Germination of stress-tolerant Eucalyptus pollen

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Abstract
Earlier reports have indicated that the pollen of Eucalyptus is mechanically robust and unusually resistant to the osmotic stress imposed by immersion in water. We have investigated some of the features of the germination mechanism in the pollen of E. rhodantha with a view to clarifying the role of pollen-wall specializations in determining this resistance. Cultured in vitro, the pollen showed erratic germination, with a scatter of germination times up to 24 h. This was associated with variation between individual grains in the rate of hydration and dispersal of the pectins of the oncus, the thickened outer component of the intine present at each aperture. The oncus is itself differentiated, with a refractive outer layer lying within a sporopollenin operculum and itself overlying the protein-bearing layer of the intine. The outer layer, interpreted as a compacted pectin, undergoes only slow dissolution in aqueous media after the lifting of the operculum, and it is this that apparently protects the grain from the effects of short-term osmotic stress. The rate of dissolution varies between grains, possibly as a consequence of minor differences in developmental rate in the final stages of differentiation in the anther, and this contributes to the wider scatter of germination times. The dehydrated pollen gave one-third of the potential germination after 24 h exposure to 60°C, and a small proportion survived 24 h at 70°C. This degree of heat tolerance must primarily reflect properties of the protoplast of the vegetative cell, not examined in the present study; but the wall specializations may well provide a guard against extreme desiccation, and it is noteworthy that the function of the germination mechanism is not prejudiced by exposure to high temperatures.