Non-orientable Lagrangian surfaces in rational 4-manifolds

We show that for any nonzero class $A$ in $H_2(X;{\mathbb{Z}}_2)$ in a rational 4-manifold $X$, $A$ is represented by a nonorientable embedded Lagrangian surface L (for some symplectic structure) if and only if $P(A)\equiv (L) (mod\ 4)$;where$P(A)$denotesthemod4valuedPontrjaginsquareof$A$.