Abstract.Resynthesis of the photosynthetic apparatus and resumption of CO 2 assimilation upon rehydration is reported for the monocotyledonous and poikilochlorophyllous desiccation-tolerant (PDT) plant Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae). During desiccation there was a complete breakdown of chlorophylls whereas the total carotenoid content of air-dried leaves was reduced to about 22% of that of functional leaves. The prerequisites for the resynthesis of photosynthetic pigments and functional thylakoids were the reappearance of turgor and maximum leaf water content at 2 and 10 h after rehydration, respectively. The period of increased initial respiration after rewetting leaves (rehydration respiration) lasted up to 30 h and was thus 6 to 10 times longer than in homoiochlorophyllous desiccation-tolerant plants (HDTs) in which chlorophylls are retained during desiccation. Accumulation of chlorophylls a+b and total carotenoids (xanthophylls and 13-carotene) started 10 h after rehydration. Normal levels of chlorophyll and carotenoids were obtained 72 h after rehydration. Values for the variable-fluorescence decrease ratio (Rfd690 values), an indicator of photochemical activity, showed that photochemical function started 10 h after rehydration, but normal values of 2.7 were reached only 72 h after rehydration. Net CO2 assimilation started 24 h after rewetting and normal rates were reached after 72 h, at the same time as normal values of stomatal conductance were obtained. The increasing rates of net CO 2 assimilation were paralleled by decreasing values of the intercellular CO2 concentration. All photosynthetic parameters investigated showed values normal for functional chloroplasts by 72 h after the onset of rehydration.Abbreviations: c = 13-carotene; c~ = intercellular CO2 concentration; Car x + c = total carotenoid content x + c; Chl a + b = total chlorophyll a+b content; gs-stomatal conductance; HDT = homoiochlorophyllous desiccation tolerant; LWC = leaf-water content; PN = net photosynthesis rate; PDT = poikilochlorophyllous desiccation tolerant; R a = dark respiration; Rfd = variable fluorescence decrease ratio (Rfd-fd/fs); x = xanthophyllsCorrespondence to: H.. Lichtenthaler; FAX: 49 (721) 608 4874 Fully regreened leaves of the presumed C 3 plant X.scabrida exhibited a net CO 2 assimilation rate which was in the same range as that of other C 3 plants and higher than that of recovered HDT plants. The fundamental difference between air-dried PDT plants, such as X.scabrida, which have to resynthesize the photosynthetic pigment apparatus, and air-dried HDT plants, which only undergo a functional recovery, is discussed.

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Summary 1.There have been no reports of the long-term responses of the desiccation-tolerant (DT) plants to elevated CO 2 . Xerophyta scabrida is a DT woody shrub, which loses chlorophylls and thylakoids during desiccation: a so-called poikilochlorophyllous desiccation-tolerant species (PDT). When the leaves of X. scabria are allowed to desiccate, the species shows many of the normal features of (P)DT plants. 2. However, the duration of photosynthesis in X. scabria is prolonged by 300% when the measurements are made at 700 as opposed to 350 p.p.m. CO 2 . The implication is that the carboxylating enzymes must still have been active at this time to enable appreciable photosynthetic activity. This response could have far-reaching implications for the success of such species in a future climate. 3. Lichens and mosses, representing the homoiochlorophyllous DTs (HDT), retain their chlorophyll content and photosynthetic apparatus during desiccation. We show the desiccation responses of two common HDT species (Cladonia convoluta and Tortula ruralis) to elevated CO 2 for comparison. Both HDT species showed increased net CO 2 uptake in the material grown at high CO 2 by more than 30% in moss and by more than 50% in lichen. It is concluded that desiccation-tolerant plants will be among the main beneficiaries of a high CO 2 future. Key-words: Desiccation tolerance, elevated CO 2Functional Ecology (1998) 12, 39-44 39 Detached and desiccated leaves were rehydrated and regreened at their respective CO 2 concentrations (350 and 700 p.p.m.) until their photosynthetic apparatus and activity were fully reconstituted (Tuba, Lichtenthaler et al. 1994;Csintalan et al. 1996). These leaves were then desiccated slowly in an atmosphere of 350 or 700 p.p.m. CO 2 on racks over water in square glass containers kept in Plexiglas chambers at controlled photon flux density (800 µmol m -2 s -1 ), temperature (23°C) and relative air humidity (90% where they were exposed to elevated (700 p.p.m.) and ambient (350 p.p.m.) CO 2 concentrations, respectively, for 5 months. Prior to desiccation the species were rehydrated for 12 h which resulted in full photosynthetic activity. Immediately before the start of the desiccation procedure they were oversaturated by the addition of distilled water. Air humidity (70% r.h.), temperature (23°C) and photon irradiance (400 µmol photons m -2 s -1 ) were set to simulate the length of the desiccation period in natural conditions.Tissue water content was measured by a direct thermogravimetric method (Catsky 1974). Plants were dried to constant weight at 90°C.The photosynthetic pigments [chlorophylls a and b as well as total carotenoids: xanthophylls + carotenes (x + c)] were determined (in 100% acetone extract) by spectrophotometry (Lichtenthaler 1987).The variable fluorescence decrease ratios (Rfd 690) were calculated from the chlorophyll fluorescence induction kinetics measured in the 690 nm region (i.e. the fluorescence maximum of the chlorophyll fluorescence emission spectrum) by means of the He/Ne laser-e...

During a slow desiccation in photosynthetically fully active leaves of the poikilochlorophyllous desiccation‐tolerant (PDT) monocotyledon Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae), thylakoid activity, CO2 assimilation and respiration decline and chlorophylls and carotenoids are successively broken down. The initially slow rate of leaf water loss is related to the large reduction in leaf area which is reflected in the decrease of specific leaf area. Chlorophylls are broken down faster than carotenoids. The ratio of the variable chlorophyll fluorescence, an indicator of photosynthetic activity (Rfd690‐values), shows that the functionality of thylakoids and chlorophylls is successively lost during desiccation. The decline in net CO2 assimilation in desiccating leaves is largely caused by stomatal closure. The complete cessation of CO2 assimilation, however, is due to the breakdown of chlorophylls and thylakoids. Respiration continued during desiccation and remained active far below ‐3.2 MPa leaf water potential. The differences during desiccation of the photosynthetic apparatus between poikilochlorophyllous and homoiochlorophyllous desiccation‐tolerant plants are discussed.

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