Does Soil under Natural Tithonia diversifolia Vegetation Inhibit Seed Germination of Weed Species ?

Pot experiment was carried out in the screen house, Ladoke Akintola University Technology Ogbomosho, Nigeria to determine the possible impact of Tithonia diversifolia on the growth of thirteen selected weed species weeds growing in its surroundings. The study consisted of two treatments (Tithonia diversifolia infested and Non-Tithonia diversifolia infested soils) and from the two media, the growth of A. hispidium, B. pilosa E. heterophylla, P. maximum and P. polystachion was significantly affected in soil infested by T. diversifolia. The number of weed seedling emergence afore mentioned was significantly lower than what was obtained in soil not infested with T. diversifolia and this accounted for about 38% of the tested weed species. Germination of four of these weeds species (23%) (A. spinosus, C. viscosa, T. procumbens and D. gayana) was enhanced by the presence of T. diversifolia. The study further revealed that weed counts in T. diversifolia infested soil is significantly lower than the ones in soil without T. diversifolia infestation. Likewise, the vegetative growth of some species (A. spinosus, C. viscosa, T. procumbens and D. gayana) was improved in this soil. This shows that T. diversifolia infested soil contains allelochemicals that performed both stimulatory and inhibitory functions.


Introduction
Weed infestation is ranked the greatest problem in agricultural systems [1], causing crop yield losses.Attention towards reducing dependency on herbicide has heightened interest in weed management strategies that combine more efficient use of herbicides with increased use of biologically based weed management methods [2].
Chemicals that are released from plants which impose negative influence on other plants are called allelochemicals or allelochemics [3].Allelochemicals that are toxic may inhibit shoot/root growth, nutrient uptake, or may attack a naturally occurring symbiotic relationship thereby destroying the plant's usable source of a nutrient [3].The consequent effects may be inhibited or retarded germination rate, reduced root or radicle and shoot or coleoptile extension, lack of root hairs, swelling or necrosis of root tips, curling of the root axis, increased number of seminal roots, discolouration, reduced dry weight accumulation and lowered reproductive capacity [4].Plants in the Asteraceae family like T. diversifolia and T. rotundifolia have been reported to exhibit allelopathic traits [5,6].
Different plant parts, including flowers, leaves, leaf litter and leaf mulch, stems, bark, roots, soil and soil leachates and their derived compounds, can have allelopathic activity that varies over growing seasons [7,8].When susceptible plants are exposed to allelochemicals, germination, growth and development may be affected.The most frequently reported gross morphological effects on plants are inhibited or retarded seed germination, deprivative effects on coleoptile elongation and on radicle, shoot and root development.
Allelopathic inhibition is complex and can involve the interaction of different classes of chemicals like phenolic compounds, flavonoids, terpenoids, alkaloids, steroids, carbohydrates, and amino acids, with mixtures of different compounds sometimes having a greater allelopathic effect than individual compounds alone [9].Furthermore, physiological and environmental stresses, pests and diseases, solar radiation, herbicides and less than optimal nutrient, moisture, and temperature levels can also affect allelopathic weed suppression [8].
The current trend in agricultural practices which discourages the use of inorganic external input in crop and animal production makes research in allelopathy important.This is because the use of inorganic input is contributory to solve many of the problems confronting adequate food production which is void of many synthethic pesticides (herbicides inclusive).Also, in organic cropping systems where synthetic herbicides are not used, crop cultivars with enhanced allelopathic activity could be part of the weed management strategy.Weed control mediated by allelopathy-either as natural herbicides or through the release of allelopathic compounds from a living crop cultivar or from plant residues is often assumed to be advantageous for the environment compared to synthetic herbicides.In view of the fact that allelochemicals are derived from natural sources, several authors were of the opinion that these allelopathic compounds will be biodegradable and less polluting to the environment than conventional herbicides [10][11][12].
Reference [16] examined the toxic effect of four legumes and reported that the aqueous leachates (1%) of all the four legumes exhibited strong phytotoxic effect on the radical growth of barnyard grass (Echinochloa crusgalli L. P. Beauv.), alegría and amaranth (Amaranthus hypochondriacus L.).Similarly, the allelopathic potential of Ipomoea was described by [17,18] identified Tricolorin A as the major phyto-growth inhibitor from the resin glycoside mixture of the plants.
According to references [19] isothiocyanates contained in Brassica spp were strong suppressants of germination on some tested weed species Spiny sow thistle (Sonchus asper L. Hill), Scentless mayweed (Matricaria inodora L.), Smooth pigweed (Amaranthus hybridus L.), Barnyard grass (Echinochloa crusgalli L. Beauv.),Black grass (Alopecurus myosuroides Huds.) and wheat crop (Triticum aestivum L.).Reference [20] studied the allelopathic effect of black mustard (Brassica nigra L.) on germination and seedling growth of wild oat (Avena fatua L.); these authors found that germination and radicle length were affected by extracting solutions and the inhibitory effect on germination increased with increasing concentration of extract solution of the fresh plant parts.
Congress grass (Parthenium hysterophorus L.) was found to show allelopathic effect due to the presence of parthenin, a sesquiterpene lactone of pseudoguanolide nature in various parts of the plant [21][22][23].Parthenin is known to have specific inhibitory effects on root and shoot growth of Crotalaria mucronata L., Cassia tora L., Oscimum basilicum L., Oscimum americanum L. and barley (Hordeum vulgare L.) [24,25].Various phenolic compounds identified in Parthenium (caffeic, vanillic, ferulic, chlorogenic and anisic acid) [21,26,27] may be responsible for growth reduction of test crops in amended soils.
Reference [28] investigated the allelopathic effects of Croton bonplandianum weed on seed germination and seedling growth of crop plants (Triticum aestivum L., Brassica oleracea var.botrytis L. and Brassica rapa L.) and weed plants (Melilotus alba Medik, Vicia sativa L. and Medicago hispida Gaertn).Leaf extract was found to be the most allelopathic and growth inhibition effect was found to increase with increasing concentrations of different aqueous extracts.
Russian knapweed (Acroptilon repens L.) is a widely distributed and problematic weed of the Western United State.[29,30] found that the roots of A. repens inhibited the root growth of many plants including some weed species such as Lactuca sativa, Medicago sativa, Echinochloa crusgalli and Panicum miliaceum by 30% at concentrations comparable to those found in the soil surrounding of A. repens plants.Moreover, the germination of Agropyron smithii and Bromus marginatus was inhibited by aqueous leaf extracts of A. repens at high level concentrations, however, according to [31], germination was induced by lower concentrations.The objective of this study, therefore, is to determine whether Tithonia diversifolia can inhibit the growth of weeds growing in its surroundings and identify the affected weed species.

Materials and Methods
The experiment was carried out in the screen house, Ladoke Akintola University Technology Ogbomosho, Nigeria

Collection of Soil Samples
A plot heavily infested by Tithonia diversifolia was selected for soil sampling for the experiment.The plot was adjudged heavily infested as a result of this weed constituting more than 90% of the total identifiable weed species.Other weed species present on the plot were Imperata cylindrica, Boerhavia diffusa, and Ageratum conyzoides.Non Tithonia infested soil has no T. diversifolia growing on it.Soil sampling of the T. diversifolia infested field and Non T, diversifolia infested field was done at the depth of 0 -15 cm of the soil.

Soil Processing
The samples were passed through just three stages of processing before being subjected to laboratory analysis: 1) Crushing; Large soil clods were crushed to facilitate drying; 2) Air drying of the soil samples from the two different locations separately for a week under a condition that prevented contamination and finally; 3) Sieving of the soil samples through a 2 mm brass sieve.

Laboratory Analysis
The physical and chemical properties of the two soil locations were determined at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria.

Pot Preparation
A total of 104 pots were used for the experiment with 52 pots for each treatment replicated four times.These pots were perforated at the base to prevent water logging and filled with 2 kg soil each.The pots were laid-out in a Completely Randomized Design (CRD).

Sowing
Twenty seeds of each of the test weeds were sown in each of the treatments and replicated four times.Pots were irrigated every other day to facilitate germination.Emergence of young seedlings was observed from two weeks after planting (WAP).

Data Collection
Data were collected every week after seedling emergence on population of weed seed that emerged in each of the soil medium.

Statistical Analysis
The collected data was subjected to Analysis of Variance (ANOVA) and means were separated using LSD at 5% probability level.The result soil chemical analysis of the two locations (Tithonia-infested and Non-Tithonia-infested soils were correlated with weed seedling emergence at 6 WAP to determine the relationship between soil chemical parameters and weed seed emergence.

Soil Physical and Chemical Parameters
The result of Physico-chemical parameters of soil in the two locations where soil samples were collected is shown in Table 1.From the result, the two soils differed in terms of pH and exchangeable acidity, but the level of Nitrogen, and Potassium were very close.The pH values show that the location with Tithonia diversifolia infestation was alkaline (8.4) while the location without T. diversifolia infestation was acidic (4.9).The T. diversifolia infested soil was higher in Ca, Zn and Fe composition (4.68, 188.59 and 136.89 ppm) respectively than the location without T. diversifolia infestation (1.07, 24.17 and 84.73 ppm) respectively but lower in Cu composition.The sand, silt, and clay composition of these locations were the same.

Weed Emergence
The performances of the 13 weed species planted in Tithonia infested and Non-Tithonia infested soil are shown in Table 2. Amongst the weeds, 69% of the thirteen species were broadleaved while the remaining 31% were grasses.Out of the weed lot, the growth of A. hispidium, B. pilosa E. heterophylla, P. maximum and P. polystachion were significantly affected in soil infested by T. diversifolia.The number of weed seedling emergence afore mentioned were significantly lower than what was obtained in soil not infested with T. diversifolia and this account for about 38% of the tested weed species.At 1 WAP A. hispidium, an average of almost five (5) plants were recorded in Non-Tithonia infested soil while less than one plant (<1) on the average was recorded in soil infested with T. diversifolia.This trend continued until 6 WAP when 16 seedlings of A. hispidium which accounted for 78% of the seeds planted/pot were recorded in Non-T.diversifolia infested soil while only approximately four seedling of A. hispidium was recorded in T. diversifolia infested soil.The same trends was observable in the growth pattern of B. pilosa, E. heterophylla, P. maximum, P. polystachion and W. indica.It was also noted that statistically, the seedling emergence of W. indica in both T. diversifolia infested and Non-T.diversifolia infested soil media were not statistically significant for 1 WAP and 3 WAP but subsequently significant difference were observed.
The presence of T. diversifolia favoured the germination and seedling emergence of four of these weeds species (23%) and these include A. spinosus, C. viscosa, T. procumbens and D. gayana.Throughout the six weeks of the experiment, A. spinosus almost have no seedling emergence on the average for the six weeks except week two when average of two seedlings were recorded but these seedlings withered before the end of the experiment Germination of Fibristylis littoralis, Hyptis suaveolens and Senna occidentalis seeds were generally poor.These weed species were not responsive in either T. diversifolia infested soil or Non-T.diversifolia infested soil.It was observed that the first emergence on Fibristylis littoralis pots was noticed on the third week when an average of less than one (<1) seedling was recorded in the two soil media.Senna occidentalis has lower germination from the first week but did not increase appreciably throughout the 6 week experimental period.
Based on the response of weed species to their growth in the two media, A. hispidium, B. pilosa, P. maximum and P. polysachion showed the highest response to the T. diversifolia infested soil media during the six weeks of the study with reduced number of seedling that emerged when compared with Non-T.diversifolia infested soil (Table 3). A. hispidium, on infested soil, has as low as 0.8 plant/pot average seedling during the first week after planting and 3.5 plants/pot was recorded at 6 WAP while in non-infested soil, 4.8 plants/pot was recorded in first weed after planting and was significantly greater than the number of seedling recorded at 6 WAP for T. diversifolia infested soil.At 6 WAP, an average of 78% (15.5 plants/pot) of A. hispidium seeds planted in soil not infested with T. diversifolia soil germinated.The same trend was observed for B. pilosa, P. maximum and P. polysachion which belong to the category of weed that were significantly affected by T. diversifolia infested soil.
Statistically, the mean of seedling recorded at E. heterophylla pot was not significant but there a slight differential in average number of seedling recorded.For example, at 1 WAP, the number of E. heterophylla seedling for both T. diversifolia infested and non T. diversifolia infested soil were 5.5 and 10.3 plants/pot respectively and at 6 WAP, the average number of E. heterophylla seedlings was 10.5 and 16.5 plants/pot for T. diversifolia infested and non T. diversifolia infested soil respectively.The seedling emergence of A. spinosus, C. viscosa, T. procumbens and D. gayana were not affected by T. diversifolia infested soil.Among all the weed seeds planted, F. littoralis had average of 0.3 and 0.8 plants/pot in both T. diversifolia infested soil and non T. diversifolia infested soil respectively at 3 WAP.There was no emergence of the weed at 1 and 2 WAP.A highest average of 1.5 plants/pot was recorded on soil not infested soil.

Correlation between Weed Seed Emergence and Some Selected Soil Chemical Parameters
Correlation between soil physico-chemical parameters and weed emergence in the two soil media were shown in Tables 4 and 5.The correlation of soil with T. diversifolia infestation and weed emergence showed that only two weed species (C.viscosa and F. littoralis) showed correlation that was significant (Table 4).It was obvious

Walteria indica 
* Key to Rating: High: < 30% of seeds planted/pot in T. diversifolia infested soil germinated by sixth week; Moderate: <50% of the seeds planted/pot in T. diversifolia infested soil germinated by sixth week; Low: <75% of the seeds planted/pot in T. diversifolia infested soil germinated by sixth week; Non: Induces the growth of the weed seeds.that there was significant positive correlation between C. viscosa and iron (Fe) content of the soil with correlation coefficient of 0.99.Moreover, F. littoralis was positively correlated to calcium (Ca) 0.95 and negatively correlated to Fe with coefficient of −0.95.In non T. diversifolia infested soil (Table 5), only two weed species showed significant correlation to soil chemical properties.C. viscosa was positively correlated to pH (0.99) and D. gayana was positively correlated Ca (0.95) and Zn (0.95) and negatively correlated to acidity (−0.94).The rest were not statistically significant.

Discussion
The presence of T. diversifolia on arable field could suggest several changes which may be taken place within such ecosystem as indicated in this study.From the soil analysis carried out on the soil of the two locations (T.diversifolia infested soil and Non T. diversifolia infested soil), the differential pH in soils from the two locations could be one of the factors that affect the germination of the weed seeds.References [32] reported that the germination of Texas weed (Caperonia palustris) increased at pH between 4 and 8 and decreasing germination at pH levels of 9 and 10.This differential pH could be as a result of impact of crop architecture of T. diversifolia which control the degree of exposure to environmental factors like erosion and other weathering activities since T. diversifolia is a shade plant.Several of the weed seeds introduced in T. diversifolia infested soil were suppressed at varying degrees.
E. heterophylla showed average susceptibility to T.diversifolia suppression as 50% of its total seeds introduced emerged.Although, it was observed that leaves of E. heterophylla appeared to be healthier in T. diversifolia infested soil than in soil without T. diversifolia.This may however be an indication of stimulatory effect of allelopathy.Reference [33]  is a potential green manure and organic fertilizer for vegetable crops.B. pilosa was the most susceptible to T. diversifolia suppression out of the selected weed plants as only 5% emerged.The result obtained in this study was similar to the work of [34] who studied the allelopathic potential of aqueous extracts from the aerial part of L. leucocephala on Desmodium purpureum, B. pilosa and Amaranthus hybridus L and found out that B. pilosa and A. hybridus were the most sensitive species to the extract in the bioassays.However, some workers have also observed inhibitory effects of some plants on other test plants.References [35] reported that Chromolaena odorata contains a large amount of allelochemicals especially in the leaves, which inhibit the growth of many plants in nurseries and plantations.Reference [36] has demonstrated that aqueous extract and shoot extract of T. diversifolia was inhibitory to the germination and growth of Amaranthus cruentus.Similarly, reference [37] has reported that the bark, leaf and leaf extract of Quercus glauca and Q. leucotricophora significantly reduced germination, plumule and radicle length of wheat (Triticum sp.) and mustard seeds.Earlier investigators have suggested that allelochemicals or toxins are released from the weed by the action of micro-organisms during decomposition, which may interfere with the plant growth processes [38].
This study reveals that weed counts in T. diversifolia infested soil is significantly lower than the ones in soil without T. diversifolia infestation.Likewise, the vegetative growth of some species (A.spinosus, C. viscosa, T. procumbens and D. gayana) was improved in this soil.This shows that T. diversifolia infested soil contain allelochemicals that performed both stimulatory and inhibitory functions.

Conclusion
It is clear from our discussion that there is immense Open Access AJPS Does Soil under Natural Tithonia diversifolia Vegetation Inhibit Seed Germination of Weed Species? 2172 prospect of allelopathic mechanism as a weed management tool.Impacts of allelopathy on different weed species have been identified.In spite of that, some factors have to be considered before application of allelochemicals as natural herbicide and such factors include soil properties, type of weed species to be controlled and time of application of the allelopathic mechanism in controlling specific weed species.

NTS LSD TIS NTS LSD TIS NTS LSD TIS NTS LSD TIS NTS LSD TIS NTS LSD
on non-T.diversifolia infested soil.When this result was compared to what was obtainable on T. diversifolia infested soil, A. spinosus seed germinated and seedling emerged from one (1) at 1 WAP to four seedings between 4 -6 WAP.The same observation was made on C. viscosa, T. procumbens and D.gayana; the number of these weed seeds (A.spinosus, C. viscosa, T. procumbens and D. gayana) that emerged on soil not infested with T. diversifolia was significantly lower than the number of weed seed that emerged on soil infested with T. diversi-folia.

Table 5 . Pearson correlation coefficients of weed emergence at 6 weeks after planting and chemical parameters in soil under non-Tithonia infested field.
reported that T. diversifolia