1 Departamento de Botanica, Universidade de Brasilia, 70910-970, Brasilia, Brasil. Email: firstname.lastname@example.org
2 Departamento de Botanica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brasil. Email: email@example.com
The Cerrado is a savannah-like vegetation covering about 2 million Km2 in central Brazil. It shows high levels of biodiversity and endemism and is characterized by extreme temperatures, fire and water stress. Allelopathic properties were investigated on five Cerrado species (Eugenia dysenterica, Qualea parviflora, Campomanesia adamantinum, Solanum lycocarpum and Trembleya parviflora). For each species, leaves were collected from natural populations, dried at 50°C, grounded, and dissolved in distilled water to obtain 5% stock solution. After 24h, these solutions were filtered and diluted to prepare solutions from 0.5 to 5%. One-day old sesame (Sesamum indicum) seedlings were disposed in Petri- dishes lined with filter-paper and the respective treatment (n>60/treatment). Water was used as control. The experiments were conducted at 30°C and photoperiod of 12h. After five days, roots and shoots of the treated seedlings were measured and compared to the control. Leaf extracts of E. dysenterica and Q. cordata at 1% reduced more than 60% the root growth of sesame seedlings. Extracts at 3% and above were necessary to significantly inhibit shoot growth. Extracts of C. adamantinum, S. lycocarpum and T. parviflora at 1% inhibited root growth by 50%, but had no effect on shoot growth up to 2%. Extracts of S. lycocarpum, T. parviflora and E. dysenterica induced lateral root development, inhibited root hair differentiation, and in some cases impaired the geotropic curvature. For all species, extracts effects on sesame growth showed a dose-response relationship, and the roots were more affected than the shoots.
The Cerrado sensu lato is the second largest Brazilian biome, covering about 2 million km2 in Central Brazil. The climate is typical of the moister savannah regions of the world, with an average precipitation of 800-2000 mm and a very strong dry season from mid-May to mid-September. High temperatures occur during the dry season, when fires are also very common. This is an ancient biome that shows a high degree of endemism and biodiversity (Ratter et al. 1997). Located quite southeast to the Amazonian Rain Forest and northwest to the Atlantic Forest and montane semi-deciduous forests, the flora of the Cerrado presents many species in common to these biomes (Ratter et al. 1997). The Cerrado has been considered a hotspot of importance for conservation (Myers et al. 2000). The typical vegetation landscape within this biome consists mostly of the Cerrado sensu stricto (a savannah-like vegetation) on the well-drained interfluves with gallery forests following the watercourses. The Cerrado sensu stricto itself presents several types of vegetation, ranging from dense grassland, usually with a sparse covering of shrubs and small trees, to an almost closed woodland with a canopy height of 12-15 m (Ratter et al. 1997). Studies have shown allelopathic properties of Cerrado species as Andira humilis (Rizzini 1970; Periotto et al. 2004), Aristolochia esperanzae (Gatti et al. 2004) and Solanum lycocarpum (Oliveira et al. 2004a, b).
In order to improve knowledge about allelopathic properties of Cerrado species, five species were selected to test the effects of aqueous extracts of their leaves on the growth of a target plant. The species were the trees Eugenia dysenterica and Qualea parviflora and the shrubs Campomanesia adamantinum, Solanum lycocarpum and Trembleya parviflora. All species are native of the cerrado sensu stricto. Sesame (Sesamum indicum) was chosen as the target plant because of the uniformity of the seed germination (Carvalho et al. 2001) and seedling growth.
Five species of common occurrence in the Cerrado vegetation, Campomanesia adamantinum Camb. Eugenia dysenterica DC. (Myrtaceae), Qualea parviflora Mart. (Vochysiaceae), Solanum lycocarpum St. Hil. (Solanaceae), and Trembleya parviflora (D. Don) Cogn. (Melastomataceae) were selected for this study. Mature leaves were collected from natural populations occurring at the Brasilia National Park (~ 48°54`0``W, 15°42`0``S, Federal District) just before their use to prepare the extracts. The leaves were oven-dried at 50°C for 24 h, grounded with a pistil and a mortar, and dissolved in distilled water to obtain 5% stock solutions. After 24 h of incubation at 4°C, the stock solutions were filtered and diluted to prepare test solutions ranging from 0.5 to 5%, depending on the species (see below). One-day old sesame (Sesamum indicum) seedlings were disposed in Petri-dishes layered with one filter-paper and the respective test solution. The number of seedlings per treatment is indicated on the Tables and Figure. Distilled water was used as control. The experiments were conducted at 30°C, and a photoperiod of 12 h (white light). After five days of incubation, roots and shoots of the treated seedlings were measured and compared to the control seedlings (Kruskal Wallis`Test, p=0.05; Biostat 2.0). Effects on root and shoot differentiation were also noticed.
Leaf extracts of Q. parviflora at 0.5% reduced by 60% the root growth when compared to the control, but extracts at 2% and above were necessary to significantly inhibit the shoot growth of sesame seedlings. Extracts of E. dysenterica at 1% reduced by more than 80% the root growth, but had no significant effect on the shoots (Table 1).
Table 1. Effect of aqueous leaf extracts of Qualea parviflora and Eugenia dysenterica (n=120/treatment) on the growth of roots and shoots of Sesamum indicum. Different letters within columns indicate significant differences in the average length (Kruskal Wallis, p=0.01). Standard deviations are in parenthesis.
Leaf extracts of T. parviflora and C. adamantinum at 1% inhibited root growth by more than 50%, when compared to the control (Table 2). On the other hand, extracts of C. adamantinum up to 2% did not affect the shoot growth, while extracts of T. parviflora up to 2% reduced shoot growth by 15% at most (Table 2).
Table 2. Effect of aqueous leaf extracts of Trembleya parviflora and Campomanesia adamantinum on the growth of roots and shoots of Sesamum indicum. N=90/treatment. Different letters within columns indicate significant differences in the average length (Kruskal Wallis, p=0.01). Standard deviations are in parenthesis.
Leaf extracts of S. lycocarpum inhibited root growth by more than 50% at concentration as low as 1%, but shoot growth was not affected at concentrations up to 3% (Figure 1). The effects of leaf extracts of both S. lycocarpum (figure 1) and Q. parviflora (table 1) on root growth showed a dose-response relationship. For both species the shoot growth was only inhibited at the higher concentrations (2-5%).
Despite of the effects of the extracts on shoot growth, no abnormality was observed in the shoot morphology of the treated seedlings. The cotyledons expanded, and the hypocotyls greened similarly to the control seedlings. On the other hand, extracts of S. lycocarpum, T. parviflora and E. dysenterica at 1% and above induced lateral root development, inhibited root hair differentiation, and in some cases impaired the geotropic curvature of sesame seedlings. Necrosis on the root tips was also observed at concentrations above 2-3%. Therefore, we do not know whether the reduction of the shoot growth was a direct effect of the extracts on the shoots or simply a consequence of their effects on root metabolism.
Figure 1: Effect of leaf extracts of Solanum lycocarpum on the growth of Sesamum indicum seedlings. The seedlings were grown for five days at 30oC under white light at different extract concentrations (w/v, in percent). N=30/treatment. Bars represent the standard deviation (Oliveira et al. 2004a).
Aqueous leaf extracts of five Cerrado species inhibited root and shoot growth of sesame seedlings. In most cases the effects were detected at concentrations as low as 0.5%. Overall, the roots were far more affected than the shoots. The effects on root morphology, as the development of lateral roots and the inhibition of root hair differentiation, open the perspective to investigate the action of the compounds on specific targets at cellular level (Macias et al. 2004). Through this kind of study it is expected to expand the use of Cerrado species as sources of new chemicals. On the other hand, these and other studies suggest that allelopathic properties of Cerrado species may be among the determinants of plant recruitment under natural conditions. These possibilities remain to be investigated.
We are thankful to Fabio N. Noda for technical assistance and to CAPES for providing scholarship to J. D. Pinheiro. The first author also thanks to the FINATEC (Fundação de Empreendimentos Científicos e Tecnológicos / Brasilia) for providing support to attend the meeting.
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