@Ironclad:
Why? Currently, Japan imports literally most of it's food. (See for instance
this newspaper article, which begins,
The government’s new target of raising Japan’s self-sufficiency in food to 45 percent in 10 years — compared with the previous target of 50 percent — may be more plausible because it’s closer to reality.
Furthermore, as a nation I believe they're pretty conscious of this fact (as the existence of the above snippet hints) so it makes perfect sense to me that it would be at the top of the national agenda in this instance.
And it's a pretty legitimate concern. Going to half-rations is already pretty serious business—Japan doesn't overeat much in the first place, I think—but agricultural yields could drop significantly if the modern farming equipment starts to fail and stocks of certain agricultural supplies go down. It's not as simple as just planting more fields; not when you've got 120 million people to feed.
(And yeah, that brings some focus back to figuring out the machinery and industrial supply problems, but opting to grab some stuff from your medieval neighbors instead seems reasonable. What isn't reasonable is... well, I already went into that.)