A crystalline lipid phase in a dry biological system: evidence from X-ray diffraction analysis of Typha latifolia pollen.

The temperature limits for germination in Typha latifolia pollen lie within the range 4-40 degrees C. These limits correlate at the low-temperature end with the 'crystallization' of endogenous triacylglycerols and on the high-temperature end with the 'melting' of a gel-like lipid component in intact pollen. X-ray diffraction analysis was used to structurally characterize and to trace the latter gel-like lipid from the intact pollen through a range of pollen lipid fractions. We tentatively identify this component as a fatty acyl sterol ester and present evidence that it resides in the exine of the pollen grain. Its thermotropic behavior is insensitive to pollen hydration. The possibility of interpreting a crystalline lipid phase as being membrane-derived when in fact it originates from contaminating non-membranous neutral lipid is discussed. The total lipid content of T. latifolia pollen is 123 mg/g dry weight, of which 37% is polar lipid. The neutral lipid consists primarily of triacylglycerols and of the aforementioned sterol ester, which represents 0.34% (w/w) of pollen dry weight. The polar lipid fraction has phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid as major components with lesser amounts of phosphatidylglycerol and phosphatidylinositol. Palmitic (16:0) and linoleic (18:2) acids, in a 1:2 molar ratio, constitute the major fatty acids of both polar and neutral lipid fractions with lesser amounts of linolenic (18:3), oleic (18:1) and stearic (18:0) acid in evidence.