Vp. Platonov, SUBGROUPS OF ALGEBRAIC-GROUPS OVER A LOCAL OR GLOBAL FIELD CONTAININGA MAXIMAL TORUS, Comptes rendus de l'Academie des sciences. Serie 1, Mathematique, 318(10), 1994, pp. 899-903
Let G be a reductive algebraic K-group, T a maximal torus of G defined
over K, and G(K) the subgroup of K-points of G. We give a description
of all subgroups of the group G(K) containing T(K) when K is a local
or global field.