Competitive Interaction of Axonopus compressus and Asystasia gangetica under Contrasting Sunlight Intensity

Axonopus compressus is one of the native soft grass species in oil palm in Malaysia which can be used as a cover crop. The competitive ability of A. compressus to overcome A. gangetica was studied using multiple-density, multiple-proportion replacements series under a glasshouse and full sunlight conditions in a poly bag for 10 weeks. A. compressus produced more dry weight and leaf area when competing against A. gangetica than in monoculture at both densities in the full sunlight and at high density in the shade. Moreover, the relative yield and relative crowding coefficients also indicated A. compressus is a stronger competitor than A. gangetica at both densities in the full sunlight and high density in the shade. It seemed that A. gangetica plants in the shade did not compete with each other and were more competitive against A. compressus as could influence A. compressus height in the shade. It is concluded that although suppression of A. gangetica by A. compressus occurred under full sunlight, irrespective of plant density, this ability reduced under shade as A. compressus density decreased. The result suggests that A. compressus in high density could be considered as a candidate for cover crops under oil palm canopy.


Introduction
Oil palm is the number one cash crop in Southeast Asia, especially in Malaysia and Indonesia. Worldwide coverage of oil palm plantations is 13 million ha of which about 5.3 million ha lies in Indonesia and 4.2 million ha in Malaysia [1]. The high demand for vegetable oil has led to the expansion of the area covered by oil palm plantations in this region.
In immature oil palm plantations, vacant space between palms creates opportunities for noxious weeds to grow ubiquitously. Noxious weeds such as Chromolaena odorata, Mikania cordata, and Mikania micrantha compete with the oil palm for nutrients, moisture, and sunlight and eventually cause yield depression [2]. Palms that grow where there is Imperata cylindrica are generally stunted and retarded in growth [2]. Yeow et al. [3] reported a 20% yield reduction in oil palm plantation output caused by weeds. Soft grasses such as Axonopus sp., Digitaria sp., and Palspalum sp. have the ability to prevent weed succession of noxious species simply because base land for the noxious weeds to colonise is less available [4].
Among different noxious weed species, Asystasia gangetica is frequently found in oil palm plantations [5]. The eradication of very dense stands of A. gangetica in an oil palm plantation resulted in a 12% increase in fresh fruit bunch production [6]. A. gangetica spreads very quickly in most Malaysian plantations and small holdings. It adapts well to almost all types of soil, especially to well aerated deep soils, peat soils, and sandy beaches [7][8][9].
Weed control in oil palm plantations contributes to 75% of the total cost of pest management. The use of herbicides in crop protection is becoming a common practice worldwide, and it was estimated that in 2007, 72% of chemicals used in agriculture in Malaysia were herbicides [10].
In tropical Asia, legume cover crops are frequently planted in oil palm plantations to provide ground cover after forest 2 The Scientific World Journal clearing [11]. The wide spacing of palms at planting exposes the newly-uncovered soil to intense rainfall resulting in soil erosion and nutrient and organic matter loss [12]. Ultimately, the legume cover crops become shaded out, and soft grasses such as Axonopus compressus, Cytococcum sp., and Paspalum conjugatum and light ferns cover the field. Finally, noxious weeds like Asystasia and Mikania can dominate in these areas because of their high tolerance to low soil fertility and shade from the palm canopy [13]. In 10-year-old palms on a coastal soil in Malaysia, 15% of light reached the ground [12].
A. compressus is one of the soft grass species that is widely used as ground cover to protect soil erosion, as turf grass for landscaping and for sports fields as well as to conserve soil moisture in Malaysia [14]. Rika et al. [15] found that the coconut yield was highest when A. compressus was used as ground cover under coconut plantations compared with other grass species used as ground cover. This grass has a high potential for use as a cover crop to suppress weeds in plantations, especially areas that are dominated by broadleaf weeds and where establishing legume cover crops is not feasible. Soft grasses like A. compressus are also better at collecting loose fruits than broadleaf legume cover crops. Therefore, we hypothesized that A. compressus might control weeds under oil palm canopy in Malaysia.
Replacement series designs are frequently used to characterize the competitive interactions of species in mixed stands [16,17]. Under this approach, species are grown in a fixed density, varying their proportions [18] to determine which species is the strongest competitor based on variables calculated from the replacement series data. Relative yield and total relative yield are variables that are frequently used to infer competitiveness between species [19]. By using multiple densities, it is possible to compare monoculture stands, allowing determination of the relative extent of intra-and interspecific competition between the species [20].
Despite the fact that Axonopus compressus is a dominant soft grass weed in oil palm plantations, there is no direct comparative study of this weeds with oil palm weeds. Therefore, the objectives of the study were to examine the interference dynamics between Axonopus compressus and the Asystasia gangetica, in different light conditions.

Experimental Site.
Two separate experiments were conducted during January to March 2011 at Universiti Putra Malaysia (UPM), Malaysia (3 ∘ 02 N, 101 ∘ 42 E; elevation 31 m). The local climate was hot, humid, and tropical with abundant rainfall throughout the year. During the experimental period, monthly average maximum and minimum temperatures and relative humidity ranged from 33.5 to 34 ∘ C, 23 to 23.3 ∘ C, and 93.4 to 96%, respectively, while sunshine hours ranged from 6.31 to 7.06 hr d −1 . Planting medium was prepared by mixing top soil, sand, and peat moss in a ratio 8 : 2 : 1 (v/v).

Plant
Materials. Axonopus compressus cuttings consisting of two nodes 4 cm long were collected from the Plants House of UPM. Asystasia gangetica seeds were collected from an oil palm field in UPM and stored at room temperature for 3 months prior to seeding.

Experimental Design and Treatments.
The competitive potential of Axonopus compressus and Asystasia gangetica ( Figure 1) was studied using multiple-density multipleproportion replacements series. One experiment was conducted in a glasshouse under shade such that the rate of penetration of light was 40% and another one was conducted in full sunlight. Before starting study, photosynthetically active radiation (PAR) was measured using an illuminometer (Extech instruments, model 407026) on the soil surface of poly bags at full sunlight and glasshouse under shade. The percentage of PAR penetrating was then calculated. Both the experiments were carried out in a randomized complete block design with four replications. Population densities used were 72 and 288 plants m −2 with five A. compressus (C) to A. gangetica (W) proportions (C 100 : W 0 , C 75 : W 25 , C 50 : W 50 , C 25 : W 75 and C 0 : W 100 ). The densities of 72 and 288 plants m −2 were considered appropriate based on the findings of previous work [21]. Polythene grow bags (poly bags) measuring 30 cm × 20 cm × 25 cm were used for growing the plants.

Plant Establishment.
Seeds of A. gangetica were directly planted in poly bags filled with planting media for the given density, and thinning was done one week after emergence. At the same time A. compressus cuttings were planted in poly bags according to the required spatial arrangement. After planting, each poly bag was treated with 0.2% benomyl. A. compressus was allowed to interact for 10 weeks after planting with A. gangetica. Plants were maintained under nonlimiting water and nutrient conditions by providing fertilizer and watering. Other weeds were removed during the experimental period.

Data Collection.
Plant heights from the soil surface to the top of the cover crops canopy were measured. Leaves of cover crops and weeds were harvested from each individual poly bag, and the leaf area of each species was determined using a leaf area meter (LI-3100, USA). Stems and leaves were placed by species into paper bags and dried at 72 ∘ C for 3 days to obtain shoot biomass. Cover crop and weed species shoot biomass data were converted to relative yield (RY) according to the following equations: yield of weed species in mixture yield of weed species in monoculture , RY weed species = yield of weed species in mixture yield of weed species in monoculture . (1) Regression of RYs on weed proportions 0, 0.25, 0.5, 0.75, and 1.0 were used to produce the replacement series diagrams to determine the competitiveness in the mixture as compared with the monoculture [18,22,23]. The shape of the replacement curve of the RY for the shoot dry weight relative to The Scientific World Journal expected yields was used as the indicator of the extent of interference between the two competing species [24]. Relative yield totals (RYTs), which predict the competition between the two species for the same resources, were calculated as described by Santos et al. [25] by using the following equation: The relative crowding coefficient (RCC), which serves as an index of competition when two species are mixed in equal proportions, was determined using the following equation [20]: where 1 and 2 are shoot dry weight per pot of crop species and weed species at C 50 : W 50 mixture and 1 , and 2 are shoot dry weight per pot of crop species and weed species in pure culture (monoculture). Equivalent yield ratios (EYR) or the proportion at which both species growing in the mixture produce the same yield was calculated for each mixture [25].
2.6. Statistical Analysis. Plant height and leaf area data were analysed by the analysis variance using SAS statistical software package version 9.2 [26], and values were further differentiated by Tukey's test at ≤ 0.05. All regressions were conducted using Sigma Plot version 11.

Competitive Ability of A. compressus against A. gangetica in Full Sunlight.
Since density by proportion interaction was significant for shoot dry weight of A. compressus, every combination was analyzed separately (data not given). A. compressus in monoculture (C 100 ), C 75 : W 25 and C 50 : W 50 produced less shoot biomass per plant than in C 25 : W 75 at two different densities (Table 1) (Figure 2(a)). The RYT value was less than 1 at 280 plants m −2 (Figure 2(b)). The Relative crowding coefficient (RCC) values of A. compressus at both densities, when grown in equal proportions, were more than the RCCs of A. gangetica (Table 3). The Scientific World Journal  (Figure 2(c)). A. gangetica responded to A. compressus to form a concave curve at 288 plants m −2 density (Figure 2(d)) that resulted in an EYR of about 0.68. The RYT value at 72 plants m −2 was greater than 1.0 (Figure 2(c)). The RYT value was less than 1.0 at 280 plants m −2 (Figure 2(d)). The RCC values of A. compressus in both densities, when grown in equal proportions, were more than RCCs of A. gangetica on them (Table 3).

Discussion
A. compressus shoot dry weight increased despite a decreasing number of A. compressus when grown with A. gangetica compared to monoculture at 72 and 288 plants m −2 in the open and at 288 plants m −2 in the shad. It appears that A. compressus produces more dry weight per plant when competing against A. gangetica than in monoculture. A. compressus shoot dry weight remained unchanged from C 100 W 0 to C 50 W 50 , hence, as proportions changed in this range both intra-and interspecific competition were counteracting each other. However, as fewer A. compressus plants were in the mixture (C 25 W 75 ), neighbouring plants apparently did not compete with each other, resulting in no intraspecific competition. Under these conditions, A. compressus was more efficient in competing against A. gangetica than against other A. compressus plants. By contrast, shoot dry weight of A. gangetica decreased with increasing A. compressus proportions in the mixture as A. gangetica had the lowest shoot dry weight in C 75 W 25 at both densities in the open and shade. These findings suggest that A. compressus responded plastically to competition, whereas A. gangetica did not. The greater biomass of A. compressus compared with A. gangetica in a mixture would result in a greater demand for resources. Other studies have also shown that more competitive species produce a higher relative yield when grown in mixtures, whereas the yield of weak competitors is lower in mixtures than in monoculture [27,28]. When one plant of basil (Ocimum sanctum) was competing with three of weed species, plant height and fresh weight plant −1 of basil increased [29]. Overyielding has been associated with higher biomass density and light interception or greater demand for resources [28,30,31].
A. gangetica at both densities in the open grew better in monoculture compared to C : W mixtures, indicating Asystasia gangetica was more affected by interspecific interactions.    Plant size suggests a potential advantage for light capture and greater penetration of PAR to the soil surface [33].
As A convex curve for one species and a concave curve for the other species in the series indicate that the species are competing for a common resource. When both curves are convex and concave, mutually stimulatory and antagonistic relations are indicated, respectively [24]. The RY of A. compressus increased in a quadratic manner or linearly to more than expected, while RY of A. gangetica in interaction with A. compressus was concave or linear but less than expected in all conditions, indicating that A. gangetica was more affected by interspecific interactions with A. compressus and was less competitive than A. compressus.
If the RY curves intersect at 50 : 50 proportions, the two competing species are relatively equal in competitiveness [25]. The RY of A. gangetica increased in a linear or nonlinear manner as its proportion in the mixture with A. compressus increased, but its RY was not equivalent to that of A. compressus when each comprised half the mixture. This resulted in EYR to equal to 0.75 at 288 plants m − An RYT value around 1 indicates that the same resource or area is being used by the two competing species (overlap in resource utilization) [20]. An RYT > 1 indicates some niche differentiation between the species, where competition is either avoided or minimized [17]. However, other processes can also produce RYT > 1, indicating facilitation where one species benefits another [34]. An RYT < 1 suggests mutual antagonism. The RYT value was less than 1 at 280 plants m −2 in the open and shade, and 72 plants m −2 in the open means that mutual antagonism is occurring with the species producing less than expected when grown together [35]. compressus with A. gangetica was < 1 for all proportions. The occurrence of allelopathic interaction would have lowered the total yield in mixtures compared with monoculture [16]. The RYT value at 72 plants m −2 in the shade was greater than 1.0 suggesting that the two species made different demands on resources leading to better growth of A. compressus or that this crop mixture was less affected by interspecific competition than by intraspecific competition, facilitating over yielding (RYT >1).
The relative crowding coefficient (RCC) value demonstrates the aggressiveness of one species towards another. The greater (RCC) values of A. compressus than the RCCs of A. gangetica at both densities in the open and shade, when grown in equal proportions, confirms the aggressiveness of A. compressus against the A. gangetica in terms of shoot dry weight production. When competing for limited resources, the species with the greater RCC in the mixture is the strongest competitor [17].
The superior competitiveness of A. compressus relative to A. gangetica is likely resource competition. Axonopus compressus became extremely dense fast, thereby limiting the space available to the weed population and suppressing A. gangetica growth. Fast growth result in a superior plant [36]. Furthermore, plant size suggests a potential advantage for light capture and greater penetration of PAR to the soil surface, making the crop less competitive against weeds [33]. Also, rapid growth by lateral spread of A. compressus through tillering seemed to be the reason for superiority of A. compressus in competition with A. gangetica. Greater tiller production is one of the factors associated with superior suppression [37]. The aggressiveness of A. compressus can also be explained in terms of its prolific rooting system, which enabled it to capture more of the limited soil water and nutrients and resulted in rapid growth in terms of biomass accumulation and canopy development [38]. Moreover, the presence of allelopathic interaction would have lowered the total yield in mixtures compared with the monocultures [16]. Axonopus compressus is known to produce allelochemicals that affect the growth of other plants [39]. There are some reports that demonstrate A. compressus had competitive ability. Oka Nurjaya [21] reported that in mixtures, A. compressus was more competitive than grass and legume species. Rika et al. [15] observed that under coconut, a local cultivar of A. compressus produced higher yields than other grasses. Axonopus sp., Digitaria sp., and Palspalum sp. are classified as soft weeds in oil palm plantation which maintain the balance of the weed flora and prevent weed succession by noxious species simply because the base land for the noxious weeds to colonise is less available [4,40]. On most oil palm plantations in the far East, the cover that establishes itself is a mixture of the fern Nephrolepis biserrata with varying components of grasses such Paspalum conjugatum and Axonopus compressus [41,42].
Based on the present findings, it can be concluded that A. compressus is highly competitive against A. gangetica. However, A. compressus under shade uses most of its energy to achieve more light for growth, and hence, Axonopus compressus density should be higher under shade to increase its competitiveness with A. gangetica. Further research into the A. compressus competitive ability with weeds is planned.