crab-apple tree, endure, bear
柰 centers on a crab-apple tree, and by extension the idea of enduring or bearing something, perhaps from the tree's hardy nature or the act of bearing fruit.
柰 combines 木 (tree) with 示, which may have originally contributed a sound component. The exact historical development is uncertain, but the character is associated with a fruit tree and the idea of endurance.
A tree 木 with a display 示 of crab-apples: the tree bears fruit, so it endures and bears the weight.
For ナ, imagine a crab-apple tree that is 'na' (nah) good at enduring harsh weather: na -> ナ, and the tree bears it all.