Supersymmetry relations and MHV amplitudes in superstring theory

We discuss supersymmetric Ward identities relating various scattering amplitudes in type I open superstring theory. We show that at the disk level, the form of such relations remains exactly the same, to all orders in α′α′, as in the low-energy effective field theory describing the α′→0α′→0 limit. This result holds in D=4D=4 for all compactifications, even for those that break supersymmetry. We apply SUSY relations to the computations of N  -gluon MHV superstring amplitudes, simplifying the existing results for N⩽6N⩽6 and deriving a compact expression for N=7N=7.