In 1.6.2 and earlier fully grown wheat/carrots/potatoes would pop-off into item form when the light level in the block was reduced to 0. Since snapshot 13w36a wheat no longer does this, this includes the current 1.7.2 release.
The screenshots show the results of placing a block above the water, reducing the light level of the wheat to 0. In 1.6.2 the wheat pops off, in the snapshot the wheat remains.
Creative or survival.