Differential longevities in desiccated anhydrobiotic plant systems

Desiccation tolerance is a wide-spread phenomenon in the plant kingdom, particularly in small propagules lacking own root or rhizome system, such as seeds, pollen, spores of spore plants, and whole moss plants, but rare in whole, vascular plants. Longevities in the desiccated state vary from a few days in some pollen and spore types to many decades in some seeds and moss spores, green vegetative tissues being intermediate in that respect. Therefore, small size of a propagule does not appear to be a factor limiting life span. The formation of a glassy state in the cytoplasm upon water loss considerably increases viscosity and slows deteriorative chemical reactions. Intermolecular hydrogen bonding strength and length in the glassy cytoplasm have been suggested to play a role in desiccation tolerance and longevity. To further explore this, a comparative Fourier transform IR study among dried anhydrobiotic plant propagules belonging to different phyla was conducted. This study indicated that strong hydrogen bonding does not correlate with long life span, but rather depends on the composition of the glass forming compounds. By contrast, a large number of double bonds in the acyl chains of the polar lipids correlated with short life span. This result suggests that deteriorative processes in membranes rather than in the glassy cytoplasm determine the rate of aging of dried anhydrobiotic propagules. This would agree with the view that lipids form the only fluid or semi-fluid phase in the dried propagules, which renders them comparatively susceptible to free radical attack.