Submitted to: Annals Of Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/9/2001
Publication Date: 9/1/2001
Citation: Wesley-Smith, J., N.W. Pammenter, P. Berjak and C. Walters 2001. The effects of two drying rates on the desiccation tolerance of embryonic axes of recalcitrant jackfruit (Artocarpus heterophyllus Lamk.) seeds. Annals of Botany 88:653-664. Interpretive Summary: The water content of embryos subjected to cryopreservation must be high enough to prevent desiccation damage, but low enough to prevent ice crystallization. The minimum water content at which embryos can survive is largely dependent on the rate at which they are dried. The mechanism of damage during slow drying is generally attributed to pathological metabolism within cells at intermediate water contents. This study relates changes in cell ultrastructure observed using electron microscopy when cells are dried rapidly and slowly with the nature of metabolism during drying, and the mechanism of cell death during slow drying.
Technical Abstract: This study compared the survival, electrolyte leakage and ultrastructural characteristics of embryonic axes of recalcitrant jackfruit dried rapidly (within 90 min) or slowly (2-3 d). Axes dried slowly showed decreased viability at higher water content than those dried rapidly, which was mirrored by an increase in electrolyte leakage at c. 0.8 and 0.4 g/g, respectively. Rapid drying conferred relatively greater tolerance to dehydration, as attested by the 100% survival attained at c. 0.4 g/g in contrast to the total mortality of axes dried slowly to this water content. Partially hydrated axes were processed for microscopy using freeze-substitution to prevent rehydration artefacts. Radicles of axes dehydrated to 0.7 g/g were examined microscopically to assess their cellular morphology and appearance of membranes following rapid or slow drying. Measurements showed that drying rate affected the distribution of water within axes, which could be relevant to the extent of stress experienced by meristematic cells. Membrane breakdown was not observed in cells dehydrated either rapidly or slowly to 0.7 g/g. Evidence suggests that the ER may play a pivotal role in the regulation of metabolism during dehydration. Autophagy and de novo vacuolation were observed in axes of both treatments. The area occupied by vacuoles increased significantly only following rehydration, particularly in axes dried slowly. Desiccation damage became evident on rehydration, and was reversible following rapid drying but more severe in axes dried slowly. Prolonged exposure to partial hydration may contribute to the greater sensitivity of vacuoles to damage during rehydration