Let $G$ be a split reductive group over a perfect field $k$ (not necessarily algebraically closed) with split maximal torus $T$ and Borel $B \supset T$.
Then there is(/should be) an inclusion-preserving bijection from the set of parabolic subgroups $P$ of $G$ containing $B$ to the set of subsets of the set of simple roots $\Delta(B)$ associated to $B$.
Does someone know a (good) reference for this?
I know that this question is more suitable for MathStackExchange but I got the impression, that there is not much activity at the moment.
Gunter Malle and Donna Testerman, Linear algebraic groups and finite groups of Lie type. Cambridge Studies in Advanced Mathematics, 133. Cambridge University Press, Cambridge, 2011.
See Proposition 12.2.