CHARACTERIZATION OF MEMBRANE-PROPERTIES IN DESICCATION-TOLERANT AND DESICCATION-INTOLERANT CARROT SOMATIC EMBRYOS

In previous studies, we have shown that carrot (Daucus carota L.) soma tic embryos acquire complete desiccation tolerance when they are treat ed with abscisic acid during culture and subsequently dried slowly. Wi th this manipulable system at hand, we have assessed damage associated with desiccation intolerance. Fast drying caused loss of viability, a nd all K+ and carbohydrates leached from the somatic embryos within 5 min of imbibition. The phospholipid content decreased by about 20%, an d the free fatty acid content increased, which was not observed after slow drying. However, the extent of acyl chain unsaturation was unalte red, irrespective of the drying rate. These results indicate that, dur ing rapid drying, irreversible changes occur in the membranes that are associated with extensive leakage and loss of germinability. The stat us of membranes after 2 h of imbibition was analyzed in a freeze-fract ure study and by Fourier transform infrared spectroscopy. Rapidly drie d somatic embryos had clusters of intramembraneous particles in their plasma membranes, and the transition temperature of isolated membranes was above room temperature. Membrane proteins were irreversibly aggre gated in an extended beta-sheet conformation and had a reduced proport ion of alpha-helical structures. In contrast, the slowly dried somatic embryos had irregularly distributed, but nonclustered, intramembraneo us particles, the transition temperature was below room temperature, a nd the membrane proteins were not aggregated in a beta-sheet conformat ion. We suggest that desiccation sensitivity of rapidly dried carrot s omatic embryos is indirectly caused by an irreversible phase separatio n in the membranes due to de-esterification of phospholipids and accum ulation of free fatty acids.