A method to characterize root morphology traits in alfalfa

Productivity in alfalfa (Medicago sativa L.) has been shown to be influenced by root morphology. Our objectives were to identify the optimum age, cultural practices, and environmental conditions to characterize taproot diameter (TD), lateral root number (LRN), fibrous root mass (FRM), and determinate taproot percentage (DTP) in alfalfa. No correlations were found between greenhouse-cultured plants and 21-wk-old field-grown plants for LRN or FRM, while TD showed low to moderate correlations between greenhouse and field environments. Fourfold more plants with determinate taproots were identified in transplanted plots compared to seeded plots. All root traits were affected by plant spacing but, no germplasm × plant spacing interactions were found. Solid seeded plants needed more time to show maximum expression of root traits and scored lower for LRN and FRM and had smaller TD than spaced plants. Only TD had a significant germplasm × location interaction. Both TD and LRN increased with N fertilizer and between the seeding and first production years, but no germplasm × N rate or germplasm × year interactions were found. Rankings of alfalfa germplasms were the same at the end of the seeding year (22 wk after planting) and at the end of the first production year (74 wk after planting). Characterization of LRN and FRM in alfalfa should be conducted in seeded field plots with uniform plant spacing at one location, with or without N fertilizer at least 22 wk after planting. A similar protocol with evaluation at more than one location would be more appropriate for characterization of TD. Key words: Medicago sativa L., root morphology, alfalfa