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FIGURE 2.3 Water uptake of leaves of Ramonda myconi (Gesneriaceae) with sealed petioles. Leaves with hairs (open triangles) and after removing the hairs (closed triangles) soaking from sprayed water; and leaves with hairs (open circles) and without hairs (closed circles) in a moist chamber. (From Gebauer, R., Losch, R., and Kappen, L., Verh. Ges. Ökologie., XVI, 231, 1987.)

Kerstiens (1996), water uptake through the cuticle is most likely, but evidence needs to be shown.

Hairs and scales can function as auxiliary structures for water uptake because they absorb water more easily than the leaf epidermis. The lower surface of the curled and folded leaflets of ferns like Ceterach officinarum, densely covered with scales and trichomes, should enhance water capture (Oppenheimer and Halevy 1962). The so-called hydathodes on the leaves of Myrothamnus may actually function as water-absorbing trichomes (Rundel 1982b). However, Sherwin and Farrant (1996) do not believe in any water uptake by leaves of this species. In addition, scales of P. polypodioides did not facilitate water uptake, but allowed the water to spread homogeneously on the leaf surface (Pessin 1924, Stuart 1968), and the scales on the leaves of several species of Ceterach and Cheilanthes retarded water uptake for several hours because of the air that was trapped between the scales (Oppenheimer and Halevy 1962, Gaff 1977, Gebauer 1986). The hairs of the leaves of R. myconi (and of other Ramonda species) had the same effect (Figure 2.3). Spraying of the detached hairy leaves resulted in less water uptake than immersion in water or spraying hairless leaves. The retarding effect of scales and hairs suggests that a very rapid water uptake after desiccation could be injurious to the leaf cells.

Problems of Resuming Water Transport

Poikilohydrous plants that possess an internal system for water transport (endohydrous bryophytes and vascular plants) are exposed to cavitation (blockade of a vessel by air bubbles) during desiccation, which compromise the functioning of the conducting tissues upon rehydration. This was particularly investigated in trees (Sperry and Tyree 1988, Tyree and Sperry 1988, Hargrave et al. 1994, Lewis et al. 1994, Kolb et al. 1996, Tyree, Chapter 6, this volume). Emboli in a fraction of the conductive elements confines water transport into a diminished number of vessels, which requires an increased tension and further increased the risk of embolism. Embolized conduits can become functional again through bubble dissolution or expulsion, which requires a positive pressurization (Zimmermann and Milburn 1982). Poikilohydrous plants face the dilemma of restoring water transport through their old, embolized tissues or investing in new conducting tissues, which reduces the resources available to be allocated elsewhere in the plant. The fact that most of the poikilohydrous vascular plants are herbaceous and smaller than 50 cm might be explained by the difficulties of restoration of conductivity of the xylem. The same mechanical difficulties of resuming water conductivity of embolized tissues might also be behind the remarkable lack of poikilohydrous species among the gymnosperms, which consist only of trees and shrubs. The low flexibility due to the xylem anatomy of gymnosperms was demonstrated by Ingrouille (1995).

Poikilohydrous vascular plants that would be able to resume water transport only if their shoot tissues have been hydrated by external water uptake perform like the endohydrous bryophytes, where water conduction in the hydroids is supported by lateral and apoplastic water transport. The imbibition of the cell walls of leaf and stem tissues generates the necessary pressure to induce dissolution of emboli in the tracheary tissues. Capillary forces in poikilohydrous plants at 40% relative humidity and under laboratory conditions could move water to a height of 2-12 cm (Galace 1974, cited in Gaff 1977). These forces are sufficient to rehydrate many of the small herbaceous poikilohydrous species. Other mechanisms to eliminate emboli are temperature-associated osmosis at the plant apex (Pickard 1989) and generation of a root pressure that is able to dissolve gas bubbles in the conduits of small herbs and grasses (Zimmermann and Milburn 1982). The latter was shown to be able to restore full liquid continuity and was assumed to be important in larger poikilohydrous species like Xerophyta eglandulosa (Gaff 1977). Reversal of almost complete embolism in stems of the homoiohydrous Salvia multiflora was related to the presence of narrow vessels and tracheids, which were better to refill than wider conduits (Hargrave et al. 1994). Very narrow vessels (approximately 14 mm) are also true for Myrothamnus flabellifolius. In addition, it has reticular perforation plates and knob-like protuberances on the outer walls of the vessels and tracheids, obviously to provide stability when the tissues swell and shrink. The hydraulic conductivity is, however, low and the shrub needs approximately 70 h to regain turgor (Sherwin et al. 1998). Water rise in the axes is substantially aided by root pressure (which develops 3-4 h after watering the plants and ceases after 4-5 days) and additionally, mechanisms that disintegrate lipid films on the lumen walls of the xylem elements, such as radial water flow along the xylem parenchyma, the phloem, and cortical cells (Schneider et al. 2000). This demonstrates that a woody poikilohydrous plant has to take great efforts for its rehydration. As soon as a leaf is reached by the waterfront, it is unfolded and hydrated to 65% RWC within 2 h and soon photosynthetically functional (Sherwin and Farrant 1996).

The slow recovery upon rehydration of whole plants when compared with detached leaves (Stuart 1968, Gebauer 1986) might be because of the time required to form new roots. This topic has been better explored in homoiohydrous plants from arid environments. For instance, the hydraulic conductivity of roots of Agave deserti and A. acanthodes, which decreased dramatically after several days of drought, rapidly recovered when water was again available, not by formation of new roots but by refilling the extant tissues, which were made up of flexible and unlignified vessels (North and Nobel 1991, Ewers et al. 1992). In fact, a flexible structure in the conductive system was identified in the poikilohydrous Blossfeldia liliputana (Barthlott Porembski 1996). Contractive tracheids evidently enable the dry, contracted, and submersed leaves of Chamaegigas intrepidus to swell by 800%-900% in water (Schiller et al. 1999). Another means of rapid water uptake is provided in this poikilohydrous species (Heilmeier et al. 2000) and in the genus Borya, Cyperaceae, and Xerophyta pinifolia (Porembski and Barthlott 1995, 2000) by a Velamen radicum, which is otherwise typical for epiphytic orchids. Earlier literature has recorded the formation of new adventitious roots in poikilohydrous plants subsequent to rehydration (Walter and Kreeb 1970). This was confirmed recently for Velloziaceae (e.g., Xerophyta scabrida), Borya sphaerocephala, and Craterostigma plantagineum, where such roots appeared after the regreening of the plant, replacing the drought-killed original roots (Tuba et al. 1993a, Porembski and Barthlott 2000, Norwood et al. 2003).

Retarding Water Loss

Poikilohydrous autotrophs can maintain their water content at a constant level only to a limited extent, but they can extend hydration into the dry period by certain, mostly structural, mechanisms. By retarding water loss, such organisms can enhance their exploitation of the transient periods of water availability. Retarding water loss could, however, counteract some advantageous aspects of the poikilohydrous strategy. For instance, extending hydration sometimes reduces water capture. Poikilohydry also provides a remarkable tolerance for desiccated autotrophs to other stresses that usually occur with drought, such as heat and excessive light (see Section "Preventing Damage and Tolerating Stresses''). Thus, if metabolic activity is extended into these harmful periods, it could not only reduce overall productivity but also compromise survival.

One way of retarding water loss in lichens is by increasing the water that can be stored within the plectenchyma (Valladares 1994b, Valladares et al. 1998). Anatomical characteristics, such as porous and thick medulla layers and rhizinae, have been suggested as means of increasing water storage in lichens (Snelgar and Green 1981, Valladares et al. 1993, Valladares and Sancho 1995). However, because water and CO2 share the pathway in lichens, enhanced water storage can hamper CO2 diffusion and consequently reduce photosynthetic carbon uptake (Green et al. 1985, Lange et al. 1996, Maguas et al. 1997). Thus, again, these plants must reach a compromise between two opposing situations. Large foliose lichens can possibly separate photosynthesis and water storage in space somewhat, as young, growing zones of the thallus optimize gas diffusion, whereas old, thick regions act primarily as water reservoirs, sacrificing gas diffusion and carbon gain (Green et al. 1985, Valladares et al. 1994). This trade-off between gas diffusion and water storage seems to be flexible, and lichens have been shown to exhibit a remarkable phenotypic plasticity in their water storage and retention capacities in response to habitat conditions (Larson 1979, 1981, Pintado et al. 1997). Dry habitats induce increased water storage (Tretiach and Brown 1995), but there are complex interactions with light availability for photosynthesis. In shaded sites without access to liquid water, the Antarctic lichen Catillaria corymbosa enhanced both water storage and photosynthesis via increased light harvesting by chlorophylls (Sojo et al. 1997), whereas in exposed sites (dry and receiving high irradiance), the lichen Ramalina capitata enhanced photosynthetic utilization of brief periods of activity via improved gas diffusion at the expense of reducing water storage capacity (Pintado et al. 1997). These problems are not faced by bryophytes because most of them have rather complex photosynthetic tissues, where the CO2 exchange surface is separated from water storage volumes (Green and Lange 1994). The capacity of bryophytes to keep high rates of photosynthesis at high water contents is a very likely explanation for the dominance of these organisms in wet habitats (Green and Lange 1994). Zotz et al. (2000) demonstrated experimentally with Grimmia pulvinata a positive relationship between cushion size and water retention capacity and also an increasing CO2 gain up to an optimum water content. However, despite mechanistic differences between mosses and lichens, in certain cases overall performance can be similar.

Discussing the role of morphological properties for lichens, Rundel (1982a) concluded that evaporative water loss can be reduced by a decrease in surface/volume ratio, but such a decrease reduces uptake of water vapor similarly. This seems not to be the case for liquid water, since structures on the lower side of the thallus such as the rhizinomorphs of the lichen family Umbilicariaceae enhance capture and storage of water from run-offs without significantly increasing water loss by evaporation (Larson 1981, Valladares 1994b, Valladares et al. 1998). Reduction of evaporative water loss by structural traits such as a tomentum on the upper surface (Snelgar and Green 1981), a thick cortex (Larson 1979, Budel 1990), or a decreased surface/volume ratio is very limited in open habitats, but can be significant in sheltered or humid places or in areas with frequently overcast skies (Kappen 1988) or under the influence of drip water from trees or from antennae on roofs. Bryophytes can also retard water loss by an increased boundary layer resistance and due to growth forms of low surface-to-volume ratios (Gimingham and Smith 1971, Proctor 1982, Giordano et al. 1993).

Comparing water retention of a terricolous moss and a terricolous lichen, Klepper (1968) found that both remained hydrated for the same period of time after the rainfall although the moss stored initially more water. Then, regardless of the initial water content, they dried out quickly when the ambient water vanished and the atmosphere became dry. Measurements on desert lichens have clearly shown that the thalli start drying as soon as the sun is rising and their period of hydration depends solely on evaporative conditions (Kappen et al. 1979, Kappen 1988).

As has been repeatedly demonstrated, the water content of vascular resurrection plants varies with the soil moisture and, like their nonvascular counterparts, they dehydrate within a few hours or days after soil water supply has declined (Gaff and Churchill 1976, Gaff 1977). In their shaded habitats some resurrection plants, especially the ferns, take profit from less evaporative stress than in the open. Water-storing tissues of the succulent Blossfeldia lilipu-tana and those within the leaves of some Velloziaceae (e.g., V. tubiflora, V. luteola, Barbacenia reflexa; Kubitzki 1998) may retard water loss. Since stomatal conductance of resurrection vascular plants is, in general, rather high (Tuba et al. 1994), water loss must be retarded by structural features such as scales, as was shown for leaves of Cetrerach officinarum (Oppenheimer and Halevy 1962). However, scales were almost ineffective in Cheilanthes maranthae (Gebauer 1986, Schwab et al. 1989). A retarding effect of hairs in leaves of Ramonda (Figure 2.4a) has long been known (Bewley and Krochko 1982) and the efficiency of leaf pubescence in retarding water loss is increased when the leaves shrink (Gebauer et al. 1987, Figure 2.4b).

FIGURE 2.4 (a) Water loss from leaves of Ramonda myconi in air (approximately 50% rh) with hairs and after hairs were removed before the experiment. (After Gebauer, R., Losch, R., and Kappen, L., Verh Ges (Ökologie., XVI, 231, 1987.) (b) Transpiration rates of leaves of R. myconi with increasing water saturation deficit. Hair density on the upper (adaxial) and the lower (abaxial) axial leaf surface increases as the leaf shrinks with increasing water loss. At approximately 40% saturation deficit, stomatal closure becomes effective. (After Gebauer, R., Wasserabgabe und Wasseraufnahme poikilohydrer höherer Pflanzen im Hinblick auf ihre physiologische Aktivitöt. Diploma thesis, Universitat zu Kiel., 97 pp., 1986.)

FIGURE 2.4 (a) Water loss from leaves of Ramonda myconi in air (approximately 50% rh) with hairs and after hairs were removed before the experiment. (After Gebauer, R., Losch, R., and Kappen, L., Verh Ges (Ökologie., XVI, 231, 1987.) (b) Transpiration rates of leaves of R. myconi with increasing water saturation deficit. Hair density on the upper (adaxial) and the lower (abaxial) axial leaf surface increases as the leaf shrinks with increasing water loss. At approximately 40% saturation deficit, stomatal closure becomes effective. (After Gebauer, R., Wasserabgabe und Wasseraufnahme poikilohydrer höherer Pflanzen im Hinblick auf ihre physiologische Aktivitöt. Diploma thesis, Universitat zu Kiel., 97 pp., 1986.)

The effect of hair density in reducing the transpiration rate strongly increased when the leaf water-saturation deficit went beyond 40%. Reduction of the exposed surface is also a typical mechanism to reduce transpiration. This is the case with poikilohydrous ferns (Pessin 1924); with Selaginella lepidophylla, which curls the whole shoot rosette (Lebkuecher und Eickmeier 1993); with Myrothamnus flabellifolius, which regularly pleats its fan-like leaves (Vieweg and Ziegler 1969, Puff 1978); and with Velloziaceae, which fold or curl their leaves; and with other xeromorphous structures in Sporobolus stapfianus (Gaff 1977, Kubitzki 1998, Vecchia et al. 1998). Leaves of Craterostigma plantagineum can reduce surface area to 15% (Sherwin and Farrant 1998) and those of Xerophyta scabrida to 30% (Tuba et al. 1997b) of the original size. Leaf or shoot movements of most of these plants are due to differential imbibition of the tissues involved, rather than to osmotic phenomena, because they still operate in dead plants. Slow drying over periods of several days may typically occur in monocotyledonous plants such as X. scabrida (Tuba et al. 1997b). Its ecological significance for hardening and conditioning is discussed later.

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