Introduction

A number of weed and crop species have been reported to possess allelopathic activity on the growth of other plant species (Rice, 1984). Chemicals with allelopathic activity are present in many plants and in many organs, including leaves, flowers, fruits and buds (Ashrafi et al., 2007). Allelopathy is derived from the Greek words “allelon "of each other" and pathos "to suffer" (Rizvi et al., 1992). Allelopathy term was coined by Molisch (1937), as chemicals with inhibitory activity are present in many plants and in many organs, including leaves, flowers, fruits and buds (Inderjit, 1996; Ashrafi et al., 2007). In agricultural practice, allelopathy is exploited for weed control (Kohli et al., 1998). Allelopathy is a natural ecological phenomenon in which different organisms affect the functioning of other organisms in their vicinity, negatively or positively (Rice, 1984) by releasing secondary metabolites (Farooq et al., 2011).

According to Farooq et al., 2011, chemicals thus released; the allelochemicals, are mostly secondary metabolites, which are produced as by products during different physiological processes in plants. In present agricultural systems heavy amounts of synthetic chemicals are being used to control weeds and other pests. But the adverse impact of these chemicals on the environment has made it necessary to search substitute weed control strategies. The current trends in agriculture production are to find a biological solution to reduce the apparent harmful impacts from herbicides and pesticides (Khanh et al., 2005). Plant allelopathy offers a great prospective to resolve this critical issue and may be used in different ways to manage weeds that includes, the use of allelopathic crop residues as surface mulch (Jung et al., 2004), water extracts (Javaid et al., 2006), intercropping (Baumann et al., 2002; Hatcher and Melander, 2003; Iqbal et al., 2007), cover crops, cropping rotations (Narwal, 2000), selection of allelopathic crop varieties (Ma et al., 2006) and identification of new herbicides chemistry (Duke et al., 2001).

Azadirachta indica, or Neem Tree, is an evergreen tree native to Southeast Asia. All parts of the tree have been used medicinally for centuries. Because neem contains a number of useful chemicals, with multiple uses and adaptability to diverse habitats and climatic conditions, interest in the tree has increased.

These compounds include nimbin (anti-inflammatory), nimbidin (anti-bacterial, anti-ulcer, analgesic, antiarrhythmic, anti-fungal), nimbidol (anti-tubercular, anti-protozoan, anti-pyretic), gedunin (vasodilator, anti-malaria, anti-fungal), sodium nimbinate (diuretic, spermicide, anti-arthritic), queceretin (anti-protozoal), salannin (repellent), and azadirachtin (repellent, anti-feedant, anti-hormonal) (Sankaram, 1987). Under certain conditions, these compounds are released into the environment, either as exudates from living tissues or by decomposition of plant residues in sufficient quantities to affect neighboring or successional plants (Ashrafi et al., 2007).

Rice husk is the natural sheath or productive cover, which forms the cover of rice grains during their growth. Rice husk represents about 20% by the weight of the rice harvested, about 80 by weight of the raw husk is made of organic components (Anonymous, 1979). During rice refining processes, the husks are removed from grains. It is of little commercial value and because of its high silicon dioxide content; it is not useful to feed either human or cattle. Incorporation of rice husk into soil mixture was found to affect many crops (Sharma et al., 1988). Soil organic matter content is gradually declining due to high cropping intensity which causes quick decomposition of organic matter.

Tomato (Solanum lycopersicum L.) is a warm-season crop with origins in elevated regions of Peru and Ecuador. A member of the Solanaceae family, tomato is the second most produced vegetable in the United States, behind the potato. The round, red-fleshed tomato predominates in the fresh market, but red- and yellow-fleshed round, plum (roma), cluster, cherry, grape, and mini-pear types are also available.

Lycopene in tomato is a fat-soluble red pigment produced by plants and some microorganisms. It represents the major carotenoid in tomatoes and is found to a lesser extent in guava, pink grape fruit, water melon and papaya. In contrast to other carotenoids, this lipophilic a cyclic isomer of b -carotene lacks vitamin A activity and, although it represents the most predominant carotenoid in human plasma that is enriched in (very) low-density lipoprotein fractions, no physiological function in humans has been described thus far.

1 Materials and Methods

1.1 Seed collection

Seeds of Tomato (Solanum lycopersicum) which were of different varieties namely NGB01301, NGB01232, NGB01363, NGB01655 utilized in the experiment were collected from The National Centre for Genetic Resources and Biotechnology (NACGRAB) in Ibadan, Oyo State. The soil was collected behind the screen-house of Adekunle Ajasin University Akungba Akoko. Plastic pots (20×30 cm; perforated at the bottom) were filled with the top soil (Sandy Loam). The treatments used include; neem leaf collected beside Pioneer hostel of AAUA and rice husk collected from rice mill at Igbemo-Ekiti, Ekiti State.

1.2 Experimental design

Factorial experiment under a complete randomized design (CRD) with four replications, three treatments and four varieties (Factorial 4×3×3) were conducted respectively. The experiment was carried out at the screen-house of the Plant Science and Biotechnology Department of Adekunle Ajasin University Akungba-Akoko.

1.3 Preparation of neem leaf aqueous extract

The part of the neem tree used is the leaf. The leaf samples were air dried for some days till it turns brown and pounded into powdery form. About 20 g of powered neem leaf was added into 2 000 ml of water and then vigorously homogenized, later soaked for 48 hours before filtering through a double folded muslin cloth. This was applied to the seedling at periods of pre-germination stage and flowering stage.

1.4 Preparation of rice husk:

The rice husk has been milled from the milling station, air dried and a quantity of 10 g is added to/mixed with the top soil prior to seed planting. This will then be applied to the seedling at periods of sowing stage, vegetative stage and flowering stage.

1.5 Soil treatment

The treatments were added to the pots a day prior to planting at 10 g of rice husk mixed with top soil and 100 ml of neem leaf extract is dispensed into the pots. Sowing of seeds was done 24 hours after treatment application. Subsequently, 500 ml of water was used to irrigate each plant every day.

1.6 Growth analysis

1.6.1 Measurement of plant height

The plant height was determined using meter rule to measure from soil level to the apical bud of the plant.

1.6.2 Measurement of total leaf area

The length and width of the leaves was determined by a digital area meter.

1.6.3 Determination of leaf numbers and branches

Leaf number was determined by counting the total number of leaves present on each branches of the plant. Numbers of branches was also counted manually on each plant.

1.6.4 Measurement of stem girth

The stem girth was measured by using digital vernier caliper at 5cm from the base of the stem.

1.6.5 Biomass determination

Leaf fresh and dry weight: The determination process was after experiment by weighing the fresh leaf using Metler Pc 180 weighing balance in gram and the oven dried leaf by using sensitive weighing balance.   

1.7 Yield components

1.7.1 Determination of fruit number

This is determined by manual counting of the number of fruits produced by each plant.

1.7.2 Fruit fresh weight and fruit dry weight determination

Measurement of the fruit fresh weight of the tomato plant was carried out after harvest using the Metler Pc 180 weighing balance in grams. The same method was used for the dried fruit; this is done after the fruit has been oven dried.

1.7.3 Length of fruits

This was determined by measuring the length of the fruits vertically using a ruler.

1.7.4 Diameter of fruits

This was determined by measuring the breadth of fruits using a ruler.

1.8 Statistical analysis

Data were statistically analysed using Statistical Analysis System (SAS) version 8.1 (SAS Institute, 2008). The treatment means were compared using a revised Least Significant Difference at the 0.05 level of Significance.

2 Results and Discussion

Allelopathy has long being in existence in plants but the study of allelopathy using plant extracts is an emerging research of interest. In this present study, there was variation in the growth of tomato (Solanum lycopersicum) using different soil treatment. Analysis of variance revealed significant differences in the plant morphology.

Allelopathic effect of the neem leaf aqueous extract (NLAE) treatment on the growth of tomato is shown in Figure 1. There were gradual increases in the plant height of the four varieties from the beginning of the experiment till the end of the experimental period. In contrast to this, Tran et al. (2004) who worked on the evaluation on phytotoxicity of neem (Azadirachta indica. A. Juss) to crops and weeds, result of the experiment showed that neem leaf strongly inhibits germination and growth of several specific crops: alfalfa (Medicago sativa L.), bean (Vigna angularis), carrot (Daucus carota L.), radish (Raphanu ssativus L.), rice (Oryza sativa L.), sesame (Sesamum indicum L.) and weeds. This present study do not support Indergit and Darkshini (1994) who also found out that the water extracts from the roots of Phichea lanceolata in the family Asteraceae inhibited the germination of tomato and mustard. However, this present study is not in agreement of Rawat et al. (2002) who observed that allelochemicals from aqueous extracts of sunflower (Helianthus annus L.) inhibited germination in some other species like linseed (Linum usitatisium) and mustard (Brassica juncea L.). Also, the water extract from tissues of sun flower (Helianthus annus) were also observed to inhibit germination of Solanum nigrum (Sedigheh et al., 2010). Nandal and Dhillon (2005) reported that the aqueous extracts of poplar leaves adversely affected the germination and seedling growth of some wheat varieties at high extract concentrations. Mulatu et al. (2006) reported that aqueous extract of Parthenium hysterophorus leaves and flower inhibited seed germination of lettuce. Preliminary investigations have revealed that the aqueous extract from the leaves of T. diversifolia retarded the germination and the radicle growth of Oryza sativa, Amaranthus cruentus, Capsicum annum and Lycopersicon esculentum (Ilori et al., 2007; Otusanya et al., 2007; Otusanya et al., 2008). Khan et al. (2009) reported that the reduction in germination counts of wheat became more pronounced with increasing levels of Eucalyptus camaldulensis aqueous extract concentration. Javed and Asghari (2008) also found that the leaf extract of Helianthus annus inhibited the rate of germination of wheat seedlings. A related work by Arshad (2011) showed that the water and methanolic extracts of Withania somnifera markedly suppressed the germination, root and shoot growth of Parthenium hysterophorus. Kushima (1998) stated that there was an inhibition of the growth of the plumule length of tomato seedlings by the application of leachate from water melon seeds. James and Bala (2003) found that dried mango leaf powder significantly inhibited the sprouting of purple nutsedge tubers while Yang et al. (2006) reported that its aqueous extract inhibited the germination and growth of some crops. Also, inhibition of germination and growth behaviour of some cowpea varieties using neem leaf water extracts revealed that the variation in the result of germination percentage is due to concentration differences. With the increase of concentration, the inhibitory effect was progressively increased in the laboratory Lawan et al. (2010). This also substantiated the works of Sazada et al. (2009) who reported that allelopathy is concentration dependent phenomenon.

The allelopathy potentials of rice husk residue RHR on the growth of four varieties of tomato is shown in Figure 2. All the varieties experienced an initial gradual increase of branches from the beginning of the experiment. Furthermore, the effect of rice husk on tomato showed rapid increases in the growth parameters at 10% level of concentration du ring the pre-germination stage but however, a slow growth was observed when the level of concentration was increased to 20% at the flowering stage. This was in compliance to Ayeni (2012), who studied the allelopathic potential of rice husk and maize root residues on the germination and initial growth of okra (Abelmoschus esculuntus L.) from this experiment, results from rice husk extracts-treated seeds revealed that while the % germination was 96% in the control experiment, % germination in 10 g, 20 g, 30 g, 40 g, and 50 g powdered extract concentrations were 92%, 92%, 92%, 76% and 64% respectively. Also in the maize root treated seeds, % germination was 96% in the control, those of 10, 20, 30, 40 and 50 g powdered extract concentrations were 92%, 84%, 80%, 80% and 68% respectively which illustrates that with increasing concentration of the extract, percentage growth will retard. This result was also affirmed by the present finding which was in accordance with the study of Bora et al. (1997) who also observed the inhibitory effects of leaf extracts of Accacia auriculiformis on the germination of some agricultural crops and contended further that the effect was proportional to the concentration of the extracts.

Study of bioassay on the allelopathic effect of neem n-hexane, acetone and water-soluble extracts on six weeds showed significant reductions in the germination and growth of the roots and shoots were observed as the extract concentration increased. The results are in agreement with previous investigations in that the activity of either water-extracts or weed residues was directly related to the concentration of the residue rates (Ashrafi et al., 2007). Four varieties of tomato subjected to the neem leaf aqueous extract (NLAE) treatment showed gradual increases in the number of leaves from the beginning of the experiment till the end shown in Figure 3.

The performance of neem leaf extract in improving the growth and yield parameters of plantain could be traced to its high N, P, K, Ca and Mg nutrients supplied to the soil which subsequently increased the soil organic matter, total N, available P, exchangeable K, Ca and Mg. This also reflected in the increased values of the plant height, leaf area, stem girth, leaf population, plantain bunch weight, fingers weight, length, diameter and population similar to what is observed in the Figure 4. The finding was also supported by Zhang et al. (2002), who reported that neem seed extract enhanced the nitrogen use efficiency in soils as well as increasing the yield of crops. The poor growth and yield performance of plantain in the control treatment was consistent with the fact that the soil was very low in nutrient contents and this observation was supported by Moyin-Jesu EI (2002), who reported poor growth and yield responses of crops in soils that are not fertilized.

Abimbola et al. (2012) described seed germination as a valuable index in allelopathic studies. Exposure of seeds to one or combined influence of organic compounds such as phenolic acids, alkaloids and volatile terpenes in the extracts or essential oils of different medicinal species often results in negative physiological effects on the germination and seedling growth (Mungole et al., 2010) (Figure 5).

3 Conclusions

The above collected information regarding the use of neem (Azadirachta indica) and rice husk in the world is matched with available literature. Recent years, allelochemical compounds, especially of plant origin received much attention as they are well tested for their efficacy on crops. This present study has helped to provide insight on their effect on tomato (Solanum lycopersicum). The general observation is that allelopathy of plants may be positive when it actually reduces the agronomic character with increasing concentration and negative when there is a gradual or rapid increases in both the growth parameters and yield component with decreasing concentration. The result concluded that four varieties of tomato (Solanum lycopersicum) showed different response in their growth and yield under the neem leaf aqueous extract and rice husk residue treatment. It is thus recommended that similar experiment should be carried out on the field so as to compare the level of allelopathy and effect on tomato raised in confinement with those raised in an open field.

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