The bryophyte paradox: tolerance of desiccation, evasion of drought

Vascular plants represent one strategy of adaptation to the uneven and erra tic supply of water on land. Desiccation tolerant (DT) bryophytes represent an alternative, photosynthesising and growing when water is freely availab le, and suspending metabolism when it is not. By contrast with vascular pla nts. DT bryophytes are typically ectohydric, carrying external capillary wa ter which can vary widely in quantity without affecting the water status of the cells. External water is important in water conduction, and results in bryophyte leaf cells functioning for most of the time at full turgor; wate r stress is a relatively brief transient phase before full desiccation. All bryophytes are C3 plants, and their cells are essentially mesophytic in im portant physiological respects. Their carbohydrate content shows parallels with that of maturing embryos of DT seeds. Initial recovery from moderate p eriods of desiccation is very rapid, and substantial elements of it appear to be independent of protein synthesis. Desiccation tolerance in effect act s as a device that evades the problems of drought, and in various adaptive features DT bryophytes are more comparable with (mesic) desert ephemerals o r temperate winter annuals (but on a shorter time scale, with DT vegetative tissues substituting for DT seeds) than with drought-tolerant vascular pla nts.