Résumé
In this work, we performed qualitative and quantitative observations of the cytological changes occurring in cells of yam (Dioscorea alata) in vitro shoot tips cryopreserved using the encapsulation-dehydration (E-D) technique. Shoot tip osmoprotection for 24 h in 1.25 M sucrose medium induced drastic changes in cellular cytological features, including high plasmolysis in all three cellular areas studied, the external cell layer (L1), one to three (L1-3) and seven to nine (L7-9) cell layers from the surface of the meristematic dome, pyknotic nuclei in meristematic area cells and disappearance of nucleoli. Nucleus size decreased significantly in all cellular areas studied. Nucleocytoplasmic ratio decreased significantly in L1-3 and L7-9 cells. Nuclear protein content increased, particularly in L1 and L1-3 cells. After physical dehydration, plasma membrane of numerous basal part cells was broken and intracellular soluble protein leakage was observed. Nucleus area and nucleocytoplasmic ratio decreased significantly in L7-9 cells. One week after cryopreservation, shoot tips showed regrowth and living cells had recovered their original morphology. In all cellular areas studied, nuclei had retrieved their original staining and nucleoli were visible. Original nucleus area values were recovered in L1-3 and L1 cells. The nucleocytoplasmic ratio retrieved its initial value in L1 cells but remained at levels observed after osmoprotection for L1-3 and L7-9 cells. The nuclear protein content had retrieved its original level. This investigation provided new insights in changes occurring in D. alata apices throughout an E-D protocol.