Let's not forget that one of the tasks of a thorough hermeneutic is to consider Jesus' original audience and how they would interpret the metaphor. His audience was not composed of plant biologists but mostly non-scientific-types who had actual experiences with growing wheat to make bread, the "staff of life" back then.
Combine this piece of the hermeneutic puzzle with another piece regarding the most common method of burial back then (viz., burial in the ground), and your notion of metaphor--perhaps, more strictly, analogy--begins to make sense. I cite your sentence,
"Or does . . . [John 12:24] just present that image metaphorically because . . . [the grain of wheat] is buried in the ground?"
In other words, what Jesus was saying to his audience was this:
"As long as a grain of wheat keeps on being a kernel in the head of a stalk of wheat, it remains just that: a lone kernel of wheat among many such kernels. Only when it is detached from the head and buried into the ground by a farmer does it have a chance of producing more grain."
In like manner, Jesus is saying,
"If you want to be productive in my kingdom, you must die to yourself. Only when you do that will you have the opportunity to be fruitful in my kingdom. Moreover, you have me as an example. I was equal to God, but I did not count my equality something to be grasped and held onto; rather, I emptied--died to--myself by taking the form of a bond-servant and being made in the likeness of men. In other words, I humbled myself by becoming obedient to the Father, even though by doing so I was writing my own death sentence."
I just paraphrased liberally the kenosis (the "self-emptying") passage in Philippians chapter 2, especially vv.6-11. In effect, Jesus was also saying,
"I am willing to die to myself because one day the Father will reward me by exalting me and bestowing on me a name above all names, so that at my name every knee will bow and every tongue confess that I am Lord. Then my Father will be glorified."
For Jesus, there was nothing pleasant about dying to himself, and his prayer in the Garden of Gethsemane tells us he would rather have had the bitter cup of Calvary taken from him. Jesus, however,
"for the joy set before him, endured the cross, despising the shame, and . . . [is now seated] at the right hand of the throne of God" (Hebrews 12:2 NASV Updated).
What Jesus was saying to his audience in John 12 was that only when you die to yourself, refusing to live life autonomously but in service to me and others, will you be amply rewarded in eternity.
