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Photosynthetic activity

- chlorophyll fluorescence (dark)

- net CO2 fixation (hatched)

1 . 1 1 1 »ill J-

Time after rewatering (h)

Fig. 11.25 Recovery of the poikilochlorophyllous desiccation tolerant Velloziaceae Xerophyta scabrida upon rehydration. The graphs in the lower three panels indicate the relative inreases of the structures and functions described in the left column. Compiled after data in Tuba et al. (1993a, b, 1994)

Time after rewatering (h)

Fig. 11.25 Recovery of the poikilochlorophyllous desiccation tolerant Velloziaceae Xerophyta scabrida upon rehydration. The graphs in the lower three panels indicate the relative inreases of the structures and functions described in the left column. Compiled after data in Tuba et al. (1993a, b, 1994)

(Kappen and Valladares 1999). Mitochondrial membranes are better preserved during desiccation than the thylakoids of chloroplasts. In X. scabrida respiration is increased above the normal rates after 12 h. This so-called rehydration respiration must be related to repair mechanisms and ceases after 30 h, when normal rates are attained again. In contrast to chromoplasts and gerontoplasts of senescing leaves, which cannot regreen, the chloroplasts of desiccation tolerant plants, which lose their entire photosynthetic apparatus, i.e. thylakoid membranes and pigments during desiccation, are totally rebuilt after rehydration. Tuba et al. (1993b) have named these plastids desiccoplasts. Some of the carotenoids, i.e. 22-28%, are preserved during desiccation, and they may play an essential role in reorganization. Reaccumulation of carotenoids and chlorophyll a + b starts after 12 h when thylakoid development also begins to appear, as indicated by the thylakoid frequency, thylakoid stacking and the ratio appressed/exposed membranes. We recall that appressed thy-lakoid regions and the membranes stacked in the grana are the sites of photosystem II (see Box 4.2). Chlorophyll fluorescence reappears at about the same time, while it takes considerably longer, i.e. about 24 h for the onset of net CO2-uptake. Recovery of photosystem I usually is faster than that of photosystem II (Kappen and Valladares 1999). After 72 h all functions have reached their normal levels again.

11.4.3.3 Structure Function Relations

Whole leaves shrink, curl, roll and fold during desiccation and the epidermis of leaves is much wrinkling (Hartung et al. 1998; Proctor and Tuba 2002; Vicré et al. 2004). This is a consequence of water loss, but it also contributes to a much reduced light absorption and irradiance stress in the dry state. In addition resurrection plants may develop accumulation of anthocyanin a sun-blocking pigment (Farrant et al. 2003). During the reduction of the volume of leaf cells and the associated extensive folding of cell walls a tight connection remains established between the plasma membrane and the cell wall, a phenomenon called cytorrhysis, which is important for recovery during rehydration (Hartung et al. 1998; Vicré et al. 1999). The cell wall is subject to considerable mechanical stress in this process. Resurrection plants develop cell walls with particular tensile strength. Arabinan polymers and arabinogalactan proteins, xyloglucans, homogalacturonan, rhamnogalacturonan and unesterified pectins are important macromolecules building up the special cell wall structure with the high mobility and water absorbing capacity required in dehy-dration/rehydration cycles and cross linking Ca2+ ions may also be involved (Vicré et al. 1999,2004; Moore et al. 2006).

Hydraulic architecture is especially important during rehydration. Conductive elements of the xylem suffer embolism during desiccation. Lipids lining the water conducting elements of the xylem may be involved in preventing complete loss of water in the dry state (Schneider et al. 2000, 2003; Wagner et al. 2000; but see also Tyree 2001). In the initial stages of moistening before xylem emboli are repaired and especially in the poikilochlorophyllous Vellozias, which lose their absorptive roots in the desiccated state (Sect. 11.4.3.2) external capillary water movement is important. In the Vellociaceae species this can occur quite effectively in the external space of the pseudostems built up of the retained leaf bases of dead leaves (Proctor and Tuba 2002; Fig. 11.26). In the xylem in addition to capillary water movement root pressure is essential for overcoming embolis and for refilling the conductive elements with water (Kappen and Valladares 1999; Proctor and Tuba 2002; Schneider et al. 2000,2003; Wagner et al. 2000). This may be the reason why desiccation tolerant angiosperms do not reach larger sizes than up to 3 - 4 m tall (Gaff 1977; Kappen and Valladares 1999; Proctor and Tuba 2002), e.g. monocotyle-donous pseudo shrubs such as Vellozia (Fig. 11.26). The homoiohydrous resurrection plant Craterostigma plantagineum (Scrophulariaceae) has desiccation tolerant roots which die two weeks after rehydration (Norwood et al. 2003).

Fig. 11.26A-C Vellozia gigantea (A) and detailed views of its pseudo stem (B, C). Serra do Cipo, MG, Brazil

11.4.3.4 Cell Physiology

At the cell physiological level during desiccation and rehydration the three most important functions are the dynamics (i) of compatible solutes for the protection of cellular components and (ii) of lipids for the maintenance of membrane structures and (iii) the control of reactive oxygen species.

(i) Compatible solutes. Compatible solutes stabilize proteins and membranes (Sect. 7.4, Box 7.1). In resurrection plants mainly sugars function as compatible solutes, predominantly sucrose but also glucose, fructose and unusual sugars, such as trehalose, glucopyranosyl-^-glyceroland arbutin (Hartung et al. 1998; Vicré et al. 2004). Increase in hexokinase activity is associated with the acquisition of desiccation tolerance (Whittaker et al. 2001). Unusual sugars, such as stachyose (Norwood et al. 2003) and in C. plantagineum the C8-sugar 2-octulose are converted to sucrose (Norwood et al. 2000; Bartels and Salamini 2001; Ramanjulu and Bartels 2002). Hydrophilic protective proteins are also important (Rodrigo et al. 2004) and are for example accumulated in plastids (Bartels and Salamini 2001). An interesting cyto-logical consequence of the accumulation of sugars with a higher degree of polymerization such as raffinose, stachyose and other galactosyl-sucrose-oligosaccharides in addition to sucrose is the suppression of crystallisation of protoplastic constituents and the promotion of glass formation or vitrification controlling metabolism in the desiccated state at low water content (Hartung et al. 1998; Proctor and Tuba 2002; Vicré et al. 2004).

(ii) Lipids. Membranes of resurrection plants appear to be well protected (Hartung et al. 1998). Dynamics of the chemical composition of membrane lipids are important to maintain the structure of membranes including the plasma membrane and chloroplast membranes during desiccation/rehydration cycles, where the unsaturation level of phospholipids and the level of total lipids decrease during desiccation (Navari-Izzo et al. 2000; Quartacci et al. 2002; Ramanjulu and Bartels 2002).

(iii) Reactive oxygen species (ROS). The formation of ROS is a particular problem during desiccation/rehydration cycles especially in homoiochlorophyllous resurrection plants (Sect. 11.4.3.1). Antioxidative defence systems such as the ascor-bate/glutathione cycle and superoxide dismutases play an important role in protection to keep functional sulfhydryl groups in the reduced state (Smirnoff 1993; Hartung et al. 1998; Kappen and Valldares 1999; Proctor and Tuba 2002; Vicré et al. 2004). Angiosperms with internal carbon concentrating mechanisms such as C4-plants (Sect. 10.1.1.2) and CAM-plants (Sect. 5.2.2.2) where oxidative stress is at least partially controlled by high internal CO2 concentrations appear to be rare among resurrection plants. A curiosity is the small desiccation tolerant cactus with CAM Blossfeldia liliputana (Barthlott and Porembski 1996; Hartung et al. 1998).

11.4.3.5 Gene Regulation

The protection of the genome against desiccation and rehydration induced damage and the down regulation and up regulation of genes follow longer time constants than metabolic processes (Cooper and Farrant 2002). As many as 800 to 3000 genes may be involved in desiccation tolerance of plants (Hartung et al. 1998). Screening and transcriptomics techniques show differential expression, up regulation and down regulation, of a large number of genes (Ingram and Bartels 1996; Bockel et al. 1998; Velasco et al. 1998; Garwe et al. 2003; Collett et al. 2004; Neale et al. 2000). This underlines the high complexity of molecular responses in desiccation tolerance. Many of these genes can be related to special functions, but the roles of very many others are not understood. Some of these genes are specific in desiccation tolerance many others are seen to be involved in general in various stress responses. Gene regulation cascades and networks in desiccation tolerance (see Ra-manjulu and Bartels 2002) are modulated by various phytohormones (Ghasempour et al. 2001; Vicré et al. 2004). The most central messenger, however, is abscisic acid (ABA) (Ingram and Bartels 1996; Hartung et al. 1998; Kappen and Valladares 1999; Bartels and Salamini 2001; Proctor and Tuba 2002; Bernacchia and Furini 2004; Vicré et al. 2004). Its levels, for example, increase rapidly in desiccating Chamaegi-gas intrepidus (Fig. 11.24). Gene expression dynamics in relation to the various cell physiological functions discussed in Sect. 11.4.3.4 have been revealed, namely carbohydrate metablosim for compatible solutes (Kleines et al. 1999; Bernacchia and Furini 2004), lipid metabolism (Bartels and Salamini 2001), photosynthesis (Phillips et al. 2002; Collett et al. 2003), and aquaporins, channel proteins facilitating water exchange across membranes (Hartung et al. 1998; Ramanjulu and Bartels 2002). Differential gene expression also addresses protective hydrophilic proteins such as the late embryogenesis abundant proteins (LEA) first discovered due to their involvement in cellular protections in dehydrating seeds, dehdrins and small heat shock proteins (Hartung et al. 1998; Bartels and Salamini 2001; Ditzer et al. 2001; Bernacchia and Furini 2004).

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