  Let G be a reductive group over an algebraically closed field of positive
characteristic. In this article we show an analogue for Morozov theorem for
characteristics that are separably good for G (and under additional hypotheses
on the group). This theorem characterises, when p= 0, subalgebras of parabolic
subgroups of G with respect to their nilradical. If now k is an arbitrary
field, let C be a smooth projective and geometrically connected curve. Let G be
a reductive C-group scheme, which is the twisted form of a constant C-group.
The aforementioned analogue is in particular useful to extend the construction
of the canonical parabolic subgroup of G proposed by M. Atiyah and R. Bott when
k is of characteristic 0, to the characteristic p>0 framework.
