Agronomic performance of okra under the effect of seed microbiolization with rhizobacteria

Plant-growth promoting rhizobacteria are an alternative to increase the productivity, adding value to the productive system of crops like okra. We aimed to evaluate the agronomic characteristics of okra genotypes after seed microbiolization with nine strains of rhizobacteria. The experimental design was in randomized blocks, in a 2x10 factorial scheme, using two okra genotypes, cultivar Santa Cruz 47 and hybrid Dardo, nine strains of rhizobacteria, LEBM 103, 105, 109, 112, 118, 134, 135, 152 and 154 and one control (without microbiolization), with four replications. We analyzed number of fruits per plant (NFP), average diameter of fruits (DM), productivity (PDT); fresh mass of fruits per plant (MFFP) and average mass of fruit (MMF). The rhizobacteria strains LEBM 109, 112, 152 and 154 provided the highest values for MFFP (1202.7; 1086.5; 1158.2 and 1175.9 g, respectively) PDT (24.1; 21.7; 23.2 and 23.5 t/ha, respectively) and NFP (78.0; 69.4; 73.27 and 76.5, respectively) in relation to the other strains and the non-use of seed microbiolization of hybrid Dardo. For cultivar Santa Cruz 47, the rhizobacteria LEBM 105, 112 and 152 provided the highest values for MFFP (896.7; 769.0 and 762.2 g, respectively). Evaluating genotypes when using each rhizobacteria, for MFFP and PDT, only for LEBM 105, no difference among genotypes was noticed, so, we concluded that okra seed microbiolization with rhizobacteria may provide productivity gain with possibility to become a feasible technique. The rhizobacteria which are the most recommended for growing hybrid Dardo are LEBM 109, 112, 152 and 154, whereas for growing cultivar Santa Cruz 47 are LEBM 105, 112 and 152.

However, this crop still does not express its total production potential due to, mainly, the low level of technology adopted by the majority of producers.Thus, the plant-growth promoting family farming, since it is a plant which shows a fast vegetative cycle, resistance to pests and diseases and with low-cost production; it is widely cultivated in Brazil, where the weather conditions are excellent for the plant development O kra (Abelmoschus esculentus) is an annual vegetable crop, with excellent nutritional value, rich in vitamins A and C, calcium, fibers, niacin, besides having medicinal properties (FAO, 2003).It is especially grown by rhizobacteria (PGPR) can be an alternative to increase the productivity, adding value to the production system.
PGPRs can increase the plant growth since they promote the mineralization of nutrients, by the solubilization of mineral phosphates and the production of growth hormones such as auxins and gibberellins.Besides, the production of low-cost inoculants with PGPRs is an alternative to reduce the use of agrochemicals and chemical products (Coelho et al., 2007).In addition to these benefits, the rhizobacteria can act as biological control agents of diseases (Zhuang et al., 2007).PGPRs act as biological control agents of diseases, since they induce systemic resistance in plants, producing antibiotics and siderophores, which inhibit the growth of several pathogens by activating the formation of physical and chemical barriers (Kloepper et al., 2004;Romeiro, 2007).
In Brazilian literature no information on the use and influence of rhizobacteria on okra crop was found.Thus, the aim of this study was to evaluate the agronomic yield of genotypes of okra, hybrid Dardo and cultivar Santa Cruz 47, after seed microbiolization with rhizobacteria.

MATERIAL AND METHODS
The experiment was carried out at Horta de Ensino, Pesquisa e Extensão from Universidade Estadual de Montes Claros, municipality of Janaúba, Minas Gerais State, Brazil (15º47'18"S, 43º18'18"W, altitude 141 m), from September, 2009 to January, 2010.This region is part of the Brazilian semi-arid region, the local climate is Aw (tropical with summer rains), according to Köppen classification (Jacomine et al., 1979).The average annual rainfall is 740 mm, 85% occurring from November to March, with average maximum and minimum temperatures of 32°C and 19.5 o C, respectively.The soil in which the experiment was installed is a Fluvic Neosol (Embrapa, 1999).
The authors used seeds of cultivar Santa Cruz 47 and hybrid Dardo, microbiolized with rhizobacteria: LEBM 103, 105, 109, 112, 118, 134, 135, 152 and 154, classified  The process of seed microbiolization started with the removal of existing impurities, being immersed in 1% sodium hypochlorite for 5 minutes and, then, rinsed three times with sterile distilled water.Afterwards, the seeds were dried in a laminar flow chamber, on sterile paper towel, and then they were placed in Erlenmeyer and immersed in 4.68 μL of the bacterial suspensions for one hour and under stirring at 150 rpm in the orbital shaker (model 430/ RDBP, Nova Ética).Afterwards, the seeds were dried for 2 hours, similarly to the previous way.Then, the seeds were kept in sterile paper bags and were immediately planted.
The experimental design was completely randomized, arranged in a 2x10 factorial scheme, two okra genotypes, nine strains of rhizobacteria and one control without microbiolization, with four replications.Each plot was composed of four planting lines, measuring 2 m long each, spacing 1.0x0.5 m, totalizing 16 plants, with a total area of 8 m 2 .The area consisted of four central plants, with a total area of 2 m 2 .
Okra seeds were planted in tubes of 12 cm length and 55 cm 3 , on September 17, 2009, using commercial substrate Plantmax, under greenhouse conditions, until the transplanting, when the seedlings showed two definitive leaves.The transplanting into the experimental area was carried out on October 8, 2009.
F e r t i l i z a t i o n w a s c a r r i e d out according to the soil analysis interpretation, taking as reference Ribeiro et al. (1999).Thus, 30 t/ha of tanned barnyard manure, 40 kg/ha of P 2 O 5 , 60 kg/ha of K 2 O and 120 kg/ha of N were applied into planting furrows.All of P 2 O 5 was applied at planting time, whereas K 2 O and N were splitted, being 40% of K 2 O and 20% of N applied at planting time, and the rest of them (60% of K 2 O and 80% of N) splitted at 20, 40 and 60 days after planting.
The seeds were irrigated using a m i c r o -s p r i n k l e r s y s t e m , t h e management was carried out using a class A evaporation pan, irrigation water depth of 5 mm/day, being irrigated once daily due to the sandy texture of the soil.No disease incidence or pest attack was observed.The weed control was carried out through hoeing.Okra fruits were harvested every three days, for twelve weeks, when the first fruit showed up, at commercial harvesting time (10 to 12 cm length), when they were still tender.
After harvesting, the fruits were packed in plastic boxes, and taken to the Laboratory of Post-harvest of State University of Montes Claros, where the analyses were carried out.
The authors analyzed: 1) number of fruits per plant (NFP) (number of fruits of the useful plot divided by number of plants of the useful plot); 2) average diameter of okra fruit (DM) (freshly harvested fruits, measured with the aid of a caliper in the median region of the fruit, the result was expressed in cm); 3) fresh mass of fruits per plant (MFFP); 4) average mass of fruit (MMF) (obtained by dividing the fresh mass of fruits of each plot by the number of fruits of their respective plots) and 5) productivity (PDT), which was estimated considering the production of useful area of each plot and plant population density, expressed in kg/ha.
The data obtained were submitted to analysis of variance.The effects of cultivars and rhizobacteria were studied by F test and Tukey test, at 5% probability, using the statistical program SAEG.

RESULTS AND DISCUSSION
The agronomic characteristics evaluated were significantly influenced by the use of cultivars and rhizobacteria.Thus, the number of fruits per plant, fruit diameter, productivity, fresh mass of fruits per plant and average mass per fruit showed significant differences for cultivars and rhizobacteria.
When evaluating the effect of microbiolization with rhizobacteria on hybrid Dardo in relation to the control, the authors verified that the use of rhizobacteria LEBM 109, LEBM 112, LEBM 152 and LEBM 154 provided higher values for number of fruits per plant (NFP), fresh mass of fruits per plant (MFFP) 1 and  2).The other rhizobacteria tested did not show significant differences among them and also, in relation to the control, for these variables: In relation to the average diameter (DM) and average mass per fruit (MMF), the effect of microbiolization with rhizobacteria on hybrid Dardo seeds was not noticed (Tables 1 and 2).
The application of some rhizobacteria on okra provided significantly higher production in relation to control, especially LEBM 109, 112, 152 and 154 on hybrid Dardo, with 19.9, 7.9, 15.4 and 16.9% more for PDT, respectively; and LEBM 105, 112 and 152 on cultivar Santa Cruz 47, with 68.9, 45.3 and 43.43%, respectively.The ability to stimulate the plant growth showed by rhizobacteria has been attributed to various mechanisms, among these mechanisms, hormone production, and, as a consequence, an increase in development of roots and aboveground area, number of leaves, leaf area and crop yield.If at least one of these modifications is observed, it can be considered as a plant growth promoter (Patten & Glick, 1996).Thus,rhizobacteria LEBM 105,109,112,152 and 154 can be defined as plant growth promoters.Harthmann et al. (2010) also verified in onions microbiolized with rhizobacteria differences in percentage, compared to the control, ranged from 15.7 to 24.4% for productivity of bulbs.
For NFP, the authors noticed a significant interaction between genotype and rhizobacteria.Comparing the varieties in each rhizobacteria and the control, the authors noticed that the hybrid Dardo was superior than the cultivar Santa Cruz 47 for all rhizobacterias tested.
When the genotype x rhizobacteria interaction was unfolded, the authors verified no significant difference for DM among the rhizobacteria on hybrid Dardo.However, the authors noticed significant difference among rhizobacteria on cultivar Santa Cruz 47, in which the rhizobacteria LEBM 103, LEBM 105, LEBM 112, LEBM 118, LEBM 135, LEBM 152 and LEBM 154 showed DM higher than the others (Table 1).The authors also observed that hybrid Dardo was superior to the cultivar Santa Cruz 47 when microbiolized with rhizobacteria LEBM 105, LEBM 109 and LEBM 152, Averages followed by the same lowercase letters in lines for each characteristic and uppercase letter in the column did not differ significantly, at 5% probability, Tukey test.Averages followed by the same lowercase letters in lines for each characteristic and uppercase letter in the column did not differ significantly, at 5% probability, Tukey test.

Table 1 .
Number of fruits per plant, diameter and productivity of okra fruits of hybrid Dardo and cultivar Santa Cruz 47, according to rhizobacteria.Janaúba, UNIMONTES, 2010.

Table 2 .
Average values of fresh mass of fruits per plant and average mass per okra fruit of hybrid Dardo and cultivar Santa Cruz 47 according to.Janaúba, UNIMONTES, 2010.