Comparative conventional and phenomics approaches to assess symbiotic effectiveness of Bradyrhizobia strains in soybean (Glycine max L. Merrill) to drought.

Symbiotic effectiveness of rhizobitoxine (Rtx)-producing strains of Bradyrhizobium spp. in soybean (cultivar NRC-37/Ahilya-4) under limited soil moisture conditions was evaluated using phenomics tools such as infrared(IR) thermal and visible imaging. Red, green and blue (RGB) colour pixels were standardized to analyse a total of 1017 IR thermal and 692 visible images. Plants inoculated with the Rtx-producing strains B. elkanii USDA-61 and USDA-94 and successive inoculation by B. diazoefficiens USDA-110 resulted in cooler canopy temperatures and increased canopy greenness. The results of the image analysis of plants inoculated with Rtx-producing strains were correlated with effective nodulation, improved photosynthesis, plant nitrogen status and yield parameters. Principal component analysis (PCA) revealed the reliability of the phenomics approach over conventional destructive approaches in assessing the symbiotic effectiveness of Bradyrhizobium strains in soybean plants under watered (87.41-89.96%) and water-stressed (90.54-94.21%) conditions. Multivariate cluster analysis (MCA) revealed two distinct clusters denoting effective (Rtx) and ineffective (non-Rtx) Bradyrhizobium inoculation treatments in soybean. Furthermore, correlation analysis showed that this phenotyping approach is a dependable alternative for screening drought tolerant genotypes or drought resilience symbiosis. This is the first report on the application of non-invasive phenomics techniques, particularly RGB-based image analysis, in assessing plant-microbe symbiotic interactions to impart abiotic stress tolerance.