How long is needed for complete recovery Processes and criteria of recovery longterm survival

The recovery processes outlined in Section 7.2.4 proceed on different time scales. What should we take as the criteria of over all 'recovery'? There may be no hard and fast answer to this question. Very short moist periods will lead to net carbon loss. Moist periods long enough for a positive net carbon balance may be insufficient for cell division and growth, but might perhaps allow significant DNA repair. This is conjectural, but indicates the kind of questions on which research is needed. The limited available measurements indicate that the moist periods experienced by desiccation-tolerant bryophytes in the field vary greatly in length (Proctor, 1990; Proctor and Smith, 1995). Maintenance of a positive net carbon balance must be important, even on a rather short time scale, whereas a bryophyte may have an entirely viable annual life cycle in which growth is largely confined to particular seasons, while for the rest of the year the plant is doing no more than maintaining its foothold in the habitat. Whether 'recovery' is seen as return to a normal rate of carbon fixation, or as requiring full restoration of all metabolic systems to their optimum moist-period activity, is a question of context - indeed, there may be no single 'optimum'. Further, bryophyte shoots commonly show progressive formation of new leaves and death of old ones; recovery from desiccation of the apical parts of the shoot may be accompanied by accelerated senescence and death of the older parts.

7.3. Vascular Plants (see also Chapter 1)

Although often observed in seeds, spores and pollen, desiccation tolerance is the exception in vegetative tissues of vascular plants. The combination of vascular tissue and intercellular spaces with cuticle and stomata allows these species to maintain a water potential higher than that of their above-ground environment (homoiohydry), avoiding the need to tolerate large fluctuations in moisture availability. Nevertheless, in intermittently arid habitats some species have adapted to survive desiccation rather than avoid it. However, of the quarter of a million or so species of vascular plants, only some 330 species, or < 0.15% of the total, have been documented as being desiccation-tolerant in their vegetative parts (Table 7.1; Porembski and Barthlott, 2000). Because the rehydration of some of these plants gives the appearance of a revitaliza-tion of apparently dead tissues, they are often referred to as 'resurrection plants'.

The first scientific report of a resurrection species was made by Hooker (1837) in a description of Selaginella lepidophylla from the southwestern USA and Mexico. Subsequent reports were made in the next century, the earliest from field observations made in dry areas of central Asia and sub-Saharan Africa. The remarkable ability of these plants to exhibit the effects of extreme desiccation and yet to remain alive and revive when rewetted was reported with interest and often careful notes (Thoday, 1921; Vassiljev, 1931; Hambler, 1961, 1964; Hornby and Hornby, 1964). Carex physodes, Myrothamnus flabellifolia and Craterostigma plantagineum were among the first desiccation-tolerant angiosperms described, while early reports confirmed the phenomenon in other pteridophytes, such as Polypodium polypodioides, Notochlaena marantae, Selaginella njam-njamensis and Platycerium stemaria (Pessin, 1924; Iljin, 1931; Hambler, 1961, 1964).

Resurrection species have been documented thus far in nine families of pterido-phytes and ten families of angiosperms (Table 7.1). They are conspicuously lacking in gymnosperms, though foliage of Welwitschia mirabilis shows some degree of tolerance (Gaff, 1972). In angiosperms, they occur among both monocotyledons and dicotyledons. Although a higher proportion of pteridophyte taxa than seed plants are tolerant of desiccation, angiosperms often exhibit a higher degree of tolerance than ferns (Gaff, 1977).

Desiccation tolerance shows a wide tax-onomic scatter, and appears to have evolved independently a number of times as an adaptation to extremes in water availability. Some genera, such as Cheilanthes, Pellaea, Selaginella and Xerophyta, have large numbers of tolerant species while others, such as Boea, have only a single species that is known to be tolerant. A

Table 7.1. Desiccation-tolerant (DT) vascular plants. With a few exceptions, names and authorities are those used by the cited authors. Species of a genus recorded in a single publication from the same geographical area are listed together; otherwise the arrangement is alphabetical within major taxonomic categories.

Species Country Reference

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