How to repair a crack along the grain in a solid block of wood from a single tree?
The attached pieces and legs may have exacerbated the effects by not moving with the seat but a piece of pine of that size was probably destined to crack regardless. You can try to weep glue down into it (run a line of yellow glue over the crack then blow it down into the void with an air nozzle) and then clamp out the space but it probably won't be permanent and you could damage your finish with the clamp pressure. Were it me, I'd fill the crack with tinted epoxy and let the piece wear the crack like a badge of honor. Think of a piece reclaimed pine:



Tape off the crack with painters tape. You don't want epoxy all over the finish, just down in the crack.
Mix a 5 minute epoxy to the manufactures specs, then add 5% by volume a Van Dyke Brown (or similar) aniline pigment. Most manufacturers will recommend a product compatible with their own on their website, I recommend West System.
Apply the mix carefully over the crack and then force it down in with a putty knife or shim. Trowel off the excess as neatly as possible and let dry. Repeat this step as necessary (sometimes is sinks in after application) until the epoxy fills the void flush with the surface.
Remove the tape preferably when the epoxy is still sort of gooey and let dry over night. (yes it's 5 minute epoxy but trust me)
Sand off the residual epoxy with 220 and a light touch and then scuff sand the entire piece with 280.
Apply a light fresh coat of your original finish.


Hope this helps, cheers!