Topics like this are why we need a seperate board for biological/geological stuff.
I like this idea. I suggested ASB simply because that's a catch-all for stuff that isn't 'plausible' - however we define it - and making rice into something like that described in the initial post just isn't going to happen with natural evolution of rice or any other cereal (i.e. the edible kernel of the rice is the _seed_ of the plant, which is not a _fruit_ evolved to be eaten, and a _grass_ is most definitely not a _tree_).
Changing the evolution of rice so that it does not require nearly as many human-hours of work per calorie requires a POD at the minimum of ~15000 BCE (for some incredibly unlikely freak mutations to make Green-Revolution varieties which have yields two or so times higher than most of the historical cultivars) or many millions of years ago (to change the species into something which does not require planting by hand, for example).
One way to make this a historical POD, rather than ASB or this proposed bio-/geo-POD class, would be to have a successful hybridization program in ancient China (for example). Farmer notices that the shorter rice stalks have bigger and more grains, saves them and grows more, etc., eventually followed by selective breeding, etc. This isn't the rice of the OP, but even having a factor of two less peasants necessary to support the population of China will massively change their society. If it happens very early, there might be less imperial unification, because individual feudal states can afford stronger armies.