Impact of MRI on chemical transport during the MS. Left panel: evolution of the ratio between the surface nitrogen abundance and surface hydrogen abundance, [N/H]_surf, as defined in Eq. (32) for selected models (see plot legends) as a function of the central hydrogen abundance, X_c. The yellow diamonds are the mean of the observed surface values found in Fig. 11 of Ekström et al. (2012), who compute models with rotation. The grey boxes encompass observed values of [N/H] for stars at the middle and end of the MS band, which are also from Fig. 11 of Ekström et al. (2012). The light grey bars extend the values of Ekström et al. (2012) to account for the large errors of the enrichment of individual stars with respect to the averaged values. Right panel: evolution of the mean molecular weight (in blue) and of D = D_shear + D_mag, O (in black) for model AM100 as a function of the Lagrangian mass coordinate, M_r, during the MS. The curves shown with different line styles are for X_c = 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.04, where the arrows, blue and black respectively, show the direction of evolution going from X_c = 0.7 to X_c = 0.04, increasing μ and restricting the region with large values of D towards larger mass coordinates with decreasing values of X_c (equivalently, increasing time).