Thank you, GWB, for the eloquent reply.In the sun means "in the sunlight", or "in the direct rays of the sun" , and it works better here than under the sun.
When I sit outdoors I prefer to sit in the shade of a tree, but Jane likes to sit directly in the sun.
Our seats in the stadium were in the sun, and after an hour we were sunburned.
The brass buttons on the uniforms of the parading soldiers sparkled in the sun.
Thank you, Angel, for the expressive and impressive essay.Strictly speaking, the leaves are located under the rays of the sun, but you wouldn't say it that way, because the action you want to portray is the leaves doing something within the area of sunlight.
You've got them glowing. The picture that leaps to my mind is that they're radioactive or something.
So what are you trying to say? What season is it? I'd not write the sentence in such a way that the leaves are doing the action, but rather I'd write it from the standpoint of the sun causing a reaction to the leaves. You could have them doing it, though, such as:
The birds are singing, the breeze is blowing, and the leaves are dancing in the sun.
It's your use of the word glowing that confuses me.
But you could also have the leaves responding to the sun. For instance: the sun captured the sparkle of color on the leaves. Or the sunshine reflected the stunning reds and oranges of fall.
But I might not be addressing the purpose of your original sentence, so I'll see what you have to say to this much.
To give you a short answer, I would not use "under the sun" in your sentence. And I would re-consider using glowing.