri dishes.
Treatments were arranged in a completely randomized design with four replications. The experiments were repeated twice. Wild out, poison hemlock, fixweed, curly dock, garden cress, white clover seeds were incubated at 15°C and other seeds were incubated at 24°C in the growth chamber for 21 days.
Germination was determined by counting and removing germinated seeds at 3 day intervals over a 21 day period and expressed as total percent germinated. Seeds were considered to be germinated when the radicle and hypocotyl length was over 2 mm.
|| Plant species used for extraction
|| List of the plants used in germination tests
Statistical analysis: The data were analysed using Analysis of Variance (ANOVA) test. The means of treatments were grouped on the basis of least significant difference (LSD) at the 0.05 probability level. The software SAS was used to conduct all the statistical analysis.
RESULTS AND DISCUSSION
The effects of plant extracts on germination of some weeds after 6 and 21 days are given in Table 3 and 4, respectively.
In all the plant extracts, the inhibition rate of seed germination was time
dependent but relatively parallel. However, compared with control, the 6th day
germination rate was lower than that of 21 day, that is those extracts inhibiting
the germination, delayed the start of germination, too. No extracts occurred
to have an effect on germination of Lepidium sativum L. a crop that sometimes
considered as a weed. Salvia officinalis L., Laurus nobilus L.
and Artemisia vulgaris L. most strongly inhibited the germination of
Lolium prenne L. Malkani et al. reported that
the plant extracts of Artemisia vulgaris and its soil where it was grown
inhibited the growth of Lolium prenne L.
|| Effects of plants extracts on seed germination on experimental
weeds at the end of 6 day incubation period (%)
|| Effects of plants extracts on seed germination on experimental
weeds at the end of 21 day incubation period (%)
In the present study, extracts of Chenopodium album L. and Reseda
lutea L. promoted the germination of Abutilon theophrastii Medik.
The other extracts inhibited germination of A. theophrastii and then
the lowest germination rate (1%) occurred in the plates treated with extract
of Ecbalium elatorium L. A. Rich. In previous studies, it was reported
that the growth of Amaranthus refroflexus L. was inhibited by the rhizom
of Sorghum halepense L. Pers. extracts and plant of Cyperus
esculentus L. extracts. In this study, germination A.
retroflexus was strongly inhibited by the extracts of Galium aparine
L. (13%), Lolium temulentum L. (12.5%), Conium maculatum L. (19.5%)
and Avena sterilis L. (26%).
Allelopathy is some plants affecting the others, either positively or negatively, by exuding chemicals. Compared with the control, all the extracts used in this study, except Lolium temulentum L., promoted emergence of Descurania sophia L. Webb. Ex. Prant. Uygur and Iskenderoglu reported that the residues of Nerium oleander L. and Melia azederach L. encouraged growth of corn although they decreased the area covered by the weeds. Some plants can exhibit both positive and negative allelopathy. For example, while S. halepense inhibits growth of corn, okra, tomato and carrot[17,18]; extracts of Raphanus sativus inhibits seed germination and rhizom growth of S. halepense, suggesting that plants may not affect each others growth negatively, but sometimes positively. In some other studies, it was reported that extracts of A. retroflexus and C. album stimulated growth of Lepinus albus and Zea mays[6, 20]. In this study, compared to control, some decreases occurred in germination of Avena sterilis, Rumex crispus and Trifolium repens.
A. sterilis is a predominant weed with economical damage threshold of 4-5 plant m2 in wheat, barley and onion production areas in Turkey[7,12]. Compared with control, germination of A. sterilis seeds was significantly decreased by the Consolida regalis S.F Gray (20%), L. tumelentum (34%) and Humulus lupulus L. (44%) extracts.
Results of present study and previous works show that use of plant extracts as herbicides to control the weeds will bring a great success in this area. Moreover, the positive allelopathic effects should also be investigated to exploit its benefit in crop production.
This study examined the effects of extracts of plants, mostly weeds, on germination of weeds in laboratory conditions. Other studies should be conducted with the same plants, used in this study, in pot and field conditions. Also, chemical compounds in extracts, having significant positive or negative allopathic effects on other plants, should be studied in detail for their specific effects on plant growth.