Seedless watermelon + PK ring? Beats me? But as for the reactions...it is his show and the Japanese are know for being good audiences for magic.
I am almost 100% sure that this is it... the seeds didn't really look like they were in the watermelon plus they left no marks. If I were to duplicate this effect I would have fake seeds that look real on the front but are flat on the back. If you noticed he also places his hands to the side and distinctly goes to his rear pocket before he shows his hands clean