All the basic metabolic processes of life take place only in the aqueous medium of a hydrated protoplasm. Drying to equilibrium with even moderately dry air is, hence, instantly lethal to most animals and plants. As a consequence, water availability is one of the most important ecological issues and evolutionary pressures on terrestrial life (Alpert 2005). Land plants have evolved three main strategies in response to drought, namely drought escape, drought avoidance and desiccation tolerance (Levitt 1980).
Drought escape refers to the ability to escape periods of drought, especially during the most sensitive periods of development. This involves the adoption of a life strategy that adapts the life cycle so that the plant dies during drought periods while desiccation-tolerant diaspores, including spores and vegetative propagules, remain viable in the ground. This strategy is best illustrated by species with an annual life cycle. Shuttle species (Section 7.3.3) are adapted to climates with a severe dry season by completing their life cycle within a few months during the wet season. Shuttle species avoid the dry season by producing large spores that have the ability to remain in the diaspore bank until above-ground conditions become favourable for growth again. This is the strategy adopted by annual mosses (e.g. in the Funariaceae and Pottia-ceae) and thalloid liverworts (e.g. Riccia; see Section 3.3) in climates with a well-marked dry season, such as the Mediterranean (Kürschner & Parolly 1999, Kürschner 2003).
Drought avoidance (also commonly referred to as drought resistance) involves plants withstanding a period of drought by maintaining a favourable internal water balance. Avoidance relies on the maintenance of a chronic disequilibrium between the water content of the cell and the outer atmosphere through water retention. Avoidance is largely the privilege of vascular plants. In the latter, avoidance is aided by a series of morphological features that help to control air exchange (e.g. stomata) and limit evaporation (e.g. a waterproof cuticle). Water is pumped up from the soil by the roots and translocated through the plant by specialized conducting tissues. In bryophytes, this strategy is rare. It is linked to a series of morphological transformations to retain and transport water. The dense shoot packing and the presence of dead hyaline cells holding large amounts of water in leaves and around stems and branches in genera such as Sphagnum and Leucobryum are, for example, typically suggestive of drought resistance. Water conduction in the most sophisticated resistant species is internal. In so-called endohydric species, internal water conduction is facilitated by the presence of specialized water-conducting cells, such as fibrillose and porous hyalocysts in Sphagnum and some Dicranaceae (Section 4.1.1), hydroids in some mosses (Section 1.4.2) and perforated cells in some liverworts (Section 1.4.2). The level of resistance to drought displayed by bryophytes is, however, comparatively low with regard to that exhibited by vascular plants. Even in endohydric species, external conduction plays a significant role in water uptake. In fact, most bryophytes have evolved an alternative strategy to drought avoidance, namely desiccation tolerance.
Desiccation tolerance refers to the ability to dry to equilibrium with air that is moderately to extremely dry and then resume normal metabolic activity after rehydration. Alpert (2005) proposed that tolerance can be operationally defined as the ability to survive desiccation to a water content of 10%, i.e. of 0.1 g of water per gramme of dry weight. This corresponds to the point at which too little water remains to surround intracellular membranes and macromolecules and, therefore, to support metabolism. Desiccation tolerance thus requires that the plants reversibly cease metabolism during drought periods and develop adaptations to desiccation at the cellular level (Alpert & Oliver 2002).
Bryophytes are poikilohydric, which means that their water content is directly regulated by the ambient humidity. Most species are termed ecto-hydric because they take up water through the whole surface of the plant. The fact, that bryophytes are poikilohydric, means that it is not necessary for them to develop a root system to draw water from the soil. This alternative strategy enables them to grow on very hard surfaces such as rocks and tree trunks that are inhospitable to most vascular plants. Except in a few cases, for example, certain epiphytic Bromeliaceae, the vast majority of the latter are indeed unable to absorb water from their aerial organs and must draw it from soil. However, the poikilohydric condition brings about a major limitation to growth during dry periods. For example, Wiklund and Rydin (2004b) recorded the growth of a colony of the epiphytic moss Neckera pennata over four years and clearly demonstrated the prime importance of precipitation on colony development (Fig. 8.1). Bryophytes desiccate at the same time as or shortly after their substratum. Physiological activity and, hence, growth is restricted to periods of hydration, with the plant entering dormancy upon desiccation. The impact of water availability on growth is most pronounced in arid areas. For example, patches of the moss Crossidium crassinerve in the Mojave Desert experienced complete hydration only 8% of the time over a four-year period (Stark 2005). Since respiration strongly increases following desiccation, whereas photosynthesis recovers at a much slower rate (Alpert & Oechel 1985), one explanation for the relative scarcity of mosses in hot deserts is that a positive carbon balance, i.e. a positive net photosynthesis, is difficult or impossible to achieve in areas where moss patches are hydrated for insufficient periods of time.
In the absence of roots and a highly efficient internal water transport system, most bryophytes depend primarily on atmospheric water to sustain their needs. Water must be absorbed over much of their body surface, which must thus be permeable. The cuticle that seals the vascular plant body is most often reduced or even lacking on the vegetative body of bryophytes and stomata, which occur in the capsule wall of mosses and hornworts, are completely lacking from the leafy stem or the thallus.
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