Endosperm and pericarp involvement in the supercooling of imbibed lettuce seeds.

Fully hydrated lettuce (Lactuca sativa L.) seeds showed dual freezing exotherms (-9 and -18 degrees C), even after 10 hours imbibition. Only the -9 degrees C exotherm was observed in seeds imbibed for 20 hours, but without external nucleation, all water in the embryo supercooled. Results indicate that the endosperm acts as a barrier to ice propagation. Other experiments suggest that the pericarp may also protect the embryo under certain freezing conditions.