Eficiência de Bacillus cereus na promoção de crescimento e proteção do aparato fotossintético à infecção de Pestalotiopsis sp. em mudas de Coqueiro anão-verde- do-brasil

Abstract

The green-dwarf coconut tree (Cocos nucifera L.) is a palm tree of high economic importance and a productive chain intended to the coconut water production. There are factors which limits the obtaining of green dwarf coconut seedlings, highlighting the costs and phytosanitary problems, as leaf diseases. Using beneficial microorganisms in the agriculture, it is in promoting benefits to the plants increasing in quality, vigor and its protection from pathogens. In addition to allow and enable a sustainable agriculture. The objective of this work was to obtain biostimulants associated with low doses of the recommended chemical fertilizer (FQ), being able to promote growth and mitigate the leaf burning, unveil the associated physiological mechanisms and the economic agro-efficiency of the technology insertion in the brazil dwarf coconut tree seedling production system. The experiment was performed in commercial nursery in Santa Isabel-PA and in a greenhouse of Feral Rural University of Amazon, Belém-PA. Soil samples were collected and made a serial dilution which were obtained 96 distinct isolates of rhizobacteria. The greenhouse experimente was performed with model plant (Oryza sativa L.), the experimental design used was entirely random (DIC) with 97 treatment (96 rhizobacteria and control treatment with water), with three repetitions each. Aerial and radicular length and total biomass were evaluated. In nursery, the best isolated selected in model plant, was tested as growth promoter in brazil green dwarf coconut tree seedlings, the experimental design was in DIC with two control treatments with 100% of FQ and R40 with 50% of FQ. The R40 isolated was tested in vitro for phosphate solubilization, siderophore and indolacetic acid (AIA) production, in the seedlings were evaluated the biometrics, gas exchanges, hormonal concentration and nutritional content, the averages were compared by t test at 5% of probability. In nursery were collected leaf samples with leaf burning symptoms, and taken to the laboratory for analysis and isolation of associated fungi. A Pestalotiopsis sp. isolate was obtained in greenhouse. The pathogenicity test in two treatments: Control with water and Pestalotiopsis sp. (103 conidia mL-1.) sprayed plants, with three repetitions, were performed in three-months-old seedlings and treatments applied in the first leaf totally expanded, were applied suspension and kept under humid chamber. The presence of symptoms in the tested leafs were evaluated during seven days after the start of the experiment, followed by the collect and a second isolation. To the diagrammatic scale elaboration were collected 100 leafs of brazil green dwarf coconut tree seedlings, randomly, digitalized and obtaining real disease severity values (%), to build an scale proposal. The selected evaluators did not showed experience with the disease, coconut leaf images were projected to them, which were evaluated without and with the scale proposal help. From the results was made a linear regression analysis, relating the real severity and estimated severity. In nursery, was made a trial to evaluate the effect of R40 use on seedlings with leaf burning symptoms caused by Pestalotiopsis sp. Two treatments were used: control with 100% of FQ and R40 with 50% of FQ and five repetitions. The severity, leaf area, gas exchange and A chlorophyll fluorescence were evaluated monthly for six months. In addition, the chlorophyll content and percentage at the end of six months of age, the averages were compared by t test at 5% probability. To the evaluation of associated cost to B. cereus use in brazil green dwarf coconut tree seedlings, was performed an experiment with two treatments: control with 100% of FQ and R40 with 50% of FQ and five repetitions. Monthly, were evaluated during six months: Height, stem diameter and dry mass, the averages were compared by t test at 5% of probability. Tables with discriminated effective operational costs (COE) and total operational costs (COT) were elaborated, comparing the treatment with B. cereus with six-months-old nursery time with the conventional eight months seedling production. The results of this study showed that R40 isolate identified as B. cereus (UFRABC40), was phosphate solubilizer, siderophores and indolacetic acid (AIA) in vitro producer. B. cereus biostimulated seedlings combined to 50% of FQ, were superior in biometrics, gas exchange, hormonal concentration and nutritional content compared to the control seedling with 100% of FQ. Plants of the treatment with Pestalotiopsis sp. pulverization presented similar symptoms to the samples collected in nursery. The second isolation identified the fungi of Pestalotiopsis sp. genus as the leaf burning cause in Brazil green dwarf coconut tree seedlings. The proposed diagrammatic scale guaranteed, from the determination coefficient values, greater precision to 100% of the evaluators. Seedlings of the B. cereus treatment, promoted, from the third month old, reduction of leaf burning severity in coconut tree seedlings combined to greater CO2 assimilations, stomatal conductance, transpiration and carboxylation efficiency relation also occured the reduction of carbon intercellular concentration. In addition, it has provided positive changes in the photosystems evaluated from the a chlorophyll fluorescence. On these seedlings also had the greater percentage and chlorophyll content compared to control plants. In the economic efficiency trial, the biostimulated seedlings had greater biometric increment compared to the control treatment seedlings, and promoted reduction in the total operational cost and an 11% reduction in the seedling unitary value. Bacillus cereus combined with FQ reduction promotes coconut tree growth and reduces the incidence of leaf spots caused by Pestalotiopsis, the microbial technology applied in Brazil green dwarf coconut tree seedlings in the state of Pará increases the economic efficiency of the management.