Seed boron content decreased in a sequence of inferior, moderate,
and superior natto lines. Since more than 90% of boron
is in the plant cell walls, there is a possible correlation of boron with seed hardness. It was also stated that seeds with lower boron content might produce softer natto. Current research
supports this statement showing that superior natto types had lower boron content and seed hardness, while inferior natto types had high boron content and seed hardness.