For anonymity's sake, I'm only going to say that my grandfather worked in the State Department during and after WWII. He sometimes told a story that a member (or members) of a Japanese delegation urinated in a potted plant to show his (or their) disgust with the diplomatic proceedings. I mostly dismissed the story as a racist rumor passed along to dehumanize the Japanese even though my grandfather would have been in a position to witness this. My grandfather is long dead, and but the story is stuck in my head. If such an incident occurred, there must be other accounts, right? Can my grandfather's story be corroborated?
While I can't corroborate this particular story, I can give a perspective on how this story probably came to be. Japan has no public urination stigma. Part of it is due to lack of space and bathrooms, and part of is just cultural (going back to the Edo period where buckets were placed on the sides of the street.) Ed Hall talks about how the concept of privacy is different in Japan.
To me, this story sounds like it's been through a game of telephone, and somebody who witnessed a Japanese man relieving himself told someone else, and the story mutated over time into the ol' piss-in-the-plant. It's common for people to go to Japan and be shocked by people urinating right in the street. Likewise, some Japanese people don't understand the Western stigma and will let it go stateside.