Post save scene callback needed?

Hi,
I have a material that uses a RenderTexture and 2 created Texture2D's (gradients) to help draw the scenery.
When I save the scene, the shaders disassociate themselves from the textures, and the scene goes grey.

The textures still exist after save, it's just the shaders think they don't.

Can I prevent this apparent bug, or do I have to reattached the texture to the shaders.
Is there a post save scene callback extension?

Thanks.