In-field lucerne root morphology traits over time in relation to forage yield, plant density, and root disease under two cutting managements

Changes in lucerne (Medicago sativa L.) root morphology concurrent with forage growth have been little studied in multi-year field experiments with lucerne stands though they could provide explanations for differences in performance among tested treatments. Our objectives were: (i) to compare lucerne root trait development under three- and four-cut managements over a 4-year period, and (ii) to investigate relationships among root traits, and between root traits and lucerne varieties, cutting frequencies, root sampling periods, lucerne dry matter yield and evidence of root disease incidence and plant density, using multivariate methods. Treatments were a factorial combination of 15 lucerne cultivars with managements of three and four cuts per year. Each spring and autumn, plants were evaluated for root morphology traits and scored for root disease. Root morphology traits were strongly modified by sampling period and plant density, which together explained over 40% of total variability. The development over time was positively associated with an increase in tap-root diameter (TD) and root mass accumulation, whereas root density determined changes in root branching traits. The four-cut management reduced TD and root branching in association with lower forage yield. Variety effect was significant but contributed only 2.8% of total variability. Root branching showed a positive correlation with forage yield through a large increase of TD for branch-rooted plants, but more intensive root branching was associated with a significantly higher disease score than in tap-rooted plants. For optimization of evaluation of root traits, we proposed a root potential index (RPI) integrating TD and plant density. This RPI showed a closer relation to yield than TD, plant density or root biomass alone, in the arable cultivation layer. Our results reveal that monitoring of root morphology is an effective tool to characterize the production potential of lucerne as a sown field crop, and may explain yield differences among the various experimental treatments. Understanding these relationships between root morphology and lucerne stand performance can help in breeding selection as well as effective lucerne stand evaluation.