greno

There is no -en- suffix in Esperanto, so there is nothing to find inside greno. The root is gren- and the -o is the noun ending, and a learner who tries to peel off an -en- is left holding gr, which settles the question.

The distinction worth learning is the one between greno and grajno. Greno is the crop and the commodity - wheat, barley, oats and rye taken together, the stuff that fills a field, a sack or a ship's hold. Grajno is a single grain, and by extension any small hard particle: a grain of sand, a grain of salt, a grain of truth. English blurs the two under one word and then has to say "a grain of grain" if it wants to be precise. Esperanto keeps them apart, and the regular derivations follow the split: grenkampo is a cornfield, grenejo a granary, while grajneca means granular.

Note also that the English trap is live here. British "corn" means whatever cereal the region grows; American "corn" means maize, which in Esperanto is maizo and never greno. If you mean the yellow cobs, say maizo.