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CO2-Concentrating: Consequences in Crassulacean Acid Metabolism

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CO2-Concentrating: Consequences in Crassulacean Acid Metabolism Journal of Experimental Botany, Vol. 53, No. 378, pp. 2131±2142, November 2002 DOI: 10.1093/jxb/erf081 CO2-concentrating: consequences in crassulacean acid metabolism Ulrich LuÈ ttge1 Institut fuÈr Botanik, Technische UniversitaÈt Darmstadt, Schnittspahnstrabe 3±5, D-64287 Darmstadt, Germany Received 11 April 2002; Accepted 1 July 2002 Abstract crassulacean acid metabolism, oxidative stress, oxygen concentrating. The consequences of CO2-concentrating in leaf air- spaces of CAM plants during daytime organic acid decarboxylation in Phase III of CAM (crassulacean acid metabolism) are explored. There are mechanistic The overt phenomenon of internal consequences of internal CO2 partial pressures, CO2-concentrating: Phase III of CAM pCO2. These are (i) effects on stomata, i.e. high pCO2 i i Crassulacean acid metabolism (CAM) is a well-known eliciting stomatal closure in Phase III, (ii) regulation of modi®cation of photosynthesis that is covered in all the malic acid remobilization from the vacuole, malate basic textbooks of plant physiology and biochemistry and decarboxylation and re®xation of CO via Rubisco 2 which has been extensively reviewed (Osmond, 1978; (ribulose bisphosphate carboxylase/oxygenase), and Grif®ths, 1989; Winter and Smith, 1996a; Cushman and (iii) internal signalling functions during the transitions Bohnert, 1997; Dodd et al., 2002). The essential mechan- between Phases II and III and III and IV, respectively, ism of CAM is the acquisition of inorganic carbon (Ci) by in the natural day/night cycle and in synchronizing ± dark-®xation of bicarbonate (HCO3 ) via phosphoenolpyr- the circadian clocks of individual leaf cells or leaf uvate carboxylase (PEPC). This leads to an organic acid patches in the free-running endogenous rhythmicity (mainly malic acid)-concentrating effect in the dark period of CAM. There are ecophysiological consequences. when organic acid is stored in the central cell sap vacuole. Obvious bene®cial ecophysiological consequences In the subsequent light period organic acid is released from are (i) CO2-acquisition, (ii) increased water-use- the vacuole again, decarboxylated and the resulting CO2 ef®ciency, (iii) suppressed photorespiration, and (iv) assimilated in the Calvin cycle. These parts of the CAM reduced oxidative stress by over-energization of the cycle are called Phase I (nocturnal CO2-®xation via PEPC photosynthetic apparatus. However, the general and acid accumulation) and Phase III (daytime CO2- potency of these bene®cial effects may be questioned. recovery and assimilation), respectively. These phases are There are also adverse ecophysiological conse- separated by transition phases, Phase II in the morning and quences. These are (i) energetics, (ii) pH effects and Phase IV in the afternoon, respectively (Osmond, 1978; (iii) Phase III oxidative stress. A major consequence of see Fig. 1 in Dodd et al., 2002). CO2-concentrating in Phase III is O2-concentrating, Nocturnally accumulated vacuolar organic acid is a increased pCO2 is accompanied by increased pO2.Do i i CO2-store in the form of carboxyl groups and not in the reversible shifts of C /CAM-intermediate plants 3 form of free Ci. Raven and Spicer (1996) have explained in between the C ±CAM±C modes of photosynthesis 3 3 detail why the storage of Ci equivalent to the large amounts indicate that C3-photosynthesis provides better pro- of organic acid actually stored in CAM nocturnally tection from irradiance stress? There are many open (~500 mol m±3) would be a less operable and favourable questions and CAM remains a curiosity. option. Thus, strictly speaking, Phase I is not considered as CO2-concentrating, although it is an essential part of the Key words: Carbon dioxide concentrating, circadian clock, whole mechanism. 1 Fax +49 61 5116 4630. E-mail: [email protected] ã Society for Experimental Biology 2002 2132 LuÈttge Conversely, the release and decarboxylation of organic leaves of C. rosea, now known as a CAM plant (Ball et al., acid during the major part of the light period (Phase III) 1991a, b), during the light period. He writes leads to a rapid build-up of high internal CO2-concentra- `In keiner P¯anze zirkuliert vielleicht eine so ungeheure tions effecting stomatal closure in the light. The internal Menge an Luft als in der Clusia rosea. Wenn man die CO2-concentrations behind closed stomata reported in the BlaÈtter dem Sonnenlicht aussetzt, so ergeben sie ... nicht literature are compiled in Table 1. They range from a eine einzige Luftblase.' 2-fold to an over 60-fold increase of internal related to `In no other plant perhaps as much air is circulating than external CO2-concentration. in the Clusia rosea. If one exposes the leaves to the sun Thus, when talking about CO2-concentrating in CAM light, they produce ... not a single air bubble.' this refers to the Phase III phenomenon. This will be the That means there is no obvious gas exchange in the light topic of this review when the mechanistic consequences of via the leaf surface. We now know, of course, that the CO2-concentrating for leaf functions and the eco- stomata are closed in Phase III of CAM, and hence there is physiological consequences for plant performance are no gas exchange. assessed. `Aus dem verwundeten Teile des Stengels faÈhrt aber mit ungeheurer Geschwindigkeit ein Strom von perlartigen LuftblaÈschen ... aus jedem dieser GefaÈûbuÈndel aus ± ein Historical recollections: CO2-concentrating and herrliches Schauspiel'. O2-concentrating `However, at the cut end of the petiole with an immense In the February of 1800 Alexander von Humboldt velocity a stream of pearl-like gas bubbles ... is released performed ecophysiological gas-exchange measurements from each of these vascular bundles, a magni®cent on Clusia rosea at Lake Valencia in Venezuela. It may be spectacle.' debated if he thus became the discoverer of CAM in We now know that behind the closed stomata there is a Clusia. He certainly made the correct observations. Once high internal gas pressure in Phase III. again, in 1937, W Hartenburg also made the right `Setzte ich den Apparat in den Schatten, so hoÈrte der observations, but like Alexander von Humboldt he did Luftstrom auf...' not present the appropriate explanation (Hartenburg, 1937; `Did I place the apparatus in the shade, the gas stream see LuÈttge, 1995). Thus, the discovery of CAM in Clusia ceased ...' appears to be rightly acknowledged to Tinoco Ojanguren That means that Alexander von Humboldt performed and Vazquez-Janez (1983). the appropriate control. What, however, Alexander von Humboldt was certainly `Luft aus dem Innern der Clusia rosea ... besteht aus 0,35 the ®rst to discover is the high internal O2-concentration in Oxygen und 0,65 Azote'. Table 1. Maximum internal CO2-concentrations and ci /ca-ratios (where ca was taken as 0.04%) measured in various CAM-plants in the light period, at midday, in Phase III of CAM by various methods Note that the determination of pCO2 from measurements of gas exchange, i.e. leaf-conductance for water vapour, g , is only possible when i H2O stomata are open, at least to some extent, and that errors become large as stomata close in Phase III. Thus the values reported by Friemert et al. (1986) are approximations, showing the order of magnitude though. CO2 Species Family pi (%) ci/ca Method Reference Opuntia ®cus- indica Cactaceae 1.30 32.5 Thermal conductivity Cockburn et al., 1979 Opuntia basilaris Cactaceae 2.50 62.5 Thermal conductivity Cockburn et al., 1979 Opuntia monacantha Cactaceae 0.12 3.0 Gas chromatography Spalding et al., 1979 Agave desertii Agavaceae 0.80 20.0 Thermal conductivity Cockburn et al., 1979 Yucca schidigera Agavaceae 0.40 10.0 Thermal conductivity Cockburn et al., 1979 AloeÈ vera Liliaceae 0.60 15.0 Thermal conductivity Cockburn et al., 1979 Ananas comosus Bromeliaceae 0.50 12.5 Thermal conductivity Cockburn et al., 1979 Ananas comosus Bromeliaceae 0.13 3.3 Gas chromatography Spalding et al., 1979 Cattleya sp. Orchidaceae 0.15 3.8 Thermal conductivity Cockburn et al., 1979 Phalaenopsis sp. Orchidaceae 0.23 5.8 Thermal conductivity Cockburn et al., 1979 Hoya carnosa Asclepiadaceae 0.08 2.0 Gas chromatography Spalding et al., 1979 Huernia sp. Asclepiadaceae 0.14 3.5 Gas chromatography Spalding et al., 1979 KalanchoeÈ gastonis-bonnieri Crassulaceae 0.27 6.8 Gas chromatography Spalding et al., 1979 KalanchoeÈ tomentosa Crassulaceae 0.35 8.8 Gas chromatography Spalding et al., 1979 KalanchoeÈ daigremontiana Crassulaceae 0.50 12.5 Gas chromatography Kluge et al., 1981 KalanchoeÈ tubi¯ora Crassulaceae 0.09 2.3 Gas exchange Friemert et al., 1986 Sedum praealtum Crassulaceae 0.29 7.3 Gas chromatography Spalding et al., 1979 Sedum morganianum Crassulaceae 0.26 6.5 Gas exchange Friemert et al., 1986 Sempervivum tectorum Crassulaceae 0.12 3.0 Gas exchange Friemert et al., 1986 CO2-concentrating in CAM 2133 `The air from the interior of Clusia rosea ... consists of With the new technique of chlorophyll ¯uorescence 0,35 oxygen and 0,65 "azote".' imaging (Daley et al., 1989; Siebke and Weis, 1995) it is That means that the gas in the leaf air spaces had 35% now possible (i) to obtain in addition a much better O2 and 65% N2. temporal resolution, (ii) to obtain spatial resolution over a As measurements in the ®eld are always dif®cult, later leaf, and (iii) to obtain some more insight into the the chemical O2-determinations were checked in the underlying processes, including the functions of CO2- laboratory of L-J Gay-Lussac in Paris and were found to evolution behind closed stomata, rather than just recording be too high by a systematic error of 5% (Quotations after the disappearance of malic acid. KraÈtz, 2001). Chlorophyll ¯uorescence imaging allows spatiotem- Using a gas chromatograph Spalding et al. (1979) poral monitoring of the relative quantum ef®ciency of rediscovered the high internal O2-concentrations during photosystem II, fPSII.
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