We investigate the support of a distribution f on the real grassmannian Gr k (R n ) whose spectrum, namely its nontrivial O(n)-components, is restricted to a subset Λ of all O(n)-types. We prove that unless Λ is cosparse, f cannot be supported at a point. As an application, we prove that if 2 ≤ k ≤ n − 2, then the cosine transform of a distribution on the grassmannian cannot be supported inside an open Schubert cell Σ k . This immediately yields a sharpening of the injectivity theorem of D. Klain: an even, continuous, translation-invariant, k-homogeneous valuation on convex bodies in R n is uniquely determined by its restrictions to the k-dimensional subspaces in an arbitrarily small neighborhood of the complement of Σ k . We consider also the general α-cosine and Radon transforms between grassmannians.