Root Trait Measurements

At harvest, taproot depth (depth) was measured directly on the root using a ruler. This trait represents the maximum length reached by the taproot. The other root traits were measured on the images of the scanned samples using the measuring tools provided by the ImageJ software. Interbranch distance (ibd) was calculated as the average of forty-five measurements along the taproot in the distal zone starting from the first distal lateral root (Figure 1). The minimal diameter (dmin) was calculated as the average of thirty measurements performed on higher order lateral roots in the basal zone. The diameter is known to vary along the root axis (Wu et al., 2016). The maximal apical diameter (dmax) was computed as the 90th quantile in a distribution of diameters along the taproot in the distal zone. One value per plant was calculated.

The slope of the linear regression of lateral root diameters by their mother root (taproot) diameter (DlDm) was calculated for each plant based on forty-five measurements of diameters made of the mother root in the distal zone and its daughter roots, which are the first-order lateral roots. The line was forced to pass through the point of coordinates (dmin, dmin). A low DlDm reveals a high dominance of the taproot on the first-order laterals. Elongation activity (elong) provides information on the actual depth reached by a root of a given diameter during the experiment, and it is calculated for each plant as the ratio of depth by dmax and by the number of days since transplantation. Traits were calculated for each plant in the two treatments. Means for each genotype in the two P treatments were used to compute a relative response for assessing the phenotypical plasticity of the different traits: