Poikilohydry, or the lack of control of water relations, has typically been a subject studied by lichenologists and bryologists. For many years, much was unknown about poikilohydrous vascular plants, and evidence for their abilities was mostly anecdotal. A small number of these plants were studied by a few physiologists and ecologists who were fascinated by the capability of these ''resurrection plants'' to quickly switch from an anabiotic to a biotic state and vice versa (Pessin 1924, Heil 1925, Walter 1931, Oppenheimer and Halevy 1962, Kappen 1966, Vieweg and Ziegler 1969). Recently, a practical demand has released an unprecedented interest in poikilohydrous plants. The increasing importance of developing and improving technologies for preserving living material in the dry state for breeding and medical purposes has induced tremendous research activity aimed at uncovering the molecular and biochemical basis of desiccation tolerance. Poikilohydrous plants have proven to be very suitable for exploring the basis of this tolerance with the target of genetic engineering (Stewart 1989, Oliver and Bewley 1997, Yang et al. 2003, Bernacchia and Furini 2004, Alpert 2006). Consequently, much of the current literature discusses poikilohydrous plants mainly as a means of explaining basic mechanisms of desiccation tolerance (Hartung et al. 1998, Scott 2000, Bartels and Salamini 2001, Rascio and Rocca 2005) instead of exploring their origin, life history, and ecology (Raven 1999, Porembski and Barthlott 2000, Belnap and Lange 2001, Ibisch et al. 2001, Proctor and Tuba 2002, Heilmeier et al. 2005).
Many new resurrection plants have been discovered during the last 25 years, especially in the Tropics and the Southern Hemisphere (Gaff 1989, Kubitzki 1998, Proctor and Tuba 2002). This has provided new insights into the biology of these organisms. In this chapter, structural and physiological features of poikilohydrous autotrophs and the different strategies in different ecological situations are discussed. As desiccation tolerance itself is the most—but not only—striking feature, our goal is to assess in addition the life style and the ecological success of poikilohydrous autotrophs. We give attention to the productivity of poikilohydrous autotrophs, how they manage to live in extreme environments, the advantage of their opportunistic growth, and what happens to structure and physiology during desiccation and resurrection.
Poikilohydrous Constitution versus Poikilohydrous Performance: Toward a Definition of Poikilohydry
According to Walter (1931), poikilohydry in plants can be understood as analogous to poikilothermy in animals. The latter show variations of their body temperature as a function of ambient temperature, whereas poikilohydrous autotrophs (chlorophyll-containing organisms) exhibit variations of their hydration levels as a function of ambient water status (Walter and Kreeb 1970). The term autotroph is used here to comprise an extensive and heterogenous list of autotrophic unicellular and multicellular organisms (cyanobacteria, algae, bryophytes, and vascular plants), including the lichen symbiosis. Poikilohydrous performance (from the Greek words poikilos, changing or varying, and hydor, water) is applied to organisms that passively change their water content in response to water availability ("hydrolabil"; Stalfelt 1939), eventually reaching a hydric equilibrium with the environment. This fact does not necessarily imply that the organism tolerates complete desiccation (Table 2.1). There is no general consensus on the definition of poikilohydrous autotrophs. The Greek word poikilos also means malicious, which, figuratively speaking, may apply to the difficulty of comprising the outstanding structural and functional heterogeneity of this group of organisms.
It is difficult to be precise about the vast number of poikilohydrous nonvascular taxa, comprising 2000 Cyanophyta, c. 23,000 Phycophyta, c. 16,000 Lichenes, and c. 25,000 Bryophyta. The number of poikilohydrous vascular plant species could be almost 1500 if the
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