Bacteria as growth-promoting agents for citrus rootstocks

The microbial community plays an essential role in maintaining the ecological balance of soils. Interactions between microorganisms and plants have a major influence on the nutrition and health of the latter, and growth-promoting rhizobacteria can be used to improve plant development through a wide range of mechanisms. Therefore, the objective of the present study was to evaluate bacteria as growth-promoting agents for citrus rootstocks. A total of 30 bacterial isolates (11 of Bacillus spp., 11 actinobacteria, and 8 lactic acid bacteria) were evaluated in vitro for indoleacetic acid production, phosphate solubilization, and nitrogen (N) fixation. In vivo testing consisted of growth promotion trials of the bacterial isolates that yielded the best results on in vitro tests with three rootstocks: Swingle citrumelo [Citrus × paradisi Macfad cv. Duncan × Poncirus trifoliata (L.) Raf.], Sunki mandarin (Citrus sunki Hort. ex Tan), and rangpur (Citrus × limonia Osbeck). The parameters of interest were height, number of leaves, stem diameter, shoot and root dry mass, and total dry mass at 150 days after germination. The results showed that most bacterial isolates were capable of IAA production. Only one lactic acid bacterium isolate (BL06) solubilized phosphate, with a high solubilization index (PSI > 3). In the actinobacteria group, isolates ACT01 (PSI = 2.09) and ACT07 (PSI = 2.01) exhibited moderate phosphate-solubilizing properties. Of the Bacillus spp. isolates, only CPMO6 and BM17 solubilized phosphate. The bacterial isolates that most fixated nitrogen were BM17, ACT11, and BL24. In the present study, some bacteria were able to promote growth of citrus rootstocks; however, this response was dependent on plant genotype and isolate. Bacillus spp. BM16 and CPMO4 were able to promote growth of Swingle citrumelo. In Sunki mandarin plants, the best treatment results were obtained with BM17 (Bacillus sp.) and ACT11 (actinobacteria). For Rangpur lime rootstock, only BM05 (Bacillus sp.) was able to promote increase in two parameters assessed, height and number of leaves. When the bacterial isolates were used in mixture there was not promoted growth of plants on rootstocks. This fact may be associated with the different mechanisms of action of each bacteria involved or with the presence of competition among the microorganisms of the mixture.