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Research Journal of Botany

Year: 2007  |  Volume: 2  |  Issue: 1  |  Page No.: 23 - 32

Phytotoxic Effects of Tithonia diversifolia on Germination and Growth of Oryza sativa

O.J. Ilori, O.O. Otusanya and A.A. Adelusi

Abstract

The effect of phytotoxins produced from Tithonia diversifolia weed was investigated with a view to understanding the strategies used by this weed to suppress the growth of associated crops. The study examined the susceptibility of Oryza sativa Linn. to phytotoxic effects of Tithonia diversifolia (Hemsl) A. Gray, compare the phytotoxicity of the plant parts of T. diversifolia and determine the effect of drying on the phytotoxic activity of this weed. The effects of the aqueous extracts prepared from the shoots and roots of T. diversifolia on Oryza sativa were obtained by subjecting the seedlings to the different aqueous extract treatments. Results showed that the growth parameters and fresh and dry matter production of Oryza sativa were retarded by all the four different aqueous extracts applied. The retardation was more pronounced in older plants. The degree of retardatory effects of the aqueous extracts were found to follow this order: Fresh shoot aqueous extract (FSE)>dry shoot aqueous extract (DSE)>fresh root aqueous extract (FRE)>dry root aqueous extract (DRE). There was a significantly higher phytotoxic potency at p<0.05 in the fresh tissue aqueous extract compared with the dry tissue aqueous extract as well as in the shoot aqueous extract compared to the root aqueous extract. This study concluded that Tithonia diversifolia was an allelopathic weed with water-soluble allelochemicals in its plant parts and had such phytotoxic potency that could suppress the growth of associated crop plants.

LA = 0.5 (LxW) L = Length of leaf W = Maximum width

Leaf Area Ratio (LAR) was calculated using the formula of West et al. (1920)

Five seedlings were randomly harvested in each regime. Each seedling was separated into shoot and root. The fresh plant parts were then weighed on a Meltler Toledo balance to obtain the fresh weight of the plant parts. Five seedlings were randomly harvested in each regime and each seedling was separated into shoot and root. The plants parts were then packaged separately in envelopes and dried to constant weight at 80°C in a GallenKamp (Model IH-150) incubator. The dried plant parts were weighed on a Meltler Toledo balance to obtain the dry weights and then mean weights were calculated. All experiments were conducted in five replicates and the data obtained was subjected to appropriate statistical analysis. Analysis of variance (ANOVA) was carried out for all the data. Treatment means were compared using least significant difference (LSD p<0.05).

RESULTS

Percentage germination in the treatment regimes was found to be only slightly variable from that of the control regime (Table 1). Table 2 shows the effect of the application of the different aqueous extract treatment on the plumule length of the germinating seedlings of Oryza sativa L. The plumule length of the seedlings in the control regime was found to be significantly higher than that of the seedlings in all the other treatment regimes at p<0.05. The plumule length of the seedlings in the dry shoot aqueous extract regime was found not to be significantly different from that of the seedlings in the fresh shoot aqueous extract treatment regime at p<0.05.

The effect of the application of the different aqueous extract treatments on the radicle length of germinating seedlings of Oryza sativa L. is shown in Table 3. The radicle length of the control seedlings was significantly higher than the seedlings in the fresh shoot and dry shoot aqueous extract treatment regimes. The seedlings treated with fresh shoot aqueous extract regime were observed to have a radicle length that was significantly shorter than that of the seedlings in other treatment regimes. The radicle length of the seedlings in the fresh shoot aqueous extract (FSE) regime was significantly different from that of the seedlings in the dry shoot aqueous extract (DSE). The radicle length of the seedlings in the fresh root aqueous extract (FRE) regime and that of the seedlings in the dry root aqueous extract (DRE) regime were not significantly different at p<0.05.


Table 1: Effect of aqueous extracts treatments on the germination of the seed of Oryza sativa L.

Table 2: Effect of the application of fresh shoot, dry shoot, fresh root and dry root aqueous extract treatments on the Plumule length of the germinating seedlings of Oryza sativa L.
FSE = Fresh shoot aqueous extract treatment, DSE = Dry shoot aqueous extract treatment, FRE = Fresh root aqueous extract treatment, DRE = Dry root aqueous extract treatment

Table 3: Effect of the application of fresh shoot, dry shoot, fresh root and dry root aqueous extract treatments on the radicle length of the germinating seedlings of Oryza sativa L.
FSE = Fresh shoot aqueous extract treatment, DSE = Dry shoot aqueous extract treatment, FRE = Fresh root aqueous extract treatment, DRE = Dry root aqueous extract treatment

Table 4: Effect of aqueous extract of Tithonia diversifolia on shoot fresh weight of Oryza sativa
Fresh weight: g

The fresh weight of shoot of the seedlings in the fresh shoot aqueous extract treatment regime remained lower than that of the seedlings in the other extract treatment regimes throughout the duration of the experiment. The fresh weight of the shoot of the seedlings in the control regime was significantly higher than that of the seedlings in all the aqueous extract treatment regimes. There were also significant differences between the fresh weight of the shoot of the seedlings belonging to the FSE and DSE regimes and between that of the seedlings belonging to the FRE and DRE regimes at p<0.05 (Table 4). The fresh weight of the root of the seedlings in the control regime was higher than that of the aqueous extracts treated seedlings during the last three weeks of the experiment (Table 5). During the last three weeks of the experiment, the fresh weight of the root of the seedlings in the control regime was significantly higher than that of the seedlings in the treatment regimes at p<0.05. Significant differences were also observed between the root fresh weights of the seedlings in the FSE and DSE regimes and between the fresh weight of the root of the seedlings in the FSE and FRE regimes in the last three weeks of the experiment. The fresh tissues being more phytotoxic than the dry tissues.

The effect of different aqueous extracts of Tithonia diversifolia (Hemsl) A. Gray on dry weight of the shoot of Oryza sativa L. is shown in Table 6. The dry weight of the shoot of the seedlings treated with fresh shoot extract (FSE) remained almost constant during the first three weeks of the experiment and then increased gradually until the end of the experiment. The dry weight of the shoot of the seedlings in the control regime remained higher than that of the seedlings in the extract treatment regimes during the last three weeks of the experiment while the dry shoot weight of the seedlings treated with fresh shoot aqueous extract was lower than that of the seedlings in the other treatment regimes throughout the duration of the experiment. The shoot dry weight of the seedlings in the FSE and DSE regime were lower than that of the seedlings in the FRE and DRE regimes respectively. Significant differences were observed between the dry weights of shoot of the seedlings in the control regime and that of the seedlings treated with different aqueous extracts, between the dry weight of shoot of the seedlings in the FSE and FRE regimes and finally between the dry weight of the shoot of the seedlings in the DSE and DRE regimes at p<0.05. Table 7 shows the effect of different aqueous extracts of Tithonia diversifolia (Hemsl) A. Gray on the dry weight of the root of Oryza sativa L.


Table 5: Effect of Aqueous Extract of Tithonia diversifolia on root fresh weight of Oryza sativa
Fresh weight: g

Table 6: Effect of aqueous extract of Tithonia diversifolia on shoot dry weight Oryza sativa
Dry weight: g

Table 7: Effect of aqueous extract of Tithonia diversifolia on root dry weight of Oryza sativa
Dry weight: g

Table 8: Effect of aqueous extract of Tithonia diversifolia on plant height of Oryza sativa
Plant height: cm

Table 9: Effect of aqueous extract of Tithonia diversifolia on root length of Oryza sativa
Root length: cm

The dry weight of the roots of the seedlings treated with Fresh Shoot aqueous Extract (FSE) was lower than that of the roots of the seedlings treated with the other aqueous extracts (Table 7). There was a significant difference between the dry weight of the root of the seedlings in the control regime and that of the seedlings in the different aqueous extract treatment regimes at p<0.05. The dry weight of the roots of the seedlings in the FSE and FRE regimes and that of the seedlings in the DSE and DRE regimes were significantly different at p<0.05.

The shoot height of the seedlings in the control and all the treatment regimes increased steadily from the beginning until the end of the experiment. The shoot height of the control seedlings was observed to be slightly higher than that of the seedlings in the two regimes treated with root aqueous extracts (FRE and DRE) which in turn were higher than that of the seedlings in the two regimes treated with shoot aqueous extracts (FSE and DSE) throughout the duration of the experiment. The height of the seedlings treated with the dry shoot aqueous extract and dry root aqueous extract were found to be slightly higher than those treated with fresh shoot aqueous extract and fresh root aqueous extract respectively (Table 8). The seedlings in the control regime had root length that was longer than that of the seedlings in the FRE and DRE regimes which in turn were longer than that of the seedlings in the FSE and DSE for the greater part of the experiment (Table 9). The root lengths of the seedlings in the regimes treated with the dry shoot aqueous extract and the dry root aqueous extract were found to be slightly longer than that of the seedlings in the regimes treated with the fresh shoot aqueous extract and fresh root aqueous extract respectively. The root length of the seedlings in the control was significantly different from that of the seedlings in the FSE, DSE and FRE regimes at p<0.05. There were significant differences at p<0.05 between the root lengths of the seedlings belonging to the FSE and DSE and between the root lengths of the seedlings in the FRE and DRE regimes p<0.05.

The leaf area of the seedlings in the control regime was higher than that of the seedlings in the other regimes while the leaf area of the seedlings treated with fresh shoot aqueous extract had the lowest value (Table 10). The leaf area (LA) of the seedlings in the control regime was significantly higher than that of the seedlings in the other regimes at p<0.05. There was significant difference between the LA of the seedlings treated with fresh tissue aqueous extracts and that of the seedlings treated with dry tissue aqueous extracts at p<0.05.


Table 10: Effect of aqueous extract of Tithonia diversifolia on leaf area of Oryza sativa
Leaf area: cm2

Table 11: Effect of different extracts on leaf area ratio of Oryza sativa
Leaf area ratio: cm2/g

There were also significant differences between the LA of the seedlings treated with the fresh shoot aqueous extract and that of the seedlings treated with the dry shoot aqueous extract and between the LA of the seedlings treated with fresh root aqueous extract and that of the seedlings treated with dry root aqueous extract at p<0.05. The leaf area ratio did not show a regular pattern during the period of treatment (Table 11). The leaf area ratio of the seedlings in the fresh root aqueous extract attained the highest peak. Significant differences were observed between the leaf area ratio of the seedlings in the control and that of the seedlings in the different aqueous extract treatment regimes, between the FSE and FRE treatment regimes and the LAR of the seedlings in the DSE and DRE regimes and finally between the LAR of the seedlings in the FSE and DSE regimes at p<0.05.

DISCUSSION

Evident from the results obtained from this study was the fact that the mean percentage germination of Oryza sativa L. seeds in the control regime was not significantly different from that of the seeds in the treatment regimes. This clearly showed that germination of Oryza sativa L. was not significantly retarded by the allelochemicals present in the extracts prepared from the shoot and root of Tithonia diversifolia. This finding correlated with that of Tongma et al. (1998) who observed that the germination of Oryza sativa L. seeds was not affected when grown in soil previously planted with Tithonia diversifolia.

The findings of this study indicated that plumule length of Oryza sativa L. seedlings in all the four aqueous extract treatment regimes was inhibited by the aqueous extract of Tithonia diversifolia. Though the allelochemicals in the aqueous extracts had no remarkable retardatory effects on the germination of Oryza sativa L. the growth of the plumule of the germinating seedlings was inhibited. The radicle growth of germinating Oryza sativa L. seedlings treated with the aqueous extract prepared from fresh and dried shoot of Tithonia diversifolia was observed to be inhibited. A similar result was obtained by Rahman (1995) on the effect of aqueous extract derived from the inflorescence, stem and leaves of Barthenium hysterophorus L. on the growth of radicle and plumule of Cassia sophera L. However, in this study, the aqueous extracts prepared from the fresh and dried root of Tithonia diversifolia did not affect the radicle growth of germinating seeds of Oryza sativa L. This probably could be attributed to low concentration of allelochemicals in the two root aqueous extracts. In support of this was the finding of Miller (1996) who stated that water extract of top growth of Medicago sativa L. produced more allelopathic effect on seedlings than extracts from the roots.

The fresh weight and dry weight of the shoot of the control seedlings of Oryza sativa L. remained highest in most parts of the experiment and was significantly different from that of the shoot of the seedlings in the different aqueous extract treatment regimes. This result agreed with that of Ahn and Chung (2000) who found that aqueous extract of rice hull inhibited the shoot fresh weight of Barnyard grass (Echinochloa crusgalli). The root fresh weight of aqueous extract treated seedlings of Oryza sativa L. were observed to be significantly reduced when compared to that of the control seedlings. Huber et al. (2002) had earlier observed that exogenously applied phenolic acids reduced root fresh weight and dry weight of soybean. Although the aqueous extracts prepared from the shoot and root of Tithonia diversifolia were observed to retard the shoot height of Oryza sativa L., it was however evident that the shoot extracts were more phytotoxic and had more inhibitory effect on the shoot height of the treated seedlings than the root aqueous extracts. The shoot height of Oryza sativa L. seedlings treated with the dry shoot aqueous extract and dry root aqueous extract were higher than those treated with the fresh shoot and fresh root aqueous extracts respectively. The drying process could have reduced the amount of volatile allelochemical in these plant tissues hence the low inhibitory effect of the extract prepared from the dried tissue. It has been fairly well established that root length was more sensitive to phytotoxic compounds than either seed germination or shoot elongation in many crops (Hall and Hendrlong, 1989; Kuiters, 1980; Luu et al., 1989; Hedge and Miller, 1990). Huber et al. (2002) showed that exogenously applied phenolic acids reduced root length of soybeans. In this work, the root length of the treated seedlings of Oryza sativa L. was markedly reduced by aqueous extract treatments applied. This indicated that the extracts applied contain some growth inhibitory substances in amount sufficient to suppress the growth of the root of these seedlings. Variation in the root length of the control and treated seedlings followed the same pattern as observed for the shoot height. The shoot aqueous extract regimes had seedlings with shortest root length during most part of the experiment. This observation was supported by the findings of Eze and Gill (1992) who stated that Chromolaena odorata L. a related weed also in the family Asteraceae had a high concentration of allelochemicals especially in its leaves.

Canston and Venus (1981) were of the opinion that leaves are the most important photosynthetic producers of the plant. According to these workers, light interception and photosynthetic rate depend to a large extent upon the available leaf area. Therefore, the amount of light intercepted is assumed to be directly proportional to the leaf area. In this study, the leaf area of seedlings in the control regime was significantly higher than that of seedlings in all the aqueous extract treatment regimes. That is, the application of the different aqueous extract was observed to have reduced the leaf area of these seedlings. This observation was consistent with the findings of Patterson (1981) who detected that the application of some synthetic allelochemicals reduced the leaf area of soybean.

The growth of Tithonia diversifolia in association with cultivated crop many lead to reduction in growth of these crops. It is therefore required that the weed be controlled in the crop fields.

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