High Throughput Phenotypic Characterization and Classification of Rice Breeding Lines for Early-Season Vigor.

Holistic and stage-specific cultivar screening is needed to improve crop genotypes for tolerance to abiotic stresses. Early-season vigor that includes the whole-plant system is critical to canopy development for maximizing efficiency of the photosynthetic machinery. The objectives of this study were to evaluate root and shoot morphology and growth of over 100 key rice-breeding lines used in the Mississippi rice breeding program during early season and develop a screening tool to classify them into different vigor groups. An outdoor experiment was conducted in pots, with five replications per line, under optimum water and nutrient conditions. Above- and below-ground growth and developmental parameters were assessed at 30 days after transplanting. Root morphological traits namely, root weight, root/shoot ratio, cumulative root length, surface area, average root diameter, root volume, root length per volume, and number of roots, tips, crossings, and forks, and longest root length, and crossings, were assessed using the winRHIZO root image analysis system and growth measurements. Above-ground plant growth parameters such as plant height, leaf area, and leaf and tiller numbers were also measured. Rice breeding lines varied significantly for many of the traits measured. Individual and cumulative vigor response indices were developed using all the traits and means and standard deviations were used to classify the breeding lines into different vigor groups. The combined vigor indices for the lines tested ranged from 11.2 (RU14041194) to 20.4 (JES). Only 5% and 6% of the rice lines showed high to very high and very low vigor indices, respectively. Majority (81%) exhibited low and moderately low vigor indices. The early-season vigor index database will be useful in developing new varieties most suitable to the US Mid-south rice-growing environment.