Divergent Alfalfa Root System Architecture is Maintained across Environment and Nutrient Supply

Abstract

Plant root system architecture can alter, and be altered by, soil fertility and other environmental conditions. We developed alfalfa (Medicago sativa L.) populations selected for tap‐ or branch‐rooted architectures and determined responses to P supply and K placement in field experiments. In Exp. 1, populations representing the parental composite and progeny from first and second cycles of divergent selection were seeded on soils testing low to medium for available P and K, a sandy loam (Becker, MN) and a silt loam (Rosemount, MN). Experiment 2 was conducted at Becker and included only second cycle progeny. Phosphorus was added to one‐half of the plots by injection of KH2PO4 (224 kg P ha−1) into the upper 40 cm of soil. On the other plots, equal K rates (270 kg K ha−1) as KCl were topdressed. Added P did not affect herbage dry mass in the first and second years following stand establishment, but increased herbage P concentration and uptake at Becker. There was no effect of K placement, but the tap‐rooted germplasm had lower K concentration in herbage than the parent. Although progeny did not differ in total root mass, the ratio of thickened‐to‐total root mass was larger below 30 cm for the tap‐rooted selection, which also produced more fine roots. Selected germplasms did not differ in P or K uptake. Selected root system architecture characteristics were expressed under, and apparently not modified by, growing conditions that covered three site‐years on two contrasting soils with differential P supply and K placement.