Imbibitional leakage from anhydrobiotes revisited

Dry desiccation-tolerant organ(ism)s leak cellular solutes when placed in w ater. Elevated temperatures at imbibition and elevated initial moisture con tents reduce the leakage and promote growth. We have re-examined the effect s of imbibitional stress imposed on cattail (Typha latifolia L.) pollen as a model anhydrobiotic system. A nitroxide spin probe technique and electron microscopy were used, allowing study of the permeability of the plasma mem brane together with its visual intactness. Imbibitional leakage can be tran sient, or prolonged when associated with membrane damage. During the first 15 s of rehydration in medium, plasma membranes of pre-humidified pollen we re highly permeable but became less permeable thereafter. The resulting tra nsient leakage may affect vigour as measured by the rate of fresh weight in crease, but did not reduce germination. A permanent, high permeability was observed when dry pollen was plunged into medium at low temperatures. This led to cell death and is associated with a phase change of the membranes fr om gel to liquid crystalline during imbibition. Freeze-fracture images indi cate that the damage to plasma membranes is mechanically imposed by the pre ssure of the penetrating water rather than occurring structurally by a phas e separation of membrane components. We suggest that a high rigidity of the plasma membranes in the gel phase at imbibition underlies imbibitional dam age.