I first observed Leaf Folding behavior in a plant called Cassia Alata aka. Senna Alata. And upon digging, found myself introduced to Nichynastic, Thermonastic, Photonastic and Chemonastic behaviors in plants. In Cassia Alata it is also called Nichynastic Sleep Behavior and its reasons are surely debatable.

The reasons for Photonastic and Thermonastic behaviors is obvious from the name of the behavior, but what may not be obvious is why your particular plant folds or rolls up its leaves.

I've recently found that Banana leaves are photonastic and fold when in direct sunlight.

And here ... astonishingly, after planting my seed grown Canna x generalis in the ground in a sunny spot, I observe leaf rolling ...

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I have seen others post to this forum, wondering what is wrong with their Canna that the leaves should be rolling up and well, I didn't have a clue and so did not respond to any of those posts.

Here's another shot ...

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These Canna were planted along a wall with a Western exposure and therefore get a whole lot of direct sunlight. But folks, this is NOT photonastic behavior ... but instead, thermonastic. It was particularly hot today, touching on 100 degrees F, about 38 degrees C. In days prior, temps were in the low 90s F, about 34 degrees C. In those temps, the leaves DID NOT ROLL or FOLD. I only noticed today when temps were up. True, temps were down here in BKF because of the haze and smoke from the Zaca fire in Santa Barbara County providing a natural sunshade, but ... I don't think this is photonastic.

In fact, I have two different species planted along that wall, and both species exhibit the leaf rolling.

Therefore, for all you who have seen this happen to your Canna and felt that there was something wrong, it is likely just the heat. Provide them shade and you might be remarkably impressed that the rolling will cease.

I seem to be becoming the Leaf Folding guy here. The behavior is much more common than I thought, especially, in tropicals.