Crassulacean Acid Metabolism Cycle Research Articles

Nocturnal citrate accumulation and its response to environmental stress in the CAM plant Kalanchoe pinnata (Lam.) Pers.

Abstract. Kalanchoe pinnata (Lam.) Pers., a plant having crassulacean acid metabolism (CAM), was grown in high light (16–23 mol photons m−2 d−1) and in the shade (0.8–2.1 mol photons m−2 d−1), respectively. Plants were stressed in three ways, i.e. by transfer from high light to shade or vice versa just before measurements, and by withholding nitrogen and/or water. During the day‐night cycle of CAM, K. pinnata showed day‐night changes of citrate levels (Δ citrate) in addition to malate changes (Δ malate). Changes of leaf‐cell sap osmotic pressure. Δπ, were linearly correlated with these changes of organic‐acid anion levels with a relation of Δπ/(Δ citrate +Δ malate) = 1/1. The environmental stressor, i.e. limited N‐nutrition, drought and higher or lower irradiance than experienced during growth, affected the absolute and relative contributions made by Δ citrate and Δ malate to total nocturnal organic‐acid accumulation. In the high‐light‐grown plants transferred to the shade, changes of citrate levels were much less affected than changes of malate levels by the generally decreased metabolic activity and inhibition of CO2 uptake. In the shade‐grown plants, Δ citrate increased in response to stress imposed by interactive effects of the three stressors.

Regulation of Phosphoenolpyruvate Carboxylase in Crassulacean Acid Metabolism: In vitro Phosphorylation of the Enzyme

Summary The in vitro phosphorylation of PEP-carboxylase of the Crassulacean Acid Metabolism (CAM) plants Kalanchoe daigremontiana and K. blossfeldiana is reported in this paper. Phosphorylation was achieved by endogenous protein kinases. It leads to lowering of enzyme inhibition by the allosteric effector L-malate. The results support the hypothesis that reversible phosphorylation of the PEP-carboxylase protein is involved in the regulation of this enzyme during the day/night cycle of CAM.

Anatomy of Succulence and CAM in 15 Species of Senecio

The ability of some species of the genus Senecio to perform Crassulacean acid metabolism (CAM) in plants growing both under controlled conditions and outdoors and the anatomy of their photosynthesizing organs, leaves, and/or stems were investigated. The studied species differed in leaf morphology and in degrees of leaf and stem succulence. Malate accumulation differed with environmental conditions. The carbon isotope ratio δ13C indicated that, in most of the species, the C3 carboxylation pathway was operating, although malate fluctuations indicated that CAM cycling occurred. In some species, carbon isotope ratios ranged between C3 and C4 values, indicating switching of photosynthetic pathways. Only two leafless species that were apparently obligate CAM species had a δ13C typical of plants assimilating CO2 through the phosphoenolpyruvate carboxylase pathway.

Mass-spectrometric evidence for the double-carboxylation pathway of malate synthesis by Crassulacean acid metabolism plants in light

Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix (13)CO2 in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After (13)CO2 fixation in darkness, only singly labelled [(13)C]malate molecules were found. Fixation of (13)CO2 under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during (13)CO2 fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the (13)CO2 application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.

Studies on carbon flow in Crassulacean acid metabolism during the initial light period

In the Crassulacean acid metabolism (CAM) plants Kalanchoë tubiflora and Sedum morganianum a shift in the pathways occurs by which external CO2 enters the metabolism during the initial light period (phase II of the diurnal CAM cycle). At the beginning of phase II, CO2 is fixed mainly by the C4 pathway; during late phase II, however, it is fixed mainly via the C3 pathway. The C3 pathway contributes to the phosphoenolpyruvate-carboxylase-mediated CO2 fixation by the provision of three-carbon skeletons. Since the shift in the carbon-flow pathway is delayed after a CO2-free night when malic-acid accumulation in the vacuoles is prevented, it is very likely that the amount of malic acid in the vacuole is integrated in the mechanism which controls CAM during the initial light period. A light-on signal at the beginning of phase II is not required to bring about the shifts in the carbon-flow pathways, as is shown by the reaction of plants to a prolonged dark period. A model of carbon flow during phase II is proposed.

Diurnal changes in the regulatory properties of PEP‐carboxylase in Crassulacean Acid Metabolism (CAM)

Abstract. The CAM plants Kalanchoe tubiflora and K. blossfeldiana were grown under photoperiodically controlled conditions (short days). In these plants, phos‐phoenolpyruvate carboxylase capacity and the sensitivity of the enzyme to the effectors L‐malate (inhibitor) and glucose‐6‐phosphate (activator) were measured throughout the diurnal CAM cycle. In K. tubiflora, enzyme capacity was higher if measured at pH 7.0 than at pH 8.0 and displayed a rhythmical behavior with highest values at the end of the light period. As reported earlier, in K. blossfeldiana PEP‐C capacity was higher during the night. It was more pronounced when plants were kept in CO2‐free air during the dark period. In both plants, the sensitivity of the enzyme to the effectors showed very clear diurnal changes: inhibition by malate and activation by glucose‐6‐phosphate were strikingly higher during the day than during the night; the effect depended on PEP concentration. The changing activation of the enzyme by glucose‐6‐phos‐phate reflects diurnal changes of the Km for PEP which was found to be higher during the day than during the night. Manipulations of malate accumulation by nocturnal application of CO2‐free air did not influence these effects. The results are discussed in context with the metabolic control of CAM.

Crassulacean acid metabolism (CAM) in Kalancho�: Changes in intercellular CO2 concentration during a normal CAM cycle and during cycles in continuous light or darkness

In the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana, the internal CO2 concentrations were measured throughout CAM cycles by gas chromatography. Under normal dark-light cycles, the internal CO2 concentration was near that of the ambient air and increased up to 0.5% during the phase of maximum malate decarboxylation. A sharp increase in internal CO2 concentration occurring after the first 12 h of the cycle was exhibited by the plants both when there was a normal day-night cycle and when the night was replaced by illumination, and also when the light period was replaced by darkness. Thus, the increase in internal CO2 in the morning does not appear to be primarily determined by a light-on signal or by alterations of temperature rather than by inherent factors of the leaves. This view is supported further by a steep increase in (14)CO2 production from labeted malate occurring during extended darkness at a time when the light period would normally begin. The results are discussed in particular in relation to of how CAM can control stomata movement.

Metabolic Control of Crassulacean Acid Metabolism: Evidence for Diurnally Changing Sensitivity Against Inhibition by Malate of PEP-Carboxylase in Kalanchoe tubiflora HAMET

Summary Properties of PEP-carboxylase were followed together with other physiological features of Crassulacean Acid Metabolism (CAM) along the diurnal CAM cycle. There was a significant diurnal change in the susceptibility of PEP-carboxylase against inhibition by L(-)malate. Inhibition was lowest during the night when the phyllodia showed net dark CO, fixation and accumulated malic acid in vacuoles, and inhibition was highest during the day when malate was released from vacuoles, i. e. when the phyllodia deacidified. This changing sensitivity of PEP-carboxylase against inhibition by malate seems to be a property of the native enzyme rather than to be caused by the extraction procedure.

Crassulacean acid metabolism (CAM) in leaves of Aloe arborescens mill

In the succulent leaves of Aloe arborescens Mill diurnal oscillations of the malic acid content, being indicative of Crassulacean Acid Metabolism (CAM), were exhibited only by the green mesophyll. In contrast, the malic acid level of the central chloroplast-free water-storing tissue remained constant throughout the day-night cycle. Apart from malate, the green tissue contained high amounts of isocitrat which was lacking in the water tissue. There was no significant transfer from the green mesophyll to the water tissue of (14)C fixed originally via dark (14)CO2 fixation in the mesophyll. Both isolated mesophyll and water tissue were capable of dark CO2 fixation yielding mainly malate as the first stable product. Both tissues have phosphoenolpyruvate carboxylase. However, the enzymes derived from the both sources could be distinguished by their molecular weights and by their kinetic properties, suggesting different phosphoenolpyruvate carboxylase proteins. The conclusion drawn from the experiments is that in a. arborescens the CAM cycle proceeds exclusively in the green mesophyll and that the water tissue, though capable of malate synthesis via β-carboxylation of phosphoenolpyruvate, behaves as an independent metabolic system where CAM is lacking. This view is supported by the finding that the cell walls bordering the green mesophyll from the water tissue lack plasmodesmata, hence conveniant pathways of metabolite transport.