I don’t know that any percentages are known or given and there would be no need to take plants aboard the Ark if they could regenerate but other than the fresh olive leaf brought back by the dove I think the bible is silent on the matter of plants and the Ark.
Well I think Noah would have had to have brought most of land vegetation onto the ark because they wouldnt survive being underwater for too long. There are many species of plant that will be too badly damaged after being submersed in H2O for extended period of time. Or maybe he just collected the seeds? That would have saved a lot of space.