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Allelopathic Effects of Rice Cultivars on the Growth Parameters of Different Rice Cultivars



Saeid Ghahari and Mohammad Miransari
 
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ABSTRACT

Eight and ten rice cultivars were tested in laboratory and greenhouse bioassay, respectively to evaluate the allelopathic effects of rice (Oryza sativa L.) hull extracts on the growth parameters of different rice cultivars. Extracts of rice hulls at different concentrations including 0, 5, 10 and 15% were produced and used to treat seeds of different rice cultivars. The growth parameters of germinated rice seeds after incubation for 12 days in the germinator were evaluated. In the greenhouse experiment, the treated seedlings were harvested at 21 days after planting and growth parameters determined. Treatments were combined factorially in both experiments, which were performed on the basis of a complete randomized design. Both positive and adverse effects of rice extracts on the growth of different rice cultivars in both experiments were observed. Although, the growth of genetically modified cultivars were greatly superior to the local ones, the inhibitory effects of their extracts were very much clear on the growth of different cultivars. This indicates that more modification of these cultivars, with respect to the related rice phytotoxicity potential and the response of rice cultivars to phytotoxins is necessary. Thus, there are some kinds of interactions between different rice cultivars, greatly influencing their production efficiency.

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  How to cite this article:

Saeid Ghahari and Mohammad Miransari, 2009. Allelopathic Effects of Rice Cultivars on the Growth Parameters of Different Rice Cultivars. International Journal of Biological Chemistry, 3: 56-70.

DOI: 10.3923/ijbc.2009.56.70

URL: https://scialert.net/abstract/?doi=ijbc.2009.56.70
 

INTRODUCTION

Allelopathy is the combination of Allelo and pathy, meaning mutual unfavorable effects. Later, it was found that plants like rice or microorganisms can positively or negatively (inhibitory and stimulatory effects, respectively) affect the growth of another plant by the production of a chemical or allelochemical (Rice, 1984). The most recent research, regarding allelopathy is related to crop plants such as wheat, rice and sorghum (Rector, 2008). Scientists indicated, that although the allopathic potential of different rice cultivars was different, it was economically recommendable for weed control, though the yield production was less than expected (Gealy et al., 2003). With the help of molecular biology techniques different allelochemicals and their corresponding genes have been recognized, indicating the likelihood of producing crop plants with higher allelopathic potential (Liang and Pardee, 1992; Nomura et al., 2003; Belz and Hurle, 2005; Rector, 2008).

Plants are able to release secondary metabolites that are not usually involved in plant growth, compared with primary metabolites (D’Auria and Gershenzon, 2005). However, it has been known that many secondary metabolites are of ecological significance, because of their important effects on plant adaptation to natural conditions (Field et al., 2006).

To show the great significance of these natural products in protecting the plant kingdom, the genomic map of the genes, involved in the production of such products have been made. For example, more than 25% of Arabidopsis genes, making approximately 170 different substances from 7 distinguished classes, are responsible for the production of such natural compounds (D’Auria and Gershenzon, 2005).

Many of these compounds such as salicylic acid, are involved in plant systematic acquired resistance (Lian et al., 2000), establishment of microbial symbiosis, including rhizobium (Eckardt, 2006) and arbuscular mycorrhiza (Akiyama and Hayashi, 2006) under stress-free and stressful conditions (Miransari and Smith, 2007, 2008; Miransari et al., 2006, 2007, 2008). The other related functions include plant protection versus pathogens including soil fungi (triterpene) and weed allelochemicals (flavonoids), disease resistance and allelopathy in maize and wheat (Hydroxamic acid), allelopathy in sorghum (quinone), allelopathy in rice (diterpene) and plant resistance (steroid) (Field et al., 2006).

Some crop and weed plants are able to release some exudates into the environment, suppressing the growth of plants of their own kind, other plants or weeds. This is called the allelopathic effect (Narwall and Willis, 2006). Scientists have stated that the growth of some plant species may be affected due to the allelopathic potential of different weed and crop plants (Rice, 1984). This may be related to the exudates of the living and decaying material of different plants (Inderjit, 1996; Noguchi-Kato, 2000). The allelopathic potential has been found for weeds affecting crop plants (Singh et al., 1988; Das and Choudhury, 1996) and vice versa (Narwal and Sarmah, 1996) and also weeds affecting weeds (Tripathy and Vaishya, 1997, 1999) and crop plants affecting crop plants (Putnam and Duke, 1978; Barnes and Putnam, 1986). Such plant or microbe exudates are called allelochemicals and can be used as natural herbicides (Singh et al., 1999).

Water-soluble substances, released as leachate from different plant parts especially plant leaf may adversely affect seedling growth due to the following reasons: (1) allelopathic effects as allelochemicals, (2) immobilizing N and (3) increased microbial population and hence enhanced competition with plants (Inderjit et al., 2004; Inderjit, 2006). Parameters such as the ability of soil colloids to absorb allelochemicals, O2 quantity affecting the oxidation potential of soil and also the microbial activity may all determine the effectiveness of allelochemicals (Bertin et al., 2003; Inderjit and Bhowmik, 2004).

In the special kind of allelopathy, called autotoxicity, the crop will adversely affect the growth of its own kind with the production of some exudates. Such a phenomenon is common for some weeds and crop plants in agroecosystems resulting in soil exhaustion. In some plants there are mechanisms, developed to deal with the problem, however, some other, have been adapted to such a phenomenon through structural changes (Singh et al., 1999). Autotoxicity and the release of autotoxins may decrease plant population and regeneration and also reduce plant yield (Liu et al., 2007).

This phenomenon may have important implications, since it may significantly determine the selection of the most appropriate strategies, for an agricultural system. For example, it may be necessary to recognize the extent to which a plant such as rice (Oryza sativa L.) adversely affects the plant growth of its own and also a weed such as barnyard grass (Echinochola crus-galli). It may also be important to determine whether it is really necessary to rotate rice with other crop plants such as wheat to avoid soil exhaustion and crop reduction or the allelopathic property of rice on weed plants may be a more superior parameter affecting the selection of the appropriate strategy.

With respect to the economical and environmental significance of biological methods to suppress the activity of organisms, such as weeds and pests, competing with crop plants, the importance of using such ways becomes even more important. Weed and pest control in the field is of great importance, since the competing weeds and pests may diminish crop yield. Usually, weed growth and pests are chemically controlled in the field, however, as stated, due to economical and environmental impacts, use of biological methods may be superior (Jung et al., 2004).

Scientists have suggested some ways to either enhance the allelopathic potential of plants through increasing allelochemicals or identify and introduce genes, responsible for the production of such compounds (Singh et al., 2003). Different plant cultivars may differ in their resistance to allelochemicals, which is a cultivar dependent trait.

To better address the rotation strategy of rice, due to the importance of rice as a very important crop feeding a large number of people in the world and since there is little finding on rice autotoxicity these experiments were performed. The hypothesis was, that different rice cultivars may differ in their autotoxicity potential. The objective was to evaluate the effects of rice (Oryza sativa L.) hull extracts on the interaction among different rice cultivars. Natural products are of great importance in the environment and hence in a sustainable agriculture.

MATERIALS AND METHODS

Origin of Rice Seeds
The experiment was conducted in 2006, in the college of Agricultural Sciences, Shahed University, Tehran, Iran. The common local and inbred cultivars, with superior properties such as high yield production were selected. The rice seeds were collected at harvest from rice fields of Mazandaran province, Iran. After preparation and before the start of the experiments the hulls and seeds were kept at 5°C. In the laboratory bioassay, 8 cultivars of rice including Khazar, Gerdeh, Fajr, Neda, Shafagh, Behnam, Tarem Hashemi and Tabesh and in the greenhouse bioassay in addition to these cultivars, Nemat and Zarak, were also tested. Six cultivars of Khazar, Gerdeh, Fajr, Shafagh, Neda and Behnam were obtained from Amol Rice Research Center of Mazandaran province.

Extraction Method
Since Chung et al. (1997) stated that the extracts of rice hulls are more active than the extracts of rice leaf, the rice hulls were extracted according to Ahn and Chung (2000). Using a shaker, 30 g of ground rice hulls were mixed with 200 mL of distilled water for 1 h. To remove the debris, the extract was filtered using four layers of cheesecloth and was then centrifuged at 3000 rpm for 4 h. Whatman filter paper No. 42 was used to filtrate the supernatants, which were then kept in the fridge at 4°C using sealed dark bottles. Ten milliliter of this stock solution with the concentration of 15% was diluted to 30 and 15 mL, respectively, to make the concentrations of 5 and 10%.

Fajr, Neda, Khazar and Shafagh are inbred cultivars and the color of their extract is the same (yellow to reddish). Behnam and Gerdeh are both local and premature cultivars with the same extract pH (4.5). The hull color of both is yellow to reddish.

Laboratory Bioassay
Rice paddies (hulls and seeds) were surface sterilized using commercial hypochlorite bleach (1:10 v/v) for 10 min and were then rinsed with distilled water several times. The paddies were placed on a filter paper to dry. Using Petri dishes, 25 paddies of different rice cultivars were treated with different concentrations of extracts at 10 mL, including control, 5, 10 and 15%. The experimental design was a factorial on the basis of a completely randomized design with three replicates. The Petri dishes were incubated in a germinator at 30°C for 12 days and the following parameters were measured:

Seed germination (%); percentage of germinated seeds
Rate of seed germination was calculated using this equation:

where, RC is the rate of seed germination, NT is the certain time for the germination of seeds and N is the total number of germinated seeds

Root length; the root lengths for the seedlings in each Petri dish were determined and the mean was calculated (mm)
Shoot length, was measured similar to root length
Root dry weight, using an oven at 65°C for 4 h the dry weight of each seedling and the mean for all seedlings were determined
Shoot dry weight, was calculated similar to root dry weight
Seedling dry weight (seed, shoot and root dry weights), was determined for each seedling using an oven at 65°C for 4 h and the mean was calculated

The inhibition percentage was calculated using the following equation (Chung et al., 2002):

Inhibition (%) = [1 – (sample extracts)/control] x 100

Greenhouse Bioassay
Plastic pots (10x10 cm) without drains were filled with 15 g of silicate rock in the greenhouse at 28°C under natural light. In each pot, 50 seeds were placed on the soil surface. The pots were saturated during the experiment and the hull extracts were applied with the irrigation water at seeding. After seedling, growth, seed germination (%) and rate of seed germination were determined. Shoot growth was used as the seed germination parameter. After thinning, 20 seedlings were kept in each pot. At days 14, Hoagland’s solution (Hoagland and Arron, 1950) was applied to each pot at 20 mL (Table 1). At days 21, the seedlings were harvested and after washing, the same growth parameters, measured in the laboratory bioassay, were determined.

Statistical Analysis
Both experiments were designed as factorial including two factors, cultivars and extract concentrations, on the basis of completely randomized design in three replicates. Hence, 32 and 40 treatments were tested in the laboratory and in the greenhouse experiments, respectively. Using SAS (SAS Institute Inc., 1988), the data were subjected to analysis of variance and covariance (correlation coefficients were determined) and using the GLM method and the protected Least Significant Difference (LSD) test the means were compared at p = 0.1. Also the matrices of contrast comparisons were used to compare the effects of different extracts with control on the growth of different plant parameters (Steel and Torrie, 1980).

RESULTS

Effect of Rice Hull Extracts on Rice Seed Germination Rate
In both experiments, cultivars Fajr at control and Neda at 10% germinated the highest, significantly different from cultivars such as Tabesh, which showed to be very sensitive to the extracts. Behnam and Tabesh at 5% in the laboratory experiment and Gerdeh at 10 and Tarem at 15% in the greenhouse experiment germinated the lowest. In both experiments, with increased extract concentration, seed germination decreased for Fajr, Khazar and Tabesh. Compared with control, the extracts significantly reduced seed germination (Table 1, 2).

Seed germination was the highest for inbred cultivars. In both experiments, the effect of cultivar on seed germination was significant and in the laboratory experiment the effects of 5 and 10% extracts on seed germination was significantly different from control. Also, the interaction effects were non significant in both experiments (p = 0.13 and 0.15, respectively) (Table 1, 2).

In the laboratory experiment, Fajr, Neda and Tarem and in the greenhouse experiment Fajr and Neda had the highest rate of germination. In the laboratory experiment the 5% extract significantly reduced the germination rate and the effect of the 10% extract was non-significant (p = 0.12). The effects of cultivar on the germination rate were significant in both experiments. The interaction effects of cultivar and extract concentration were non-significant (p = 0.14) in the laboratory experiment and significant in the greenhouse experiment (Table 1, 2).

Inhibitory Effect of Hull Extracts on Rice Seed Germination Rate
In both experiments, the highest inhibition was related to Tabesh, Khazar and Gerdeh and in the laboratory experiment, while 5% extract was inhibitory to Behnam, 10 and 15% and all concentrations in the greenhouse experiment resulted in the highest enhancement of seed germination for Behnam (Fig. 1a, b).


Table 1: Comparing the phytotoxicity potential of different rice cultivars, based on different concentrations (R) of hull extracts, on plant growth parameters, including germination percentage (GP), rate of germination (RG), root length (RL), shoot length (SL), root dry weight (RDW), shoot dry weight (SDW) and total dry weight (TDW), in the laboratory bioassay
Rice cultivars including, B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, S: Shafagh, T: Tarem, TB: Tabesh, ns: Not significant, *Significant at p = 0.1, **Significant at p = 0.05, ***Significant at p = 0.01, mean (SD), n = 3

Table 2: Comparing the phytotoxicity potential of different rice cultivars, based on different concentrations (R) of hull extracts, on plant growth parameters, including germination percentage (GP), rate of germination (RG), root length (RL), shoot length (SL), root dry weight (RDW), shoot dry weight (SDW) and total dry weight (TDW), in the greenhouse bioassay
Rice cultivars including B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, N: Nemat, S: Shafagh, T: Tarem, TB: Tabesh, Z: Zarak. ns: Not significant, *Significant at p = 0.1, **Significant at p = 0.05, ***Significant at p = 0.01, mean (SD), n = 3

In both experiments, the extracts were inhibitory to some cultivars and stimulatory to some others. The stimulatory effects of the extracts on the germination rate were clearer under greenhouse conditions. The 5% extract resulted in the highest rate of germination inhibition in Tabesh and Neda in the laboratory and greenhouse experiments, respectively and resulted in the highest rate of germination stimulation in Nemat (Fig. 2a, b).


Fig. 1: The inhibitory effects of different hull extract concentrations on rice seed germination in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including, B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, S: Shafagh, T: Tarem, TB: Tabesh

Fig. 2: The inhibitory effects of different hull extract concentrations on the rate of seed germination in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including, B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, S: Shafagh, T: Tarem, TB: Tabesh

Table 3: Correlation coefficients for growth parameters of the cultivars used in both experiments
GP: Germination Percentage, RG: Rate of germination, RL: Root length, SL: Shoot length, RDW: Root dry weight, SDW: Shoot dry weight, TDW: Total dry weight, 1: Laboratory, 2: Greenhouse

Effects of Rice Hull Extracts on the Seedling Root and Shoot Length
In both experiments the highest root lengths were related to Fajr, Neda and Tarem and the extracts significantly decreased root length (Table 3). The effects of cultivar on root length were significant in both experiments and the interaction effect between cultivar and extract concentration was non-significant in both experiments (p = 0.1 and 0.18, respectively). The lowest root length in the laboratory experiment is related to Gerdeh at 15% and in the greenhouse experiment is related to Khazar at 10%. The inbred cultivars had longer roots (Table 1, 2).

The highest and the lowest shoot lengths are related to Fajr and Behnam, respectively, at control and 5% extracts in the laboratory experiment and to Zarak and Tabesh in the greenhouse experiment. In both experiments, the effects of cultivar and the interaction effect of cultivar and concentration were significant. In addition, in the laboratory experiment all extracts and in the greenhouse experiment the 10% extract significantly reduced shoot length (Table 1, 2).

The Inhibitory Effects of Hull Extracts on the Seedling Root and Shoot Length
In both experiments, the extracts inhibited at the most the root growth of Fajr, Neda, Khazar, Shafagh and Gerdeh and stimulated at the most the root growth of Tabesh and Tarem in the laboratory and greenhouse experiments, respectively. Both extract concentrations of 10 and 15% highly inhibited the root growth of Gerdeh and Khazar (Fig. 3a, b).

In both experiments, the highest inhibition in shoot length was observed in Gerdeh, Khazar and Shafagh due to 10 and 15% concentrations. In the laboratory experiment the extracts were stimulatory on the shoot growth of Behnam and Tabesh and in the greenhouse experiment on the shoot growth of Behnam (Fig. 4a, b).

Ratio of Root/Shoot Length and Dry Weight in Different Rice Cultivars
In the laboratory experiment, the shoot growth of Fajr, Khazar and Shafagh was higher than their root growth. Behnam had a higher shoot growth, compared with its root growth. In the greenhouse experiment, the shoot growth of Gerdeh and Khazar was higher than their root growth at all extract concentrations except control. However, Tarem and Neda had the highest root growth, than their shoot growth, compared with other cultivars (Fig. 5a, b).

In the laboratory experiment, with the exception of Behnam, Neda and Tarem, all other cultivars had a higher shoot dry weight than their root dry weight. In the greenhouse experiment Behnam, Gerdeh, Khazar and Tabesh had the highest shoot dry weight, compared with their root dry weight (Fig. 6a, b).


Fig. 3: The inhibitory effects of different hull extract concentrations on the seedling root length in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including, B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, S: Shafagh, T: Tarem, TB: Tabesh

Fig. 4: The inhibitory effects of different hull extract concentrations on the seedling shoot length in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, S: Shafagh, T: Tarem, TB: Tabesh

Fig. 5: The effects of different hull extract concentrations on the ratio of seedling root length to seedling shoot length in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, N: Nemat, S: Shafagh, T: Tarem, TB: Tabesh, Z: Zarak

Fig. 6: The effects of different hull extract concentrations on the ratio of seedling root weight to seedling shoot weight in the (a) laboratory and (b) greenhouse experiments. Rice cultivars including B: Behnam, F: Fajr, G: Gerdeh, K: Khazar, ND: Neda, N: Nemat, S: Shafagh, T: Tarem, TB: Tabesh, Z: Zarak

Verification of the Laboratory and Greenhouse Experiments
Analysis of covariance (determination of correlation coefficients) was performed to verify the results of the two bioassay. High and significant correlation coefficients were determined between the growth parameters in the two experiments (Table 3).

DISCUSSION

Although allelopathy can adversely affect the growth of other plants or microbes, exudates of plants including rice can also have stimulating effects on the growth of other cultivars (Rector, 2008). There are mechanisms, such as oxidation (Wieland et al., 1998; Inderjit and Duke, 2003) by which plants are able to detoxify the effects of allelochemicals and hence, live together (Fitter, 2003). Similarly, when subjected to plant pathogens, important cytoskeletal rearrangement in plant cells happens, usually resulting in exudation of proteins and or/natural productions through vesicles (Schulze-Lefert, 2004; Koh and Somerville, 2006). It is not yet known, if the same vesicles or different vesicles mediate the production of resistance producing products (Field et al., 2006).

The differences in seed germination of different rice cultivars, when subjected to different extract concentration indicate that the extracts can differently affect seed germination of different rice cultivars and accordingly the responses change. Genetical modification increased the rate of germination and hence can increase the efficiency of cultivars production. In addition, the responses of cultivars to concentration gradient were different and obviously the 5% concentration had the highest impact on seed germination. This indicates that the new arrangement of genes in the genetically modified cultivars result in different genes expression when subjected to the extracts. Such a response can affect plant performance and hence production significantly by affecting the rate of seed germination. These results are in agreement with Gealy et al. (2003), who indicated that different rice cultivars behaved differently in their response to the extracts. Hence, genetical re-arrangement can be used as a useful tool for increased yield production when different rice cultivars are planted.

Low extract pH of 4 may also intensify the allelopathic effects of rice extract (Chung et al., 2002), which is similar to the results of these experiments. Present results verify the biological activity of rice hull extracts, through their stimulating and inhibitory effects, on the growth of different rice parameters (Chung et al., 2002), as according to Minorsky (2002) and Inderjit and Duke (2003) different crop and weed plants have the ability to produce phytotoxins at biological concentrations. Although, the extracts were phytotoxic to rice cultivars, there may be some compounds in the extract that are not initially phytotoxic but may become phytotoxic to rice growth after microbial transformation in the field. According to Chung et al. (2002) P-hydroxybenzoic acid had the highest phytotoxicity effect on seed germination and the growth of barnyardgrass (Echinochloa crus-galli).

The highest germination in Neda, when subjected to the extract, may show the capability of this cultivar, to resist the phytotoxicity of other cultivars and hence, higher growth and yield, under field conditions. This may be of great significance for the farmers in the field. Accordingly, Tabesh may not be a preferable cultivar when the phytotoxicity potential related to rice residues in the filed is high. The inhibitory effects of rice extracts on the seed germination of Tabesh, Khazar and Gerdeh may indicate that: (1) higher rate of seed may be used for seeding these cultivars at planting in the field, (2) using genetic techniques the phytotoxic potential of these cultivars be reduced or covered seeds be used. It means that the genes responsible for the phytotoxicity of different rice cultivars should be recognized and such genes be genetically modified so that their phytotoxicity potential is decreased or the seeds may be covered with some substances so that their biochemical interactions with the surrounding environment is decreased, (3) using a circular method of irrigation may reduce the phytotoxicity of rice seeds, (4) when the germination rate is not high Fajr and Neda must be used instead of Gerdeh and Khazar, (5) the residues of cultivars such as Shafagh in the field may be favorable to rice growth indicating the benefit of using Shafagh for rotation.

Cultivars such as Fajr, Neda and Tarem had the highest root length when subjected to the extracts. Although, Fajr and Neda had the highest root length, the effect of extract was inhibitory at the most on these cultivars (Chon and Kim, 2004). The most stimulatory effect of extract on the root length was related to Tabesh and Tarem. Hence, where a strong establishment of cultivars or/and higher rate of oxygen is required and there is a high population of weeds and hence high competition, these cultivars may be planted. Like the root length, Fajr and Neda had the highest shoot growth, but their growth was also inhibited at the most when subjected to the extracts. When there is a high growth of weeds the suggested cultivars may be planted, since due to their high growth rate they are able to compete with weeds.

The ratio of root/shoot length and dry weight may be a good determining factor for the selection of the most suited cultivars for planting under different conditions. For example, if the shoot growth is higher than root growth the plant may be able to compete with weeds that have a higher shoot growth and vice versa. Under stress plants usually allocate more carbon to their roots than their shoots to handle the stress (Miransari and Smith, 2007, 2008; Miransari et al., 2007, 2008).

It is likely, that different rice cultivars, which are able to grow more efficiently when subjected to allelopathic chemicals either can use some mechanisms to neutralize the unfavorable effects of such products on plant growth or their genes, are not expressed by such products. It is interesting to recognize such mechanisms, which are probably more related to genetically modified cultivars.

Scientists have stated that the allelopathic ability of rice cultivars may account for 34% of the competing ability of rice with weed plants. In addition to phenolic acids other products may also be involved in the phytotoxicity potential of rice such as momilactone (Kato-Noguchi et al., 2002). It is also worth mentioning that rice residues have a great ability to suppress the growth of plant weeds, for example Xuan et al. (2005) stated that rice residues inhibited the growth of weed plants at 70% and increased crop yield at 20%. Hence, it is very important to determine both the allelopathy and phytotoxicity potential of rice cultivars.

There are different kinds of interactions between different organisms including plant-plant interactions. Both the plants and their straw are capable of influencing the soil environment through exudation of different biochemical products. Such kind of behavior is affected by different soil, plant and climate parameters. Hence, it is of great significance to determine how different plant species or cultivars may interact in a cropping strategy.

The results of this research work clearly indicate that there are interactions among different rice cultivars that can definitely affect their growth in the field. Although such a trait may be cultivar dependent, however the results of these experiments indicate that different rice cultivars own such kind of behavior, but with different intensity. Thus, it is a necessity that farmers know that for a more efficient rice crop production such kind of responses must be exactly elucidated so that appropriate rice cultivars in a proper sequence are planted.

The high and significant correlation coefficients among different growth parameters verify the precision and accuracy of the two experimental methods. Hence, the produced extracts affected the growth of different rice cultivars in the two experiments similarly, which is another indication and verification to the effectiveness of the extracts. Accordingly, the conclusions drawn can be more generalized and be used for rice cropping under similar conditions.

CONCLUSION

According to the results, different rice cultivars behaved differently when subjected to rice extracts and the genetically modified ones behaved more efficiently when subjected to allelochemicals. Comparison of different cultivars can indicate that which cultivars be selected for plantation. The modified genotypes were less affected by the unfavorable effects of rice extracts and grew more efficiently.

The findings of this research work address the objective and indicate that there are some sort of interactions among different rice cultivars, affecting their growth and hence yield production. The results of this research work also indicate that rice extracts can substantially influence the growth and hence yield of other cultivars and their intensity is cultivar dependent. Hence, determination of rice biological effects including the stimulating and inhibitory effects on the growth of other rice cultivars is of great importance and must be used for planning different biological and agricultural strategies. This can be very important to the rice farmers to plan their cropping strategy with respect to such responses. It can also be suggested that recognition and modification of the genes that are responsible for such behaviors in rice cultivars can very much help to alleviate the adverse effects of rice extracts on the growth of other cultivars and hence, increase rice yield production. The significance of this research is that there are little data on rice autotoxicity and accordingly some interesting strategies have been suggested to enhance rice production.

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