  We discuss the possible origin of the radial truncations in stellar discs,
using measurements that we presented in an earlier paper (Kregel, van der Kruit
& de Grijs, 2002). A tentative correlation is found with the de-projected
face-on central surface brightness; lower surface brightness discs tend to have
a smaller truncation radius in units of scalelength. This and our earlier
finding that in smaller spirals the truncation tends to occur at a larger
number of scalelengths are best reproduced when the truncation is caused by a
constant gas density threshhold on star formation.
