ALLELOPATHIC POTENTIAL OF AQUEOUS EXTRACT OF LEAF AND ROOT SESAME ON SICKLEPOD AND BRACHIARIA SEEDS

Sicklepod is an important weed in Brazil pastures, but control measures are not yet effective. Thus, allelopathy can be an alternative, due to its ecological importance and for being able to provide new structures sources for agrochemicals production. This research aimed to verify the allelopathic potential of sesame (Sesamum indicum L.) on germination and seedling development of sicklepod (Senna occidentalis L.) and brachiaria (Urochloa brizantha). The treatments were root and leaf extract of sesame, and control (distilled water). Germination percentage, speed of germination, hypocotyl and radicle length were evaluated. Speed of germination, germination percentage and radicle length had no interference from the aqueous extracts. However, in brachiaria these traits had lower values than sicklepod. Although no statistical difference was found for hypocotyl length, the leaf extract showed lower growth of this feature. Allelopathy did not affect germination and speed of germination in both species, but interfered in brachiaria seedling development.


INTRODUÇÃO
Brazil has the largest commercial bovine herd in the world, standing out in the production and export of meat (CARVALHO; ZEN, 2017). It is estimated that 96.5% of slaughtered cattle are raised on pasture and the remaining 3.5% have at least one of their breeding phases maintained in this system (FONSECA et al., 2010).
Among the pastures and forages used in Brazilian livestock, the genre Urochloa stands out. However, several pastures established with this genre show signs of degradation, denoting this as one of the main obstacles faced by national livestock, considering that the estimate is approximately 80% of the areas cultivated with pastures, present degradation characteristics (DIAS-FILHO, 2014).
Several factors are related to the degradation of pastures, among than inadequate initial formation, absence of maintenance fertilization and inadequate grazing management, which consequently favor the emergence and establishment of weeds (IKEDA, 2007;MACEDO, 2009;MECHI et al., 2018).
The control of weeds in pastures, in addition to increasing the cost of production, can also intoxicate and injure animals (CLAUDINO et al., 2016). The damage of weeds to pastures occurs through competition for water, light and nutrients, in addition to producing allelopathic compounds, affecting the quality and quantity of the yield of the forage species (VICTORIA FILHO et al., 2014).
The intensity of weed interference in the pasture depends on the characteristic of the cultivated species (spacing, sowing density, soil conditions, climate, management, season and duration of the period of mutual coexistence), as well as the characteristics of the crop (JAKELAITIS et al., 2010;CAVALCANTE et al., 2017;BRIGHENTI, 2018).
The sicklepod (Senna occidentalis) is the most common weed species found in pastures. This species develops very well in tropical climate conditions and it has high reproductive and developmental capacity, in addition to being considered toxic to animals (WALKER; OLIVER, 2008).
The methods used to control the sicklepod have little efficacy, since the fires carried out on pastures stimulate their germination, a method widely used in the Brazilian Midwest, and the use of chemicals is efficient only in the initial development of the plant, being adults more resistant to this control (DIAS-FILHO, 1990;JABRAN et al., 2015;Müller-Stöver et al., 2016).
Alternative methods are being developed, which are environmentally safe and biodegradable in nature, in order to reduce the use of synthetic herbicides (BAJWA et al., 2016). Alternative control is an option for the replacement of agrochemicals, favoring the reduction of its use and minimizing the damage caused to the environment (CARVALHO et al., 2012).
Allelopathy is the ability of plants, algae and microorganisms to produce chemical substances that, released into the environment, favorably or unfavorably, directly or indirectly, affecting the develop of other species .
Allelopathic sesame (Sesamum indicum L.) extracts demonstrated an efficient action in the development of black prick seedlings and reduced the germination percentage of this weed from 80% to 28% in relation to the control (PEREIRA, 2007). Although research has shown the effectiveness of using allelopathic substances on weeds (SILVA et al., 2016;ESPINOSA et al., 2019;RIBEIRO et al., 2019), there are few literary references on the use of plant extracts to control sicklepod, requiring further research in this area (SOUZA FILHO et al., 2003).Thus, this study aimed to verify the aqueous extracts allelopathic potential of sesame leaves and roots (Sesamum indicum L.) in germination and development of sicklepod (Senna occidentalis L.) and brachiaria (Urochloa brizantha) seedlings.

MATERIAIS E MÉTODOS
The experiment were carried out in the Botany Laboratory of the Mato Grosso State University (UNEMAT). Allelopathic potential of aqueous extract of leaves and root sesame (Sesamum indicum L.) in seeds of sicklepod (Senna occidentalis) and brachiaria grass (Brachiaria brizantha) were analized by germination test trials (SGI and germination percentage) and initial seedling development (length of hypocotyl and root length). The extracts had a single concentration of 1:5 (w/v), being compared with distilled water, which served as a witness.
In this way, two treatments were obtained with aqueous extracts of leaves and roots of sesame, in the concentration 1:5, and the control, used in seeds of sicklepod and brachiaria.
Sicklepod seeds were obtained from field collection, in a rural property in Cáceres (MT), homogenized, packed in paper bags, stored at room temperature until use, and identified according to the literature by LORENZI (2000).
Brachiaria seeds also used for analysis were obtained from local stores (germination = 62%). The seeds of sicklepod and brachiaria grass underwent a process of breaking dormancy before the test, using 35 mL of concentrated sulfuric acid (H2SO4), where the seeds of sicklepod and brachiaria were kept immersed for 20 and 15 minutes, respectively, in acid, washed in running water for 5 minutes and dried in the shade (BRASIL, 2009).
Seed germination was carried out according Brasil (2009), using gerbox with four replications of 20 seeds distributed on two sheets of Germitest paper, moistened with 5 mL of aqueous extract or water, depending on the treatment. The gerboxes were kept at an average temperature of 28.5 °C (± 2 ºC), under constant light in the germination chamber.
The seeds were moistened whenever necessary.
Seed germination counts were performed daily for 20 days. The seeds were considered germinated when the protrusion of the radicle through the integument became visible (2 mm). The speed of germination index (SGI) were calculated using the following formula proposed by MAGUIRE (1962).
G is number of germinated seedlings, computed in the first, second, third and until the last count; N is number of sowing days in the first, second, third and until the last count; After the end of the germination, the hypocotyl and radicle length (cm) were measured using graph paper.
The experimental design adopted was completely randomized, with ten replications.
The germination percentage variable was transformed into a sine arc √ 100 . The variables were analyzed statistically through the normality test (Lilliefors) and homogeneity of variances (Cochran and Bartlett) and when significant, ANOVA was performed. The treatment means were compared using the Tukey test, at 5% probability.

RESULTADOS E DISCUSSÃO
The aqueous extracts used did not influence sicklepod and brachiaria germination, as shown in Figure 1. The average values obtained for germination percentage were 16.84%; 17.78% and 17.85% for sicklepod and 8.59%; 7.75% and 9.81% for brachiaria grass, being root extract, leaf extract and control, respectively. These results demonstrate that the germination percentage of both species was low, brachiaria still having the lowest percentage when compared to the sicklepod. This fact may be due to the type of experimental conduction to which they were subjected, as the test was conducted with 24 hours of light / day and an average temperature of 28 º C (± 2 º C) during the entire period because of the germination chamber that stored seeds only this configuration. Since it is recommended to use 8 to 12 hours of light / day with temperature at 25º C, for brachiaria seeds (AZANIA et al., 2003;CHIODINI;CRUZ-SILVA, 2013) and constant lighting at 25 ° C for the sicklepod seeds (BORGES et al., 1997), it can be inferred that the light factor may have caused low seed germination for both species.
Thus, it may be that the effects of allelochemicals present in the different extracts were suppressed by the experimental conditions. However, many times, the allelopathic effect may not occur on the germination percentage, but on the SGI or another process  The data confirmed that analyzing only the percentage of germination and the SGI does not provide sufficient information regarding the seedling development process, which demonstrates the importance of evaluating the length of the hypocotyl and radicle to better verify the behavior of the species in the germination and development.
However, the length of the hypocotyl was also not influenced by the treatments.
However, the sesame leaf extract applied to the brachiaria seedlings provided less growth of the hypocotyl (Figure 3). This may have occurred due to the higher concentrations of allelochemicals (lecithin, sesamine, mircine and tannins) being present in the leaves of plants, when compared to the roots (WANDSCHEER; PASTORINI, 2008;SILVEIRA et al., 2021). The fact that sicklepod seedlings do not suffer interference from both extracts may be linked to the more pronounced specificity of secondary compounds in causing negative allelopathic effect only in brachiaria grass seedlings.
The seedlings of sicklepod did not differ statistically in the length of the radicle with the different treatments to which they were submitted. In the brachiaria seedlings, the sesame leaf extract provided less root growth compared to the control treatment. However, the treatment that used sesame root extract did not differ significantly from the control treatment, as shown in Figure 4. The sesame leaf extract showed potential to delay the hypocotyl and radicle lenght seedling development (Figures 3 and 4).
According to SOUZA FILHO (2003), the allelopathy effects on the lengthening of grass root can affect the competitive capacity of these crops by factors essential to survival, such as water and nutrients, with the main consequence of decreased productivity and longevity of pasture, and with negative effects on reducing pasture maintenance costs.
The results obtained suggest that the development of the hypocotyl and radicle are more susceptible to the action of sesame extracts when compared to the germination percentage and the speed of germination. In this way, the data indicate that allelopathic compounds possibly act on the seedling development, having greater expression in brachiaria.
In general, the aqueous leaf and root sesame extract at a concentration of 1:5 (w:v) did not affect the sicklepod initial development, one of the main weeds found in brazilian pastures. However, it negatively affected the brachiaria seedlings development, reducing the length of the hypocotyl and radicle, therefore, it is not recommended to apply this extract It is also worth noting that although brachiaria is the main species cultivated in brazilian pastures (PAULA et al., 2012;BEZERRA et al., 2020), in many agricultural areas, mainly grain production, this species is considered a difficult to control weed (BIANCO; TONHÃO;PITELLI, 2005;FORTE et al., 2018), thus, for these cases, the use of aqueous leaf and root sesame extract is indicated for the control of U. brizantha in the initial phase of development.