Iðunn?
 
Following up on my version of the “Everyone lives happily ever after - forever” idea, Applejack’s claim to immortality is becoming the next Great Seedling. After years of searching for it, she finally finds him, and in return, the Seedling finds a worthy successor. I imagine the exchange of power is through a golden apple that the current Seedling grows in their antlers to give to the succeeding Seedling. Applejack accepts her prize but doesn’t eat it right away. No, I imagine she waits quite some time before eating the golden apple, weighing the benefits and downsides of immortality and all that. Rainbow Dash dying and then coming back after outflying Death is probably what convinces her to join the permanent life club.
 
Though apples are her specialty, Seedling-Applejack can easily grow pears on herself as well, and other plants a bit less adeptly.