Evolution of abundances in the thick (green curve) and thin (red curve) disc of the Milky Way. Shaded areas display corresponding 1σ dispersion. The stellar Li source is 1.2−1.6 M_⊙ red giants. Top: evolution of O/Fe. Middle: evolution of Li/H, starting with low (left) or high (right) primordial Li. Observations are from Fig. 1 and the dotted lines are fits to those data, used as approximations to derive the minimum depletion of Li (see text). Bottom: corresponding evolution of the required level of minimum depletion of Li inside stars. Shaded aereas in the bottom figure correspond to 1σ errors of 0.1 dex.