Water is evasive in many terrestrial habitats, and plants in general have to deal with the changing availability of this crucial resource. This is especially true for poikilohydrous autotrophs, which have successfully explored many different strategies within their general tolerance to water scarcity. However, some of the features that make tolerance to desiccation possible are irreconcilable with those that enhance water use. Poikilohydrous autotrophs, therefore, have had to trade-off between surviving desiccation against uptake, transport, and storage of water. Some adaptive conflicts appear, for instance, when a particular feature retards water loss. The important functional problems that arise when the plant has to resume water transport after desiccation might have limited the range of growth forms and plant sizes compatible with poikilohydry.
Plants must be efficient in acquiring water, particularly in arid regions where rainfall is scarce and sometimes the only available water comes from dew, mist, or fog. Poikilohydrous plants can outcompete their homoiohydric counterparts in dry habitats if they can rehydrate efficiently. The following section describes the different possibilities for water capture exhibited by poikilohydrous autotrophs, with emphasis on the role of the growth form and of the morphology and anatomy of the structures involved.
Aerophytic algae and lichens with green-algal photobionts can take up enough water from humid atmospheres to become metabolically active (Lange 1969b, Blum 1973, Lange and Kilian 1985, Lange et al. 1990a, Bertsch 1996a,b). Rehydration in lichens from humid atmospheres may take 1-4 days until equilibrium, whereas mist and dewfall yield water saturation within hours (Kappen et al. 1979, Lange and Redon 1983, Lange et al. 1991). Even water vapor over ice and snow serves as an effective water source for the activation of lichens in polar regions (Kappen 2000, Pannewitz et al. 2006; see Chapter 14). As a consequence, lichens in deserts can survive well with sporadic or even no rainfall (Kappen 1988, Lange et al. 1990c, 1991). Anatomical structures such as long cilia, rhizines, branching, or a reticulate thallus structure are characteristic of lichens from fog deserts (e.g., Ramalina melanothrix, Teloschistes capensis, Ramalina menziesii), suggesting that these structures are means for increased water absorption (Rundel 1982a). In lichens, liquid water is absorbed by the entire body (thallus), usually within a few minutes (Blum 1973, Rundel 1982a, 1988). The thallus swells and can unfold lobes or branches. However, there is little evidence of a water transport system in these organisms (Green and Lange 1994). Nevertheless, not all lichens have the same capacity for exploiting the various forms of water from the environment. For example, lichens with cyanobacteria as photobiont cannot exist without liquid water (Lange et al. 1988). For the Australian erratic green-algal Chondropsis semiviridis, rainwater is necessary to allow photosynthetic production because the curled lobes must be unfolded (Rogers and Lange 1971, Lange et al. 1990a).
The kinetics of water uptake seems to be similar in lichens and mosses, and the larger the surface area to weight ratio, the more rapid the water uptake (Larson 1981). Rundel (1982a) suggested that thin cortical layers of coastal Roccellaceae in desert regions may be a morphological adaptation to increase rates of water uptake. However, textural features of the upper cortex seem to be more important for water uptake than just thickness (Larson 1984, Valladares 1994a). Valladares (1994a) found that species of Umbilicariaceae that possess the most porous and hygroscopic upper cortex (equal to filter paper) are adapted to live mainly from water vapor (aero-hygrophytic), whereas species that have an almost impervious cortex were more frequently exploiting liquid water from the substratum (substrate-hygrophytic; Sancho and Kappen 1989).
Most bryophytes need a humid environment or externally adhered water to keep a level of hydration high enough for metabolic functions. Many species form cushions, turfs, or mats that aid to keep capillary water around the single shoots (Gimingham and Smith 1971, Giordano et al. 1993). At full saturation, the water content of mosses (excluding external water) can vary between 140% and 250% dry weight (d.wt.) (Dilks and Proctor 1979), which is similar to that of macrolichens. Thallose hygrophytic liverworts require higher levels of hydration, and their maximal water content can be more than 800% d.wt. In shaded or sheltered habitats, hygric and some mesic bryophytes are able to keep their water content relatively constant throughout the year, which is characteristic for a stenopoikilohydrous lifestyle (Green and Lange 1994). In more open and exposed sites, the fluctuations in water content are very large (Dilks and Proctor 1979).
The more complex and differentiated morphology and anatomy of bryophytes, in comparison with lichens, allow for more varied modes of water uptake (Proctor 1982, 1990, Rundel 1982b). Bryophytes can take up water vapor to limited extent and reach only low (less than 30% of maximum water content) relative values (Rundel and Lange 1980, Dhindsa 1985, Lange et al. 1986). Dew uptake was recorded for Tortula ruralis (Tuba et al. 1996a) and for 10 sand-dune mosses (Scott 1982). Leaves of certain desert mosses (e.g., Pottiaceae) act as focus for condensation of water vapor and mist by means of their recurved margins, papillose surfaces, and hair points (Scott 1982). However, the presence of lamellae, filaments, and other outcrops on the adaxial surface of the leaves, which is common in arid zone mosses, may act more as sun shelter rather than as means to enhance water uptake. The role of scales and hyaline structures on the midrib of desert liverworts (e.g., Riccia, Exormotheca, and Grimaldia), which is inverted and exposed to the open when the thallus is dry, is not clear, but they start absorbing rainwater and swelling to turn down rapidly and may help in storing water (Rundel and Lange 1980). Mosses of the family Polytrichaceae have so-called rhizomes or root-like structures, which are not very efficient for water uptake (Hebant 1977). In general, water uptake of mosses from the soil is poor and needs to be supplemented by external water absorption.
Two main groups of bryophytes have been described according to the mode of water transport. Ectohydrous species resemble lichens because they take up water over all or most of their thallus surface and have no internal water transport system, whereas endohydrous species have various water-proofed surfaces (cuticles), often well developed near to the gas exchange pores (stomata on the sporophytes), and have a significant water-transport pathway (Proctor 1984, Green and Lange 1994). These properties of the latter are similar to those of homoiohy-drous plants (Hebant 1977). However, they differ from vascular plants in that their conductive structures are not lignified, and all these properties are functional only in moist environments. Therefore desert mosses are typically ectohydrous (Longton 1988a), and the water transport in eurypoikilohydrous bryophytes growing in dry environments is predominantly external. However, some eurypoikilohydrous mosses (Fabronianaceae, Orthotrichiaceae) have large masses of stereom tissue (usually a supporting tissue), that is considered to be an alternative route for the conduction of water (Zamski and Trachtenberg 1976).
Proctor (1982) summarized four different pathways or modes by which water moves in a bryophyte: (1) inside elongated conductive cells (hydroids), forming a central strand in the stems of mosses and some liverworts; (2) by the cell walls, which are frequently thickened (in fact, bryophyte cell walls have higher water conductivity than those of vascular plants); (3) through intervening walls and membranes; and (4) by extracellular capillary spaces. The highest internal conduction for water in Polytrichaceae at 70% relative humidity was 67% of the total conduction (Hebant 1977).
Water uptake in poikilohydrous vascular plants can be very complex because of interactions between different organs. For instance, in the fern Cheilanthes fragrans, water uptake through the leaf surface from a water vapor-saturated atmosphere allows it to reach 80% of its maximal water content within 50 h (Figure 2.2a). Petiolar water uptake was also efficient, but only if the leaves were in high air humidity (Figure 2.2b). Stuart (1968) found that the fern Polypodium polypodioides was not able to rehydrate by soil moistening if the air was dry, and the leaves reached only 50% of their maximal water content within 2-3 days, even in a water
FIGURE 2.2 (a) Water-vapor uptake of leaves of the fern Cheilanthes fragrans with sealed petioles in a moist chamber. The different symbols stand for four replicates (L. Kappen, unpublished results). (b) Water uptake of leaves of C. fragrans placed on filter paper in a moist chamber (open circles); with petiole in a vessel with water and standing in a moist chamber (closed circles), and (open and closed triangles) with petiole in water in a room (approximately 60% rh) (L. Kappen, unpublished results).
vapor-saturated atmosphere (Stuart 1968, confirming the results of Pessin 1924). Fronds of the highly desiccation-tolerant Polypodium virginianum were, however, not able to absorb water from air as was shown with deuterium-labeled water (Matthes-Sears et al. 1993). Thus, the capacity of the leaves to take up water vapor varies significantly among species and seems not to be associated with the tolerance to desiccation. In contrast, liquid-water uptake by leaves has been shown to be a common feature in poikilohydrous vascular plants. Detached leaves of P. polypodioides regained full saturation within 20-30 min if submersed in liquid water (Stuart 1968). However, leaves attached to the rhizome needed 10 times longer for saturation than detached leaves. Stuart explained this by alluding to anaerobic conditions that impede rapid water uptake. Rapid water uptake by leaves was also shown in Selaginella lepidophylla (Eickmeier 1979). It seems that, in pteridophytes, water uptake through leaves is an important mechanism for reestablishing water relations of the whole plant and for resuming xylem function. Similarly, rehydration of the whole plant solely by watering the soil in dry air is also incomplete in poikilohydrous angiosperms (Gaff 1977).
Water uptake from mist or from saturated atmospheres is insignificant in poikilohydrous angiosperms (Vieweg and Ziegler 1969), as has been shown for isolated leaves of Ramonda myconi (Gebauer et al. 1987). In addition, exposure to dewfall could only raise the relative water content to less than 13% in Craterostigma wilmsii (Gaff 1977). Foliar water uptake by desert plants has been investigated, particularly with respect to dew uptake (Barthlott and Capesius 1974), but it seems to be insignificant in homoiohydrous plants except in the genus Tillandsia (Rundel 1982b). In contrast, foliar water uptake from rain by poikilohydrous vascular plants may be important to resume functioning of the hydraulic system, as Gaff (1977) found that leaves of resurrection plants in contact with liquid water can rehydrate within 1-14 h, depending on the species. The quickest uptake was measured in Chamaegigas intrepidus (Hickel 1967). The cuticle of vascular plants is generally considered an efficient protection against water loss. However, the cuticle of poikilohydrous vascular plants may also enhance water uptake by leaves (e.g., Borya; Gaff 1977). The permeability of the cuticle to water was assumed for C. interpidus (Hickel 1967). Barthlott and Capesius (1974) suggested that the cuticle of some of these plants seems to be more permeable to water from outside than from inside the leaf. However, this is not clear as some studies attribute permeability to the state of the cuticular layer rather than to the cuticle itself (Schonherr 1982). According to
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