Can be performance indexes used to select plant growth-promoting rhizobacteria?

The plant growth-promoting rhizobacteria (PGPR) are bacteria that colonize the rhizosphere or roots, resulting in a beneficial effect on plants. The evaluation of the PGPR effects is mostly based on growth biometric measurements, but there is little information about using the chlorophyll (Chl) fluorescence. We used rhizobacteria to treat canola seeds and evaluate their effects on leaf area, dry mass, and Chl fluorescence. In order to select PGPR, we suggest the JIP-test as a method to evaluate their effects using the performance indexes. The biometric measurements and JIP-test parameters allowed to group bacterial isolates into different groups. We observed that some PGPR increased the energy absorption efficiency/or flux. We conclude that the performance indexes can be used to differentiate the interaction of plants and PGPR and to select rhizobacteria with a potential for plant growth promotion. Additional key words: Brassica napus L.; fluorescence transient; OJIP curve. Received 29 August 2019, accepted 7 November 2019. +Corresponding author; e-mail: bacarin@ufpel.edu.br Abbreviations: ABS/RC – specific absorption flux per reaction center; cfu – colony forming units per mL; DM – dry mass; ET0/RC – maximal specific flux for electron transport further than QA; F0 – minimal fluorescence yield of the dark-adapted state; Fm – maximal fluorescence yield of the dark-adapted state; Fv/Fm= φP0 – maximum photochemical efficiency of PSII; LA – leaf area; PGPR – plant growth-promoting rhizobacteria; PIABS – performance index; PItotal – total performance index; RC – reaction center; RE0/RC – electron flux for reducing end electron acceptors at the PSI acceptor side per RC; TR0/RC – specific trapping flux at t = 0; φE0 – quantum yield for electron transport at t = 0; φR0 – quantum yield for reduction of end electron acceptors at the PSI acceptor side. Acknowledgements: This study was supported by grants and scholarships from the Brazilian National Council for Scientific and Technological Development (CNPq) and Coordination for the Improvement of Higher Education Personnel (CAPES). Introduction Soil microorganisms are essential for the maintenance and sustainability of animal and plant communities because they affect many fundamental biological processes (Ambrosini et al. 2012). Plant growth-promoting rhizobacteria (PGPR) are bacteria that colonize the rhizosphere or roots, resulting in a beneficial effect on plants (Abbasi et al. 2011). Some of the action mechanisms are: nitrogen fixation (Zhang et al. 1997, Vessey 2003, Ahemad and Khan 2012, Satapute et al. 2012), phosphate solubilization (Ramani 2011), synthesis of plant growth regulators (Ma et al. 2011, Walia et al. 2014), siderophore production (Yu et al. 2011, Noël et al. 2014), limitation of ethylene inhibitory responses through the enzyme 1-aminocyclopropane-1-carboxylate deaminase (Ahmad et al. 2013, Barnawal et al. 2013), and biocontrol agents against phytopathogens (Bhattachaeyya and Jha 2012). The complex network of interactions occurring between plants and PGPR can affect plant growth and development directly or indirectly (Liu et al. 2013). PGPR stimulate plant growth through nutrient mobilization, improvement of soil structure, and production of growth regulators (Bulgarelli et al. 2013). Indirect mechanisms include antagonism against pathogens through competition with deleterious microorganisms for nutrients and synthesis of siderophores, antibiotics, and enzymes (Karakurt and Kotan 2011). The use of PGPR in sustainable systems can be a viable technology to increase plant production with lowering the use of chemical fertilizers and pesticides, with consequent decreases in risks for the environment and humans (Souza et al. 2013, Ahemad and Kibret 2014). The effects of these microorganisms on plant development have been studied in several crops, and their benefits in specific biosystems have been reported (Mathesius and Watt 2011, Saharan and Nehra 2011). Evaluation of the PGPR effects on the plants is mostly based on biometric measurements of growth, such as plant height, number