Introduction

Among the weeds, morning glory (Ipomoea purpurea L.) is one of the most common invasive species in agricultural areas since it is widespread in almost all Brazilian territory. Belonging to the Convolvulaceae family, which includes approximately 58 genera, the genus Ipomoea has about 650 known species (Mabberley, 1997). These are climbing and herbaceous plants, which occur more frequently in moist and well-prepared soils. Its propagation is exclusively by seed (Moreira and Bragança, 2011). It is extremely harmful to many annual crops because it has a longer life cycle than cultivated species and also because it hinders mechanized harvesting, since it becomes intertwined with plants. Another important characteristic is the production of a considerable number of seeds per season, about 50 to 300, which have asynchronous germinations over time (Kissmann and Groth, 1999). According to Azania et al. (2003) and Pazuch et al. (2015), Ipomoea species have different germination flows during spring and summer, due to dormancy. In the soybean crop, losses of 27 to 45% have already been reported due to infestation by this invasive plant (Piccinini et al., 2018). In addition to what was mentioned, this vegetable can also host the bacterium Pseudomonas syringae pv. syringae and mites of the genus Brevipalpus (Moreira and Bragança, 2011).

In agriculture, there is an increasing number of plant species that are resistant or tolerant to different herbicidal molecules (Travlos et al.,2020). For Vargas and Roman (2006), the level of tolerance of plants against herbicides can be classified as susceptible, tolerant and resistant, which will depend on how the species of weeds will react after the action of the herbicides. According to Monquero et al. (2004), Ipomoea species are among the most tolerant to glyphosate. Monquero and Silva (2007) also reported that glyphosate did not efficiently control I. purpurea. Later, Pazuch et al. (2017) reported tolerance in Ipomoea spp. in the southern region of Brazil, which raises concern about the future of the management of this invasive species.

Integrated management uses several techniques to combat weeds, for example: cultural, preventive, mechanical, physical, chemical and biological (Oliveira and Brighenti, 2018). This is a strategy that makes it possible to reduce the use of synthetic agricultural pesticides; in addition, the approach not only decreases the possibility of weeds becoming resistant, but it is also a way for farmers to save on the purchase of phytosanitary products and causes less environmental impacts. Allelopathy can be cited as a management measure, which consists of any positive or negative interaction that a plant can exert over another, through chemical compounds released by them (Mehdizadeh and Mushtaq, 2020). Allelopathic compounds released by a plant can affect several intrinsic factors, for example inhibiting or delaying the germination of seeds of other plant species (Miller, 1996). Through allelopathic indications, species can be selected for use in consortium, as a source of new herbicide molecules (Oliveira and Brighenti, 2009), and by using extracts for the alternative management of pests, diseases and weeds.

Some studies have already been carried out to investigate plant allelopathy regarding Ipomoea spp., for example, Steinsiek et al. (1982) who studied the potential allelopathic effect of wheat on I. hederacea (L.) Jacq. and I. lacunosa L. Mauli et al. (2009) evaluated the effect of aqueous extract of leucena (Leucaena leucocephala (Lam.), R. de Wit.) on the germination of I. grandifolia (Dammer) O'Donell. Lima et al. (2009) also investigated lemon grass (Cymbopogon citratus Stapf.) and elderberry (Sambucus australis Cham. & Schltdl.) allelopathy in the germination and initial development of I. grandifolia. The hydroalcoholic extract of araticum-do-cerrado (Annona crassiflora Mart.) was also evaluated in the germination of I. grandifolia, by Inoue et al. (2010). According to Araújo et al. (2010), sunn hemp (Crotalaria juncea L.) extracts reduced the germination of I. grandifolia. Grisi et al. (2013) tested the influence of the aqueous extract of the soapberry root (Sapindus saponaria L.) on the germination of I. grandifolia. Silva et al. (2015a) evaluated the allelopathic potential of papaya seed extract (Carica papaya L.) on the germination of I. purpurea. Studies have shown that the aqueous extract of the leaves of Eucalyptus grandis W. Hill ex Maiden promote the inhibition and reduction of the germination of I. purpurea seeds (Silva et al., 2015b). Silva et al. (2016) described the antagonism of leachate from Asemeia extraaxillaris (Chodat) J.F.B. Pastore & J.R. Abbott on the growth of I. cordifolia.

Therefore, the objective of the present work was to evaluate the allelopathic effect of the aqueous crude extract of 24 plants on Ipomoea purpurea L.

Materials and Methods

The study was conducted in the Federal District, central Brazil (15.58 ºS, 47.73 ºW), consisting of the Cerrado biome, during the months of August 2019, March 2020 and September 2020. According to the Köppen classification, the location has a Tropical seasonal climate of megathermic savannah, with an average annual precipitation of 1,400 mm (Cardoso et al. 2014).

The seed lots of Ipomoea purpurea were purchased from a commercial supplier, already chemically treated and within the expiry date.

The plants, source of plant extracts, were collected in the medicinal and toxicology garden or in the afforestation of the college's own campus, in the morning period, and are described according to the table below (Table 1).

Table 1. Description of the plants used in this study.

Six independent experiments were carried out (1-5 in vitro):

1. 1-9 plants (4 mL of 35% extracts) (Table 1);

2. 10-18 plants (4 mL of 35% extracts);

3. 19-24 plants (4 mL of 35% extracts);

4. Plants selected from previous experiments (8 mL of 35% extracts);

5. Plants selected from previous experiments (4 mL of 50% extracts);

6. Plants selected from previous experiments for in vivo tests.

To prepare the aqueous crude extract, 35 g or 50 g of young leaves and branches of the mentioned plants were used, which were crushed in 100 ml of distilled water.

In the in vitro experiment, a gerbox (11 x 11 x 3.5 cm) containing three sheets of autoclaved germitest paper was used, where 50 seeds were deposited and 4 ml or 8 ml of the extract of each plant (treatments) were added. The gerboxes were sealed with plastic wrap to prevent drying, and then they were kept at room temperature and in light. As moisture was lost, the seeds were moistened. The readings of the experiment were daily, where the number of seeds germinated each day was recorded.

Based on the germination data, the average germination time (equation 1) and the germination index (equation 2) were calculated using the following equations (Santana and Ranal, 2004):

 (days), average germination time                          (Eq 1)

              (Eq 2)

Where f_i = number of seeds germinated on the i-^th day; and x_i = number of days counted from sowing to the day of reading.

The experiment was conducted using a completely randomized design (CRD). The data were submitted to analysis of variance (ANOVA), followed by the Tukey test, at 5% significance level, using the Sisvar 5.6 program (Ferreira, 2014).

Results and Discussion

Experiment 1

The results of this experiment showed a delay in the radicle emission of Ipomoea purpurea for all treatments (Figure 1A), varying between 0.28 and 0.96 days. The rosemary crude extract, which took 3.44 days to germinate, stood out when compared to the control treatment, which germinated in 2.48 days, causing a significant delay of almost one day.