On the first eigenvalue of invariant K\"ahler metrics

Given a simply connected compact generalized flag manifold M together with its invariant K"ahler Einstein metric g, we investigate the functional given by the first eigenvalue of the Hodge Laplacian on smooth functions restricted to the space of invariant K"ahler metrics. We give sufficient and necessary conditions so that the metric g is a critical point for this functional. Moreover we prove that when M is a full flag manifold, the metric g is critical if and only if M= SU(3)/T^2 and in this case g is a maximum.