Info

100-200 500-750 100-500 150 >2000 400-500

Minimum Water Content for Net Photosynthesis

80% desiccation

20% RWC

Source

Quadir et al. 1979 Kallio and Kärenlampi

1975, Green et al. 1984 Lange et al. 1998 Lange et al. 1994 Rundel and Lange 1983 Tuba 1987

Gildner and Larson 1992 Gebauer 1986 Gebauer 1986 Nobel 1978 Eickmeier 1979 Gebauer 1986 Tuba et al. 1994 Larcher 1980

Abbreviation: d.wt., dry weight; RWC, relative water content.

length of the period of desiccation (Eickmeier 1979). In Ramonda myconi, net photosynthesis decreased to one-third after the fifth cycle of drying and wetting (Gebauer 1986). In S. lepido-phylla, intermediate drying speeds (52-94 h until complete curling of the plant was reached) led to maximal recovery, whereas either rapid (5.5 h) or very slow (175 h) drying was associated with significantly reduced photosynthetic rates (Eickmeier 1983). Rapid drying implied increased membrane dysfunction, whereas slow drying caused retarded de novo protein synthesis.

Poikilochlorophyllous plants typically undergo a long hiatus in photosynthetic activity during the periods of drying and rehydration because of the destruction of the photosynthetic apparatus and resynthesis of the desiccoplasts. Under natural conditions, the desiccation process may take several days or weeks (Hetherington and Smillie 1982, Tuba et al. 1997). In slowly drying leaves of Xyrophyta scabrida, net photosynthesis became negative after 3 days when leaf RWC was 54%, and respiration ceased after 15 days at 8% leaf RWC. A similar value limiting respiratory activity (less than 10%) was reported for X. humilis and also the homoihydrous Craterostigma wilmsii and Myrothamnus flabellifolius (Farrant 2000). Accordingly, respiration of X. scabrida was activated within 20 min of rehydration and reached full rates within 6 h, even before turgor was restored in the cells (Tuba et al. 1994). However, chlorophyll resynthesis started only after 12 h of rehydration and was not complete until 36 h had elapsed. The time required to fully restore photosynthetic capacity upon rehydration was not different whether the plants were exposed to air with 350 or 700 ppm of CO2, but downregulation of photosynthetic rates was found at 700 ppm (Tuba et al. 1996b). During regreening upon rehydration, photosystem I activity appeared to recover faster than that of photosystem II (Gaff and Hallam 1974, Hetherington and Smillie 1982).

Available studies on poikilohydrous plants have mainly focused on the cellular and molecular mechanisms underlying inactivation and reactivation of photosynthesis and respiration. With a few exceptions, seemingly relevant aspects of the recovery of plant CO2 exchange such as stomatal responses (Lichtenthaler and Rinderle 1988, Schwab et al. 1989, Tuba et al. 1994), leaf performance (Stuart 1968, Matthes-Sears et al. 1993), and whole plant behavior (Eickmeier et al. 1992) have been widely neglected. Field studies of the CO2 exchange during the dehydration/rehydration cycles, such as those conducted with lichens (see Section "Opportunistic Metabolic Activity In Situ"), are necessary to reach a better ecological understanding of the carbon economy of poikilohydrous plants.

Different Strategies

Poikilohydrous life style does not obligatively mean that the organism is a perennial, as several of the constitutively poikilohydrous bryophytes and a few lichen taxa are ephemeral (Longton 1988a). Compared with the strategy of homoiohydrous plants, it can be characterized by the hydration range and rapidity with which the poikilohydrous plant enters into the anabiotic state during desiccation and with which it recovers normal activity during rehydra-tion (Figure 2.10). According to the features discussed throughout this chapter, we can discern four kinds of strategies among the eurypoikilohydrous organisms.

The Ready type is exhibited by constitutively poikilohydrous nonvascular species (cyano-bacteria, algae, lichens, and some bryophytes), which can easily lose and absorb water, switching their metabolism on and off very quickly. Their cellular structures are highly strengthened and do not need extensive, time-consuming repair processes, which allows them to oscillate frequently (daily or even within a few hours) between anabiosis and active state.

Certain bryophytes and vascular plants need variable periods of time (hours in the case of the former, days in the case of the latter) to recover completely from desiccation and therefore represent a Repair type. Preventing rapid water uptake may protect from deleterious effects during rehydration. The most drastic example of this strategy is poikilochlorophyllous plants. The partial destruction of organelles and photosynthetic pigments during desiccation seems

Homoiohydrous

Poikilohydrous (constitutional or secondary)

Stenohydrous (mesophytic, hygrophytic)

Euryhydrous (xerophytes)

Stenopoikilohydrous (moist environment) not tolerating >90% water loss; 40% rh

Eurypoikilohydrous (wide habitat range) preferably semiarid

96% rh aquatic, >40% rh Transient wet environments moist or mesic (temporal)

(algae, bryophytes, environment

Hymenophyllaceae) (algae, bryophytes, lichens)

environment

Permanent

Prepair Repair Ready type type type

FIGURE 2.10 Water status-related plant performance.

to be disadvantageous, since it reduces the period of activity and limits photosynthetic carbon gain. However, this response type prevents oxidative membrane deterioration, particularly in plants that grow in open-exposed habitats. Such plants need rather long and continuous periods of activity and oscillate between dry and wet states only once or a few times per year (Gaff and Gies 1986). Homoiochlorophyllous plants manage better with repeated changes in hydration.

Certain bryophytes and vascular plants are not capable of tolerating extreme desiccation without a previous acclimation or preconditioning and are therefore typical of the Prepare type. Tolerance to desiccation is increased if they are exposed to a slow water loss, or if water loss occurs under a low vapor pressure deficit (e.g., Bryum caespititium and Pohlia elongata). Such plants either make use of structural features that retard water loss or grow in sheltered habitats where the evaporative potential is low (rock colonizing ferns, Borya nitida, some Velloziaceae).

The Transient type includes certain bryophytes and ferns that acquire a eurypoikilohydrous character only temporarily by hardening as their fronds become extremely desiccation-tolerant during the winter or the dry season (mesic bryophytes, Polypodium vulgare, Asplenium species).

A type with mixed strategy is realized with plants that act poikilohydrously only with a major part of the individual. By shifting between functioning with larger "dolichoblasts" in the rainy season and small, extremely desiccation-tolerant "brachyblasts" in the dry season, the small shrub Satureja gillesii can reduce its transpiring leaf surface (Montenegro et al. 1979). Brachyblasts have a mesophytic anatomy and are covered by filamentous trichomes. Among the plants that live in ephemeral rock pools, those species (Aponogetum desertorum and Limosella species) that are preserved by desiccation-tolerant rhizomes or corms (Gaff and Gies 1986) may also reveal a mixed strategy type. However, the heterophyllous Chamaegigas intrepidus typically does. It survives with dry rhizome and contracted conic basal leaves most of the year. On flooding the pool the basal submersed leaves do expend (it is not clear whether they carry out significant photosynthesis). The floating leaves can be produced within a few days and are the productive part of the plant. They perform according to the repair-type, as they can pass periods of desiccation repeatedly (up to 20 times) during one season and are able to regain full photosynthetic capacity within 18 h (Woitke et al. 2004).

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