Was not difficult to find a paper that supports this. Here is an abstract

Here is a link that might be useful: Stomatal behaviour towards four classes of herbicides as a basis of selectivity to certain weeds and crop plants

No, the paper refers to "urea ... type herbicides" (diuron) which must refer to the molecular structure because diuron contains no urea. Urea is not an herbicide.

Do you have a microscope handy? Would only take an hour or so to test this hypothesis.

"Do you have a microscope handy? Would only take an hour or so to test this hypothesis."

No, do you? You have correctly assumed that I have reagent grade urea.

I have access to microscopes but not the urea. A common experiment we perform in our botany labs is to take a leaf peel and observe stomatal opening/closure under treatments of auxin, 20% sucrose or distilled water. Tradescantia (Spiderwort), Commelina (Dayflowers), Zebrina (Wandering Jew) are all good plants to use for observing stomata. I am only familiar with the effect that solute concentration or hormones have on guard cell turgidity. I suspect that growth/vigor is the primary response of interest when urea is applied to a plant, so finding a primary source for this claim may require looking back quite far in the literature...or conduct the experiment yourself.

That is all that I can offer. Perhaps someone will eventually come along with an answer to your question.