versão impressa ISSN 0102-3306
Acta Bot. Bras. vol.25 no.4 Feira de Santana out./dez. 2011
SCIENTIFIC NOTE NOTA CIENTÍFICA
Fungos solubilizadores de fosfato isolados de área semiárida cultivada com melão (Cucumis melo L. cv. gold mine)
Flavia Paiva CoutinhoI, 1; Maria Auxiliadora de Queiroz CavalcantiI; Adriana Mayumi Yano-MeloII
IUniversidade Federal de Pernambuco, Centro de Ciências Biológicas, Departamento de Micologia, Recife, PE, Brasil
IIUniversidade Federal do Vale do São Francisco, Campus de Ciências Agrárias, Colegiado de Zootecnia, Petrolina, PE, Brasil
Considering that little is known about the occurrence of phosphate-solubilizing fungi from areas cultivated with melon, the phosphate solubilization ability of filamentous fungi isolated in these areas was evaluated. Three hundred and eighteen filamentous fungal isolates belonging to 23 genera were evaluated, besides Aphyllophorales and Mycelia sterilia. From those, 52 were able to solubilize P: Aphyllophorales (2), Aspergillus (34), Penicillium (10) and Rhizopus (6). These results will contribute to subsidizing further research regarding the capacity of these fungi to solubilize other sources of phosphate applied to the melon crop, as well as indicate the need for a screening program to select those with higher capacity and potential for solubilization.
Key words: fertilization, filamentous fungi, microorganism, San Francisco Valley
Considerando que pouco se conhece sobre a ocorrência de fungos solubilizadores de fosfato de áreas cultivadas com melão, foi avaliada a habilidade de solubilização desse nutriente por fungos filamentosos isolados dessas áreas. Foram avaliadas 318 amostras de fungos filamentosos pertencentes a 23 gêneros, além de Aphyllophorales e Mycelia sterilia. Dessas amostras, 52 apresentaram habilidade para solubilizar o fosfato: Aphyllophorales (2), Aspergillus (34), Penicillium (10) e Rhizopus (6). Esses resultados contribuem para subsidiar pesquisas que testem a capacidade desses fungos em solubilizar outras fontes fosfatadas aplicadas na cultura do melão, assim como indicam a necessidade de selecionar isolados com maior capacidade e potencial para solubilização.
Palavras-chave: adubação, fungos filamentosos, micro-organismos, Vale do São Francisco
Melon is one of the most demanding cucurbitaceous crops, usually demanding potassium, nitrogen and phosphorus fertilization (Vitti et al. 1994). Phosphorus stimulates growth and is essential to root formation, flowering and fructification, acting as a decisive factor in the quality and quantity of fruits (Crisóstomo et al. 2002).
Plant productivity is often limited by low levels of soil nutrients in absorbable forms. Phosphorus is possibly the most limiting nutrient in tropical regions, considering its high stability and, consequently, low solubility (Moreira & Siqueira 2006). Microbes have the ability to make insoluble P available by the processes of solubilization or mineralization. The solubilization process occurs through the production of organic acids that directly dissolve phosphate minerals or by a chelating action over soil cations liberating soluble phosphates (Silva Filho et al. 2002). Therefore, P solubilizing microorganisms play a key role in the biogeochemical cycle of phosphorus and its availability to plants (Carneiro et al. 2004).
With the development of organic agriculture in fruit production in the San Francisco Valley, many organic residues have been used as fertilizers. However, little is known about the effect of these residues on the soil mycobiota, especially on phosphate-solubilizing fungi. Organic residues that favor the increase of the P-solubilizing fungal population are of utmost importance for improving the nutritional condition of the crops in the region. The aim of this study was to evaluate the ability of phosphate solubilization by filamentous fungi isolated from the rhizosphere of melon plants (Cucumis melo L. cv. gold mine) fertilized with organic composts.
Fifty-four rhizosphere soil samples were collected at random from an irrigated semiarid area cultivated with melon (C. melo cv. gold mine) in the San Francisco Valley, Petrolina, Brazil (09º32´09´´S, 40º55´28´´W). Aliquots (1 mL) of serial dilutions (1:1000) of the soil samples were plated on Sabouraud Agar medium with chloranphenicol (500 mg l-1), and incubated at 28 ºC for up to 96 h. The fungal colonies were transferred and maintained on Potato Dextrose Agar (PDA). Fungal species were identified based on colony morphology on different media (Czapeck, malt extract agar and PDA) and micromorphological characteristics. The evaluation of the ability of these fungi to solubilize phosphorus was carried out by removing a 5 mm diameter disc from the edge of the colony and transferring it to a Petri dish containing GAGES solid medium (glucose, arabinosis, glycerol, soil extract, CaCl2, MgSO4, NaCl and agar) supplemented with 10% K2HPO4 (50 ml) and 10% CaCl2 (50 ml), to produce inorganic phosphate precipitate, CaHPO4 (Souchie et al. 2006). The plates were kept at room temperature (28 ºC) for 12 days and the colonies that formed a clear zone around them, due to solubilization of CaHPO4, were considered P-solubilizing fungi.
From the soil samples, 318 filamentous fungi (Tab. 1) were isolated, belonging to 23 genera and 78 species, besides Aphyllophorales and Mycelia sterilia. Fifty-two of them were able to solubilize P, as follows: Aspergillus flavus (2), A. japonicus var. aculeatus* (3), A. japonicus var. japonicus* (8), A. niger (16), A. niger var. niger* (2), A. tamarii* (3), Penicillium citreonigrum* (1), P. crustosum* (1), P. decumbens* (4), P. janthinellum* (4), Rhizopus microsporus var. chinensis* (6) and Aphyllophorales (2). Those marked (*) are reported for the first time as P-solubilizers.
Among the identified species, 15% are P-solubilizers, which agrees with values usually found soil (8 to 58% of all fungi), as reported by Nahas et al. (1994). Aspergillus and Penicillium species stood out as phosphorus solubilizers, and these two genera are the most cited in the literature with this capability (Silva Filho et al. 2002; Wakelin et al. 2004; Souchie et al. 2006). However, most works have not identified the species, making comparisons only possible at the genus level. All 16 samples of A. niger were able to solubilize phosphate. This species has been isolated from soils cultivated with a wide variety of plants, such as coconut, beets, olive, wheat, chickpea, soybean, broad bean, rice, sugar cane, Pinus, Eucalyptus and Avicennia germinans (Wahid & Mehana 2000; Abd-Alla & Omar 2001; Rashid et al. 2004; Zayed & Abdel-Motaal 2005). Two samples of Aspergillus flavus solubilized P, which has been reported for the species (Abd-Alla & Omar 2001; Sapatnekar et al. 2003; Rashid et al. 2004). Aspergillus fumigatus, A. nidulans, A. terreus, Emericella nidulans, Fusarium oxysporum, Sordaria fimicola and Thielavia terricola were not able to solubilize P, but other isolates of the same species from Indian soils were reported as phosphorus solubilizers (Sapatnekar et al. 2003). Similarly, Penicillium citrinum and Trichoderma viride did not show the capacity to solubilize P in this study, although P. citrinum, isolated from wheat, soybean and broad bean rhizospheres (Abd-Alla & Omar 2001), and T. viride, isolated from black soil of Maharashtra, India, and sugar cane plantations (Sapatnekar et al. 2003; Zayed & Abdel-Motaal 2005), have been reported as solubilizers. A screening program to select those isolates with high capacity and potential for solubilization is planned, with the further goal of a program for controlled inoculation.
The authors would like to thank the CAPES, CNPq and Embrapa Semiárido for supporting the project and Dra. Elaine Malosso and Leonardo Costa for reviewing the English text.
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Recebido em 21/10/2008
Aceito em 9/09/2011