Altered Phase Behavior in Membranes of Aging Dry Pollen May Cause Imbibitional Leakage

Aging of dry pollen has been shown to coincide with increases of free fatty acids and lysophospholipids. These compounds reduce the integrity of hydrated liposomes made from isolated pollen phospholipids but do not lead to their total destruction. However, a massive, instantaneous leakage occurs upon imbibition of dry cattail pollen (Typha latifolia) that has aged to the point of complete loss of viability. To resolve the apparent discrepancy in stability between hydrated and dry membranes, the lyotropic phase behavior of two liposome systems containing lysophospholipid (12 mol%) was studied with differential scanning calorimetry and Fourier transform infrared spectroscopy. In both systems dehydration caused phase separation of the lipids. Fourier transform infrared data concerning phase behavior of isolated membranes from aging pollen and of membranes in situ did not show phase separations, probably because the assay technique was not sufficiently sensitive to detect them. However, aging of the pollen resulted in a permanent increase in the gel-to-liquid crystalline phase transition temperature (Tm) of isolated membranes and in a broadening of the transition in situ. We conclude that the increase in Tm of hydrated membranes may be more closely related to the leakage.