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Allelopathic Effects of Invasive Acacia mangium on Germination and Growth of Local Paddy Varieties



N.A.N. Ismail and F. Metali
 
ABSTRACT
Laboratory and plant house experiments were conducted to study the allelopathic effects of Acacia mangium (Fabaceae), an invasive plant species in Brunei Darussalam, on germination and growth of two local paddy varieties. Germination, relative growth rates and/or biomass allocations of paddy Laila and Pusu were determined using a series of aqueous leaf extract concentrations and aqueous soil extract concentrations from A. mangium plantation and heath forest. Laila appears to be the most sensitive target species as its germination and relative growth rates based on elongations lengths (RERs) were affected by all the three types of extracts. Mean percentages germination of seeds treated with A. mangium leaf and A. mangium and heath forest soil extracts significantly decreased in Laila and Pusu as extract concentration increased to 10/12%. Mean RERs of seeds treated with 12% of A. mangium leaf extract were significantly slower in both Laila and Pusu as compared to the control. Both types of soil extracts significantly decreased RERs in Laila but not Pusu. At 5% of leaf extract concentration, Laila seedlings allocated a higher proportion of dry mass to roots but a lower proportion of dry mass to shoots (or a higher root-to-shoot ratio) than in other treatments but this differential allocation did not translate into greater final total dry biomass or faster growth rates. Acacia mangium negatively affected germination and growth of paddy. It is suggested that careful planning needs to be undertaken before using invasive species in any integrated land use systems.
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N.A.N. Ismail and F. Metali, 2014. Allelopathic Effects of Invasive Acacia mangium on Germination and Growth of Local Paddy Varieties. Journal of Agronomy, 13: 158-168.

DOI: 10.3923/ja.2014.158.168

URL: http://scialert.net/abstract/?doi=ja.2014.158.168
 
Received: July 09, 2014; Accepted: August 25, 2014; Published: December 13, 2014

INTRODUCTION

Genus Acacia (Fabaceae) is regarded as one of the most successful alien species that has invaded many areas worldwide (Henderson, 2007; Richardson and Rejmanek, 2011), including tropical countries in South East Asia. Acacia auriculiformis was first introduced to Brunei Darussalam in the 1950s to solve the problems of soil erosion occurring along the Tutong-Muara highway highway levee (Osunkoya et al., 2005). Then, A. mangium was also planted to accomplish the long-term objective of making Brunei self-sustainable in timber and furniture industries. In the late 1980s and early 1990s, A. mangium was used for revegetation and rehabilitation work with the aim to increase the productivity of secondary forest and highly degraded heath forest in Brunei (Mr Alex Cheng, Brunei Plantation Unit, pers. comm.).

The setback of planting Acacia in Brunei is that there have been evidences showing that the species have spread quite extensively and rapidly into the open, disturbed and degraded areas and forests over the years and they have outgrown many native plants in particular heath forest species, such as Melastoma malabathricum, Ploairium alternifolium, Commersonia bartramia and Gymnostoma nobili (Osunkoya et al., 2005). This invasion may result in the loss of biodiversity and disruption of structure and dynamics of the natural ecosystems (Osunkoya et al., 2005; Peh, 2010; Hussain et al., 2011a). They have also trespassed the fruit and coffee farms in Tutong district in Brunei, where they were initially planted at the edge of the farms in order to provide shade to the fruit trees (Osunkoya et al., 2005), also reported by K. Omar Ali, Agronomist in Department of Agriculture and Agrifood, pers. comm.

Acacia mangium is an evergreen tree, which may grow up to 15-20 m in height (Coode et al., 1996). Acacia mangium has larger phyllodes, which are the flattened leafy petioles than that of A. auriculiformis (Lakshmi and Gopakumar, 2009). Acacia mangium has creamy-white flowers and coiled pods, which contain fruits and seeds (Lakshmi and Gopakumar, 2009).

The term ‘Allelopathy’ was first introduced in 1937 by an Australian plant physiologist, Hans Molisch (Bhowmik and Inderjit, 2003). Allelopathy in plants is a biological process by which plants release allelochemicals that affect germination, growth and reproduction of other plants (Olofsdotter, 1998). These allelochemicals may be released to the environment by different plant parts, such as leaves, phyllodes, flowers, bark and twigs, through volatilization and leaching as well as from the soil through the decomposition of plant residue and root exudation (Chou, 1983; Rice, 1984; Oyun, 2006; Das et al., 2012). The release of allelochemicals by plants is regulated by environmental conditions, such as soil moisture content, temperature, light intensity, nutrients and other microorganisms, such as fungi and viruses (El-Khawas and Shehata, 2005).

Plants possessing allelopathic potential are able to inhibit or stimulate the germination or growth of another species (Crafts and Robbins, 1962; Whittaker, 1970; Harborne, 1977; Rice, 1984) or may also impose no effect on the target species (Reigose et al., 1999). Many reports have shown that Acacia have a phytotoxic effect on the agricultural crops, which may decrease crop yields (Korner and Nicklisch, 2002; Leu et al., 2002). Acacia auriculiformis have shown to have hindered germination and growth of herbaceous plants, Callistephus chinensis (aster) and Chrysanthemum coronarium (chrysanthemum), if the A. auriculiformis leaf extracts were soaked long enough (Barman et al., 1997), thus suggesting that prolong exposure to the allelochemicals would inhibit the germination and growth of certain species.

Kamal et al. (1997) have reported that the growth of wheat was highly inhibited by the leaf leachates of A. auriculiformis and A. nilotica. The lower A. nilotica leaf extract concentrations (0.25 and 0.5%) have promoted the growth of shoots and roots in peas, while as the concentration increases, there was a decrease in the shoot and root length and fresh and dry weight of shoot and root (Al-Wakeel et al., 2007). Three native tree species in India, namely Ficus infectoria (white fig), Emblica officinalis (Indian gooseberry) and Acacia leucophloea and a crop plant, Cicer arietinum (Indian chickpea), were proposed to be used in agroforestry, however all the three species have showed a phytotoxic effect on the germination and growth of the Indian chickpea in the laboratory setting (Siddiqui et al., 2009).

The main aim of this study is to investigate the allelopathic effects of Acacia mangium on germination and growth of two local paddy varieties. The encroachment of Acacia mangium to many disturbed forests in Brunei Darussalam as well as its potential ability to inhibit the growth of certain native tree species and agricultural plant species have been quite distressing, so, the present study attempts to determine the effects of allelochemicals from Acacia mangium (test species) on crops in laboratory and plant house settings.

We are interested in answering the following questions:

Does the aqueous leaf extract of Acacia mangium and aqueous soil extract of Acacia mangium plantation affect the percentage germination and relative elongation rates of selected local paddy varieties?
Does the aqueous leaf extract of Acacia mangium increase the Relative Growth Rate (RGR) and affect the biomass allocations of selected local paddy varieties?

MATERIALS AND METHODS

Target species: Oryza sativa L. (Poaceae) or commonly known as paddy is a monocot grass, which is able to grow all-year round and may grow to a height of 2 m (Duke, 1983). Ensuring optimum growth of rice is imperative as rice is a staple food for most Asians (Olofsdotter, 1998). Paddy Laila, a new hybrid rice variety developed by the International Rice Research Institute in the Philippines, was introduced in 2009 by the Department of Agriculture, Brunei Darussalam (Ministry of Industries and Primary Resources Brunei) to meet the local demand for rice, so the people will be self-dependent on the country’s own resources (K. Omar Ali, Agronomist in Department of Agriculture and Agrifood, pers. comm.). Paddy Pusu is a hill paddy variety native to Borneo and it has been traditionally planted by Bruneian rice farmers. The seeds of paddy Laila and Pusu used in this study were supplied by the Department of Agriculture, Ministry of Industry and Primary Resources, Negara Brunei Darussalam.

Sampling of leaves and soils: Leaves of Acacia mangium were collected in September-October 2013. The term leaf or leaves will be used to represent phyllode or phyllodes. The leaves were washed with distilled water and air-dried in an air-conditioned room for a week before they were grinded into fine powders. Soil cores were collected in December 2013-January 2014 from Acacia mangium plantation site (04.59305°N, 114.515.80°E) and primary heath forest (04.60006°N, 114.51611°E) at the Andulau Forest Reserve, respectively. The soils from each site were bulked and passed through sieve to remove any debris.

Preparation of aqueous leaf and soil extracts: Dried powdered leaves of A. mangium were used to prepare a series of aqueous extract concentrations (0.5, 1.0, 2.5, 5.0, 7.5, 10.0 and 12.0%) following the methods described by Hussain et al. (2011a). The leaves were soaked in distilled water and left for 24 h in the laboratory before the solution mixtures were vacuum-filtered. Fresh soil samples were used to prepare the aqueous soil extracts of the following concentrations; 1, 5 and 10% using the techniques described by Conway et al. (2002). Soils were soaked in distilled water and left for 36 h in the refrigerator before the solution mixtures were vacuum-filtered. The filtrates were refrigerated at 4°C until needed. Distilled water was used as the control for this experiment.

Seed germination and elongation experiments: An experimental design of 4 replicates of 25 seeds for each paddy variety (n = 2 paddy varieties) were used in this study. In each treatment, 5 mL of test solution was added to all the respective petri dishes at initial stage, followed by 3 mL of the same test solution for every two days. The petri dishes were placed in the plant house (25-33°C) and seed germination was scored every two days until the end of the experiment (20 days). Radicle emergence of at least 1 mm was used as the criterion for seed germination. When the seeds germinated, the initial radicle length and shoot length were measured as well as their final radicle and shoot lengths on day 14 after germination, which were used in the calculation of the relative growth rates. Relative growth rates (RGR, mm mm-1 day-1) based on elongation lengths over a period of time were calculated by following equation (Hunt, 1982):

RGR = (loge EL2-loge EL1)/(t2-t1)

where, EL2 and EL1 are final and initial mean elongation lengths (mm), respectively and t2-t1 was 14 days.

Growth and biomass allocation of paddy Laila in response to A. mangium aqueous leaf extract: Only paddy Laila was used in this investigation because of insufficient supplies of seeds. Germination was carried out in the laboratory (20-21°C). The seedlings (n = 50) of 6-7 cm in height were chosen at random and transferred to plastic pots (base diameter = 11 cm, height = 13 cm), which were filled with black loamy soils (about 900 g) that have been sieved and autoclaved. All pots were kept in the plant house (25-33°C) and left to acclimatise for one week. Ten seedlings (n = 10) with mean height of 16-17 cm were selected at random for the initial harvest in February 2014 to provide estimates of initial size.

Only A. mangium leaf extracts with the concentrations of 1.0, 5.0 and 10.0% were used for the seedling growth experiment because these concentrations showed a negative effect on germination and relative elongation lengths of paddy Laila. Following the method of Orr et al. (2005), the seedlings (n = 10 seedlings per treatment, including distilled water for control) were hand-watered with 5 mL of leaf extracts or distilled water every morning for a period of three weeks while in the final 10 days, 10 mL of extracts or distilled water were added to the seedlings for every 2 days. A final harvest of all surviving seedlings was conducted after 30 days. At both harvests, the lengths of the shoot and root were determined. Each seedling was separated into shoot and root fractions and each fraction was oven-dried at 60°C for 7 days (until the dry mass was constant).

Biomass allocations of seedlings were determined using dry mass ratios (dry mass of plant part divided by total plant dry mass) for shoot (shoot mass ratio, SMR) and root (root mass ratio, RMR) as well as root-to-shoot mass ratio per seedling from the final harvest only (Hunt, 1982). The total dry mass of shoot and root was also calculated for each seedling from the final harvest only. Relative Growth Rate (RGR) in terms of height and biomass were calculated following Hunt (1982):

RGR = (loge W2-loge W1)/(t2-t1)

where, W2 and W1 are final and initial total height (cm) for RGRheight and final and initial total seedling dry masses (g) for RGRbiomass and t2-t1 was 30 days. The calculations of RGR values were based on height and biomass of seedlings harvested in March 2014.

Statistical analysis: The mean final germination percentage±standard error of mean (SE) was calculated for each leaf extract or soil extract concentration per paddy variety. Final percentage germination was relatively expressed to initial seed numbers used in the experiment. Prior to analysis, germination percentages and biomass allocations were arcsine-square transformed, while relative growth rates were log10-transformed if residuals were not normally distributed after fitting models to the untransformed data. The germination percentages between treatments and within each paddy variety were analysed using a one-way analysis of variance (ANOVA) and Tukey’s honest significant difference (Tukey’s HSD) tests. However, for convenience in interpretation, untransformed data appear in all tables and figures. All statistical analyses were conducted using R version 3.0.3 (R Development Core Team, 2014).

RESULTS

Effects of Acacia mangium leaf extract: The mean final percentages germination were significantly different among A. mangium leaf extract concentrations for paddy Laila (F7, 24 = 3.17, p<0.05; Fig. 1a) and paddy Pusu (F7, 24 = 4.50, p<0.01, Fig. 1b). The mean percentages germination of paddy Laila and paddy Pusu were low at 7.5% (94±2%) and 12% (93±1%) A. mangium leaf extract concentrations, respectively (Fig. 1a and b). The mean relative growth rates based on elongation lengths were significantly different among A. mangium leaf extract concentrations for the two paddy varieties; paddy Laila (F7, 72 = 27.11, p<0.001; Fig. 2a) and paddy Pusu (F7, 72 = 6.03, p<0.001, Fig. 2b). There seems to be a general decreasing trend in the mean RERs of paddy Laila and paddy Pusu as the different concentrations of A. mangium leaf extract increase (Fig. 2a and b). The mean RERs of paddy Laila and Pusu were significantly greater by 41 and 15% in the control than at 2.5 and 12% A. mangium leaf extract concentrations, respectively (Fig. 2a and b).

Fig. 1(a-b):
Boxplots of final germination percentages of: (a) Paddy Laila, (b) Paddy Pusu in different aqueous concentrations (0% or control, 0.5, 1.0, 2.5, 5.0, 7.5, 10.0 and 12.0%) of Acacia mangium leaf extracts. Each treatment involved four replicates of 25 seeds and the experiment was conducted over 20 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

Fig. 2(a-b):
Boxplots of relative growth rate based on elongation lengths (RER, mm mm-1 day-1) of: (a) Paddy Laila and (b) Paddy Pusu in different aqueous concentrations (0% or control, 0.5, 1.0, 2.5, 5.0, 7.5, 10.0 and 12.0%) of Acacia mangium leaf extracts. Each treatment involved 10 seedlings (n = 10) and the experiment was conducted over 14 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

Fig. 3(a-b):
Boxplots of final germination percentages of: (a) Paddy Laila and (b) Paddy Pusu in different concentrations (0% or control, 1.0, 5.0 and 10.0%) of aqueous extracts of soils from Acacia mangium plantation. Each treatment involved four replicates of 25 seeds and the experiment was conducted over 20 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

Effects of Acacia mangium soil extract: The mean final percentages germination were significantly different among A. mangium soil extract concentrations for paddy Laila (F3, 12 = 29.18, p<0.001; Fig. 3a) and paddy Pusu (F3, 12 = 46.11, p<0.001, Fig. 3b). Generally, there was a decreasing trend of mean percentages germination of Paddy Laila (from 100% at 0 to 80±2% at 10%) and Paddy Pusu (from 99±1% at 0 to 65±2% at 10%) as Acacia soil extract concentration increases (Fig. 3a and b). The mean percentages germination for paddy Laila and Pusu seeds started to decrease at 1% (Fig. 3a and b). The mean percentages germination of paddy Laila and paddy Pusu at 1% (Laila: 95±3% and Pusu: 74±4%), 5% (Laila: 84±2% and Pusu: 67±4%) and 10% (Laila: 80±2% and Pusu: 65±2%) were significantly lower than the control (0%; Laila: 100% and Pusu: 99±1%) treatment (Fig. 3a and b).

The mean relative growth rates based on elongation lengths (RER) were significantly different among A. mangium soil extract concentrations for paddy Laila (F3, 36 = 11.55, p<0.001; Fig. 4a) but not for paddy Pusu (p>0.05, Fig. 4b). The aqueous extract of A. mangium soil extract seem to have a stimulating effect on the RER of paddy Laila as the mean RER at 1% (0.29±0.01 mm mm-1 day-1), 5% (0.27±0.01 mm mm-1 day-1) and 10% (0.26±0.01 mm mm-1 day-1) were higher than control (0.18±0.02 mm mm-1 day-1) (Fig. 4a).

Effects of primary heath forest soil extract: The mean final percentages germination were significantly different among the primary heath forest soil extract concentrations for paddy Laila (F3, 12 = 5.47, p<0.05; Fig. 5a) and paddy Pusu (F3, 12 = 41.62, p<0.001; Fig. 5b). Generally, there was a decreasing trend of mean percentages germination of Paddy Laila (from 100% at 0 to 89±3% at 10%) and Paddy Pusu (from 99±1% at 0 to 71±3% at 10%) as primary heath soil extract concentration increases (Fig. 5a and b). The mean percentages germination for paddy Pusu and paddy Laila seeds started to decrease at 1% (Fig. 5a and b). The mean percentages germination of paddy Pusu at 1% (82±1%), 5% (73±3%) and 10% (71±3%) were significantly lower than the control (0%; 99±1%) treatment (Fig. 5b). As for paddy Laila, the mean percentages germination at 5% (91±4%) and 10% (89±3%) were significantly lower than the control (100%, Fig. 5a).

The mean relative growth rates based on elongation lengths (RER) were significantly different among primary heath forest soil extract concentrations for paddy Laila (F3, 36 = 5.40, p<0.01; Fig. 6a) but not for paddy Pusu (F3, 36 = 0.974, p>0.05; Fig. 6b). Generally, there was an increasing trend in the mean RER of paddy Laila (from 0.18±0.02 mm mm-1 day-1 at 0% to 0.25±0.01 mm mm-1 day-1 at 10%) as the concentration of the primary heath forest soil extract increases (Fig. 6a).

Growth and biomass allocation of paddy laila: The mean shoot mass ratios (F3, 12 = 5.47, p<0.05; Fig. 7c), root mass ratios (F3, 12 = 41.62, p<0.001; Fig. 7d) and root-to-shoot ratios (F3, 24 = 7.11, p<0.01, Fig. 7e) of paddy Laila seedlings were significantly different across aqueous extracts concentrations of A. mangium leaves.

Fig. 4(a-b):
Boxplots of relative growth rate based on elongation lengths (RER, mm mm-1 day-1) of: (a) Paddy Laila and (b) Paddy Pusu in different aqueous concentrations (0% or control, 1.0, 5.0 and 10.0%) of Acacia mangium soil extracts. Each treatment involved 10 seedlings (n = 10) and the experiment was conducted over 14 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

Fig. 5(a-b):
Boxplots of final germination percentages of: (a) Paddy Laila and (b) Paddy Pusu in different concentrations (0% or control, 1.0, 5.0 and 10.0%) of aqueous extracts of soils from the primary heath forest. Each treatment involved four replicates of 25 seeds and the experiment was conducted over 20 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

Generally, there was an increasing trend of biomass allocations to roots (from 0.23 at 0% to 0.32 at 5%) but a decreasing trend of biomass allocation to shoots (from 0.77 at 0% to 0.68 at 5%) of paddy Laila seedlings as leaf extract concentrations increase to 5% (Fig. 7c and d). After 5%, there was a decline trend of biomass allocations to roots but not shoots in the Laila seedlings (Fig. 7c and d). At 5%, plants allocated a higher proportion of dry mass to roots and lower proportions of dry mass to shoots (or a higher root-to-shoot ratio) than in the 0, 1 and 10% treatments (Fig. 7c, d and e), but this did not translate into greater final total dry biomass or faster growth rates (Fig. 7a, b and f). Relative growth rates (RGRbiomass and RGRheight) and total final dry biomass of paddy Laila were not significantly affected by the A. mangium leaf extracts (p>0.05; Fig. 7a, b and f).

Fig. 6(a-b):
Boxplots of relative growth rate based on elongation lengths (RER, mm mm-1 day-1) of: (a) Paddy Laila and (b) Paddy Pusu in different aqueous concentrations (0% or control, 1.0, 5.0 and 10.0%) of aqueous extracts of soils from the primary heath forest. Each treatment involved 10 seedlings (n = 10) and the experiment was conducted over 14 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level


Fig. 7(a-f):
Differences in (a) Relative growth rate based on biomass, RGRbiomass (g g-1 day-1), (b) Relative growth rate based on height, RGRheight (cm cm-1 day-1), (c) Shoot mass ratio, SMR, (d) Root mass ratio, RMR, (e) Root to shoot mass (root:shoot) and (f) Total final dry biomass (g) of paddy Laila seedlings at different concentrations (0% or control, 1.0, 5.0 and 10.0%) of aqueous Acacia mangium leaf extracts. Each treatment involved one replicate of 10 seedlings and the experiment was conducted over 30 days. Different letters within a panel indicate significant differences between the species mean values at 5% significance level

DISCUSSION

Effects of Acacia mangium leaf and soil extracts on germination and growth of crops: A. mangium and other Acacia species have been implicated as allelopathic species because they release allelochemicals that affect the germination and development of other plant species, for example A. melanoxylon and A. dealbata inhibited the germination and suppressed root growth of weeds, such as Dactylis glomerata (orchard grass), Lolium perenne (ryegrass) and Rumex acetosa (sorrel) and crops, such as lettuce (Hussain et al., 2011a). However, none of these studies look into the allelopathic of A. mangium in paddy.

The mechanism of allelopathy depends on three factors, namely, the concentration of the test species, the part or organ of the test species at which the extract is obtained and finally, the target species that may respond to the allelochemicals (Hussain et al., 2011a). Chou et al. (1998) reported that A. confusa and some other Acacia species have shown phytotoxic effect at an aqueous extract concentration of as low as 0.5%. The phytotoxic present in A. confusa are water-soluble (Chou et al., 1998), thus suggesting that aqueous extract solutions of plants can be used in the study of allelopathy. The aqueous extracts used to study allelopathic effects of a species may derived from different plant parts, such as roots, stems, flowers and leaves as well as soils and the degree of allelopathic potential may differ for every plant parts (Hussain et al., 2011a). Leaf is the most common part of plants, which is used in many allelopathic studies (Conway et al., 2002; Kato-Noguchi et al., 2011; Baratelli et al., 2012), so this is why we use Acacia leaves (phyllodes) in our current study.

The present findings reported that A. mangium leaves have phytotoxicity effect on the two local paddy varieties in Brunei Darussalam (paddy Laila and paddy Pusu). For example in paddy Laila and paddy Pusu, the mean percentages germination were significantly lower at 7.5 and 12%, respectively, suggesting that the higher the concentration of the extract, the more prominent the degree of germination inhibition is. Similar studies also reported the negative effects of increasing leaf extract concentrations of Acacia species on germination and growth of crops (Bora et al., 1999; Oyun, 2006; Hussain et al., 2011a, b). We also observed a delay in the germination of the first seed, for example in paddy Laila, which showed a delayed by a day as compared to the control treatment. This observation was similar to Rafiqul-Hoque et al. (2003), who reported that A. auriculiformis leaf leachates delayed the germination process of target plant species. We observed that the A. mangium leaf extracts seemed to promote the growth of fungus, thus affecting the germination of the first seed and final germination percentages of target species. The test solutions can be autoclaved, however, autoclaving the aqueous extracts is not advisable as high temperatures may cause an alteration or loss of active allelochemicals (Orr et al., 2005).

In the present findings, the decreasing trend of percentages germination as the concentration of A. mangium leaf extract increases also reflect a decreasing trend in the mean RERs of some species. Jadhav and Gayanar (1992) and Kamal et al. (1997) also reported that A. auriculiformis inhibits the growth of rice and cowpeas. Blum et al. (1999) reported that root is the first organ affected in allelopathy by disrupting its uptake of water and ions. This is in agreement with the study by Hussain et al. (2011a), who reported similar findings, where the root lengths were significantly affected by the aqueous extract of A. melanoxylon flowers. Oyun (2006) also reported that the inhibition of seedling growth is related to the inhibition of nutrient uptake, which affect the growth the plants shoot and root. Tannins, wax, flavonoids and phenolic acids are the potential major allelochemicals that inhibit the seed germination of the crop plants, to some extent (Oyun, 2006). This coincides with the findings of El-Khawas and Shehata (2005), who reported that the same chemicals were responsible in the deleterious effect of Acacia species on the germination and growth of other plants.

In the present study, the germination percentages of Paddy Laila and Pusu were negatively affected by both types of aqueous soil extract concentrations. This is not surprising as allelochemicals may also be passed into the soils via the fallen leaf litters (Chou et al., 1998). The Acacia plantation in Brunei was established since 1998 and it is very likely that the soils may contain allelochemicals that are released by the invasive A. mangium trees. The amount of the phytotoxic chemicals present in the soil depends on the density of the fallen leaves, the rate of leaf litter decomposition, the distance of Acacia trees from other plants, rainfall, soil type and soil pH (Mann, 1987; Saxena et al., 1996; Escudero et al., 2000; Nilsson et al., 2000).

Our findings showed an increasing trend of RERs as the concentration of soil extract from Acacia plantation and primary forests increases. There were no A. mangium trees present in the primary heath forest at Andulau Forest Reserve but we can deduced that allelochemicals are also present in soils with A. mangium and heath forest species. However, it is not certain as to whether the allelochemicals in both aqueous soil extracts are the same or not until the active compounds in aqueous soil extracts are identified. Being an invasive species, the allelopathy mechanism in Acacia helps them to survive and avoid competition with other plants, especially the native species. As for primary forests, there seem to be certain degree of allelopathy happening in this forest type (Chou et al., 1998). The species in natural forest may possess allelopathic potential even though it was not as dominant as the A. mangium soil extract. This might be due to the plants interacted with the allelochemicals are able to stimulate or inhibit the release of allelochemicals and may also affect the effectiveness of allelopathy in the soil as well as their ability to choose for low or high concentrations of allelochemicals (Inderjit et al., 2011).

Based on this study, paddy Laila and paddy Pusu seem to be a good candidates for the study of allelopathic potential of A. mangium and other allelopathic species as the germination at control was consistent at 100%, hence the allelopathic effect of A. mangium on these species is more convincing.

Effects of Acacia mangium leaf extracts on growth and biomass allocation of crops: Biomass allocations (SMR, RMR and shoot-to-root ratio) showed significant results at 5% A. mangium leaf extract concentration. The RMR was significantly low at 10% compared to at 5%, suggesting that at 10%, the roots were the most affected tissues by A. mangium leaf extract. This coincides with other findings, which reported that root elongation was more sensitive in response to the extracts of allelopathic plants (Meissner et al., 1982; Rafiqul-Hoque et al., 2003). As a result, plant shoots were affected and they were unable to attain optimum shoot growth due to the disruption in root elongation, thus reducing water and nutrient uptake (Rafiqul-Hoque et al., 2003). None of the increases in biomass allocations were translated into faster RGRs or greater final total dry biomass, thus suggesting that the total dry mass and the height were less affected by the allelochemicals present in A. mangium leaf extract or the need to extend the duration of seedling growth experiments.

Future study should focus on extending the list of target species to other common crops, fruit trees and native plants in Brunei and lengthening the duration of the seedling experiment because seedlings may take a long time to response to environmental changes. Since all plant parts may be potential sources of allelopathy, it might be useful to investigate the different parts, such as stems, barks and flowers, of the A. mangium tree on seed germination, growth and biomass allocations of target species. It is also recommended to use extracts from mixed plant parts as allelopathic activity always work when there are more than one allelochemical (Einhellig, 1995). Finally, it is recommended that the allelopathic studies to be replicated in-situ as it would be interesting to observe the natural patterns of seed germination and seedling growth in the field.

Implications of study: Acacia, such as A. auriculiformis, has been recommended worldwide as a potential agroforestry species, which are planted with shrubs and crops (Petmak and Williams, 1991). However, it is crucial to investigate the allelopathic effects of Acacia species on target shrub/crop species that will be used in agroforestry sites (King, 1979). Allelopathic species, such as A. mangium and A. nilotica, have the potential to be used as natural herbicides because of their abilities in killing weeds (El-Khawas and Shehata, 2005). However, we need to treat this matter with caution because we do not want to introduce invasive plant species as agroforestry candidate and natural biological control.

CONCLUSION

From the present findings, A. mangium leaf extract seems to exhibit allelopathic potential on local rice varieties by decreasing germination percentages and relative growth rates in terms of elongation lengths of paddy Laila and paddy Pusu. Acacia mangium soil extract also decreased the germination percentages of paddy Laila and paddy Pusu and decreased RERs of paddy Laila, to a certain extent but not paddy pusu. Biomass allocations of paddy Laila seedlings were significantly affected by the A. mangium leaf extracts to a certain degree but not their relative growth rates and total final dry biomass.

ACKNOWLEDGEMENTS

This study was funded by the research grant provided by Universiti Brunei Darussalam. We thank the Department of Forestry and Department of Agriculture and Agrifood for their kind assistance in making this study possible.

REFERENCES
Al-Wakeel, S.A.M., M.A. Gabr, A.A. Hamid and W.M. Abu-El-Soud, 2007. Allelopathic effects of Acacia nilotica leaf residue on Pisum sativum L. Allelopathy J., 19: 411-422.
Direct Link  |  

Baratelli, T.G., A.C.C. Gomes, L.A. Wessjohann, R.M. Kuster and N.K. Simas, 2012. Phytochemical and allelopathic studies of Terminalia catappa L. (Combretaceae). Biochem. Syst. Ecol., 41: 119-125.
CrossRef  |  

Barman, D., L.C. De and C.K. Sharma, 1997. Allelopathic effects of Acacia auriculiformis A. Cunn. on germination of aster (Callistephus chinensis) and chrysanthemum (Chrysanthemum coronarium). Ann. Plant Phys., 11: 212-213.

Bhowmik, P.C. and Inderjit, 2003. Challenges and opportunities in implementing allelopathy for natural weed management. Crop Prot., 22: 661-671.
CrossRef  |  

Blum, U., S.R. Shafer and M.E. Lehman, 1999. Evidence for inhibitory allelopathic interactions involving phenolic acids in field soils: Concepts vs. an experimental model. Crit. Rev. Plant Sci., 18: 673-693.
Direct Link  |  

Bora, I.P., J. Singh, R. Borthakur and E. Bora, 1999. Allelopathic effect of leaf extracts of Acacia auriculiformis on seed germination of some agricultural crops. Ann. For., 7: 143-146.

Chou, C.H., 1983. Allelopathy in Agroecosystems in Taiwan. In: Allelochemicals and Pheromones, Chou, C.H. and G.R. Waller (Eds.). Institute of Botany, Academia Sinica, Taipei, Taiwan, pp: 27-64.

Chou, C.H., C.Y. Fu, S.Y. Li and Y.F. Wang, 1998. Allelopathic potential of Acacia confusa and related species in Taiwan. J. Chem. Ecol., 24: 2131-2150.
Direct Link  |  

Conway, W.C., L.M. Smith and J.F. Bergan, 2002. Potential allelopathic interference by the exotic Chinese tallow tree (Sapium sebiferum). Am. Midland Natur., 148: 43-53.
CrossRef  |  

Coode, M.J.E., J. Dransfield, L.L. Forman, D.W. Kirkup and I.M. Said, 1996. A Checklist of Flowering Plants and Gymnosperms of Brunei Darussalam. Ministry of Industry and Primary Resources, Brunei Darussalam, ISBN: 9991731008, Pages: 477.

Crafts, A.S. and W.W. Robbins, 1962. A Text Book and Manual: Weed Control. 3rd Edn., McGraw Hill Book Co., New York.

Das, C.R., N.K. Mondal, P. Aditya, J.K. Datta, A. Banerjee and K. Das, 2012. Allelopathic potentialities of leachates of leaf litter of some selected tree species on gram seeds under laboratory conditions. Asian J. Exp. Biol. Sci., 3: 59-65.
Direct Link  |  

Duke, J.A., 1983. Handbook of energy crops: Oryza sativa L. Electronic Publication on the NewCROPS Web Site. http://www.hort.purdue.edu/newcrop/duke_energy/oryza_sativa.html.

Einhellig, F.A., 1995. Allelopathy: Current Status and Future Goals. In: Allelopathy: Organisms, Processes and Applications, Inderjit, S., K.M.M. Dakshini and F.A. Einhellig, (Eds.). American Chemical Society, USA., ISBN-13: 9780841230613, pp: 1-24.

El-Khawas, S.A. and M.M. Shehata, 2005. The allelopathic potentialities of Acacia nilotica and Eucalyptus rostrata on monocot (Zea mays L.) and Dicot (Phaseolus vulgaris L.) plants. Biotechnology, 4: 23-34.
CrossRef  |  Direct Link  |  

Escudero, A., M.J. Albert, J.M. Pita and F. Perez-Garcia, 2000. Inhibitory effects of Artemisia herba-alba on the germination of the gypsophyte Helianthemum squamatum. Plant Ecol., 148: 71-80.
Direct Link  |  

Harborne, J.B., 1977. Introduction to Ecological Biochemistry. Academic Press, London, ISBN-13: 9780123246707, Pages: 243.

Henderson, L., 2007. Invasive, naturalized and casual alien plants in southern Africa: A summary based on the Southern African Plant Invaders Atlas (SAPIA). Bothalia, 37: 215-248.
Direct Link  |  

Hunt, R., 1982. Plant Growth Curves: The Functional Approach to Plant Growth Analysis. Edward Arnold, London.

Hussain, M.I., L. Gonzalez and M.J. Reigosa, 2011. Allelopathic potential of Acacia melanoxylon on the germination and root growth of native species. Weed Biol. Manage., 11: 18-28.
CrossRef  |  

Hussain, M.I., L. Gonzalez, C. Souto and M.J. Reigosa, 2011. Ecophysiological responses of three native herbs to phytotoxic potential of invasive Acacia melanoxylon R. Br. Agrofor. Syst., 83: 149-166.
CrossRef  |  

Inderjit, D.A. Wardle, R. Karban and R.M. Callaway, 2011. The ecosystem and evolutionary contexts of allelopathy. Trends Ecol. Evol., 26: 655-662.
CrossRef  |  Direct Link  |  

Jadhav, B.B. and D.G. Gayanar, 1992. Allelopathic effects of Acacia auriculiformis on germination of rice and cowpea. Indian J. Plant Phys., 1: 86-89.

Kamal, S.R., R.C. Dhiman and N.K. Joshi, 1997. Allelopathic effects of some tree species on wheat (Triticum aestivum L.). J. Res. Bisra Agric. Univ., 9: 101-105.

Kato-Noguchi, H., H.L. Thi, T. Teruya and K. Suenaga, 2011. Two potent allelopathic substances in cucumber plants. Sci. Hort., 129: 894-897.
CrossRef  |  

King, K.F.S., 1979. Agroforestry and the utilisation of fragile ecosystems. For. Ecol. Manage., 2: 161-168.
Direct Link  |  

Korner, S. and A. Nicklisch, 2002. Allelopathic growth inhibition of selected phytoplankton species by submerged macrophytes. J. Phycol., 38: 862-871.
CrossRef  |  Direct Link  |  

Lakshmi, M.N. and S. Gopakumar, 2009. Morphological keys for four Australian Acacia species grown in Kerala, India. J. Trop. Agric., 47: 62-66.
Direct Link  |  

Leu, E., A. Krieger-Liszkay, C. Goussias and E.M. Gross, 2002. Polyphenolic allelochemicals from the aquatic angiosperm Myriophyllum spicatum inhibit photosystem II. Plant Physiol., 130: 2011-2018.
PubMed  |  

Mann, J., 1987. Secondary Metabolism. Clarendon Press, Oxford. UK., ISBN-13: 9780198555292, Pages: 374.

Meissner, R., P.C. Nel and N.S.H. Smit, 1982. The residual effect of Cyperus rotundus on certain crop plants. Agroplantae, 14: 47-53.
Direct Link  |  

Nilsson, M.C., O. Zackrisson, O. Sterner and A. Wallstedt, 2000. Characterisation of the differential interference effects of two boreal dwarf shrub species. Oecologia, 123: 122-128.
CrossRef  |  

Olofsdotter, M., 1998. Allelopathy in rice. Proceedings of the Workshop on Allelopathy in Rice, November 25-27, 1998, Manila, Phillippines, pp: 1-5.

Orr, S.P., J.A. Rudgers and K. Clay, 2005. Invasive plants can inhibit native tree seedlings: Testing potential allelopathic mechanisms. Plant Ecol., 181: 153-165.
CrossRef  |  

Osunkoya, O.O., F.E. Othman and R.S. Kahar, 2005. Growth and competition between seedlings of an invasive plantation tree, Acacia mangium and those of a native Borneo heath-forest species, Melastoma beccarianum. Ecol. Res., 20: 205-214.
CrossRef  |  

Oyun, M.B., 2006. Allelopathic potentialities of Gliricidia sepium and Acacia auriculiformis on the germination and seedling vigour of maize (Zea mays L.). Am. J. Agric. Biol. Sci., 1: 44-47.

Peh, K.S.H., 2010. Invasive species in Southeast Asia: The knowledge so far. Biodivers. Conserv., 19: 1083-1099.
CrossRef  |  

Petmak, P. and E.R. Williams, 1991. Use of Aacacia species in agroforestry systems. Proceeding of International Workshop on Tropical Acacias, February 11-15, 1991, Thailand, Bangkok, pp: 1-16.

R Development Core Team, 2014. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria.

Rafiqul-Hoque, A.T.M., R. Ahmed, M.B. Uddin and M.K. Hossain, 2003. Allelopathic effect of different concentration of water extracts of Acacia auriculiformis leaf on some initial growth parameters of five common agricultural crops. Pak. J. Agron., 2: 92-100.
CrossRef  |  Direct Link  |  

Reigose, M.J., A. Sanchez-Moreiras and L. Gonzalez, 1999. Ecophysiological approach in allelopathy. Crit. Rev. Plant Sci., 18: 577-608.

Rice, L.E., 1984. Allelopathy. In: A Discipline Called Allelopathy: Basic and Applied Aspects, Rizvi, S.J.H., H. Haque, V.K. Singh and V. Rizvi (Eds.). Chapman and Hall, New York, pp: 1-9.

Richardson, D.M. and M. Rejmanek, 2011. Trees and shrubs as invasive alien species-a global review. Diver. Distrib., 17: 788-809.
CrossRef  |  

Saxena, A., D.V. Singh and N.L. Joshi, 1996. Autotoxic effects of pearl millet aqueous extracts on seed germination and seedling growth. J. Arid Environ., 33: 255-260.
CrossRef  |  

Siddiqui, S., M.K. Meghvansi, R. Yadav, F.A. Wani, K. Yadav, S. Sharma and F. Jabeen, 2009. Phytotoxic effects of some agro-forestry trees on germination and radicle growth of Cicer arietinum var.-pusa-256. Global J. Environ. Res., 3: 87-91.

Whittaker, R.H., 1970. Communities and Ecosystems. MacMillan Co., London, Pages: 162.

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