I've just looked at my alder-bodied EX-1 and whist I don't have any actual cracks in mine, what look like very fine cracks on yours running at right-angles to the growth-rings, I've also got on mine and those just appear to be a grain pattern in the wood that join up with grain features on the face and rear of the guitar.

If you were going with a solid colour over the top, then there are lots of things you could fill the cracks with, including the method Dedman suggested. But I'm not sure how well that would take a stain finish, as PVA glue spots are often the cause of anguish when staining. So if staining or DTing, I'd suggest fill using Timbermate, as that usually takes a stain well.

Otherwise Alder has a tight grain structure with closed pores, so there isn't normally a need to grain-fill the wood to get it smooth and sanding is sufficient (at least Stewmac say it doesn't need to be grain filled).