Do phospholipids and sucrose determine membrane phase transitions in dehydrating pollen species?

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Abstract
Gel‐to‐liquid crystalline transition temperatures (Tm) of phospholipids of five desiccation tolerant pollen species in vivo were compared at various levels of dehydration. In an attempt to explain chemically the differences in Tm between these species, phospholipids and soluble carbohydrate contents were examined. We observed a negative correlation between the number of double bonds per phospholipid and Tm values for the intact pollen. This negative correlation also applied to Tm and the relative amount of linolenic acid. For the purpose of comparing Tm values of pollen and of unsaturated PCs (from the literature), the relative amounts and degree of unsaturation of pollen PCs were determined. A discrepancy between Tm values of individual PCs and intact pollen is discussed. Sucrose is the major soluble carbohydrate in 15 pollen species tested, generally making up a considerable part of the dry weight. A positive correlation between sucrose content (either as a percentage of the dry weight or as grams sucrose per grams phospholipid) and Tm was established. This unexpected result was explained in terms of alternative adaptive strategies. We suggest that, for desiccation tolerance, pollen either has to contain sucrose for the protection of its phospholipids, or have a high degree of fatty acid unsaturation in its phospholipids. The advantages and disadvantages of the two options are discussed.

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