- The leaf is soft and drooping due to lack of water, may recover after watering.
- The leaf has turned brown and crumbling.
For the first meaning there is a specific verb: "wilt" To droop or become limp and flaccid (as a dying leaf or flower). (wiktionary)
For the second meaning I would use "The leaves turned brown" as the simplest way of expressing this idea.
You might also use "brown" as a verb. For example, a houseplants website mostly uses "turned brown" but also has "Browning leaf tip tips are as common as they are challenging to diagnose." in which "browning" is a gerund.
You could also use "wither" to mean "dry up and die". It is a bit more dramatic in mood.
First of all just some minor corrections to your original sentences:
The leaf is soft and drooping due to lack of water, but may recover after watering.
The leaf has turned brown and is crumbling.
You could use the words "withered" and "withering" like this:
The leaf is withering due to lack of water, but may recover after watering.
The leaf has withered.
However, the word "withered" in connection with a plant doesn't usually mean the same as "crumbling" which was in your original sentence. I'm no botanist, and perhaps that is the expert meaning, but in general use "withered" means limp or lifeless, whereas "crumbled" means broken to pieces, beyond rescuing.