Usually, parenchyma cell walls of monocotyledons do not develop secondary walls; however a few days after harvesting, the yam tuber of Dioscorea dumetorum starts to harden. Two or three weeks Iater, hardness is so pronounced that the tubers cannot be eaten, even after a long cooking time.
Cytochemical studies using autofluorescence or some fluorescent dyes, such as phloroglucinol hydrochloride showed that the thin, and flexible cell walls of parenchyma tubers very quickly became fully lignified after harvesting. Ultrastructura 1 stud ies of the hardened ce 11 wa 11 s showed very thick secondary wa 11 s and very deep pit apertures. These secondary walls reacted strong ly with li gn in reactants such as potassium permanganate. The use of a radioactive (l '• C) ce llulose precursor, uri dine- 5'-d ipho sphateglucose, confirmed the formation of such secondary walls. The lignification started from the corners of the cells around intercellular spaces and proceeded along the walls.
Sealy, L.; Renaudin, S.; Gallant, D. J.; Bouchet, B.; and Brillouet, J. M.
"Ultrastructural Study of Yam Tuber as Related to Postharvest Hardness,"
1, Article 19.
Available at: http://digitalcommons.usu.edu/foodmicrostructure/vol4/iss1/19