Carbon isotope composition of intermediates of the starch-malate sequence and level of the crassulacean acid metabolism in leaves of Kalanchoe blossfeldiana Tom Thumb

Contribution of carbon fixed by Rubisco and PEPC to phloem export in the Crassulacean acid metabolism plant Kalanchoe daigremontiana

Crassulacean acid metabolism (CAM) plants exhibit a complex interplay between CO(2) fixation by phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), and carbon demand for CAM maintenance and growth. This study investigated the flux of carbon from PEPC and direct Rubisco fixation to different leaf carbon pools and to phloem sap over the diurnal cycle. Concentrations and carbon isotope compositions of starch, soluble sugars, and organic acids were determined in leaves and phloem exudates of Kalanchoë daigremontiana Hamet et Perr., and related to CO(2) fixation by PEPC and Rubisco. Three types of leaf carbon pools could be distinguished. (i) Starch and malate pools were dominant and showed a pattern of reciprocal mobilization and accumulation (85/54 and 13/48 mg C g(-1) DW, respective, at the beginning/end of phase I). The carbon isotope composition of these pools was compatible with predominant PEPC fixation (delta(13)C values of -13 and -11 per thousand for starch and malate compared to -11 per thousand of PEPC fixed carbon). (ii) Isotopic composition (-17 per thousand and -14 per thousand) and concentration of glucose and fructose (2 and 3 mg C g(-1) DW, respectively) were not affected by diurnal metabolism, suggesting a low turnover. (iii) Sucrose (1-3 mg C g(-1) DW), in contrast, exhibited large diurnal changes in delta(13)C values (from -17 per thousand in the evening to -12 per thousand in the morning), which were not matched by net changes in sucrose concentration. This suggests a high sucrose turnover, fed by nocturnal starch degradation and direct Rubisco fixation during the day. A detailed dissection of the carbon fixation and mobilization pattern in K. daigremontiana revealed that direct fixation of Rubisco during the light accounted for 30% of phloem sucrose, but only 15% of fixed carbon, indicating that carbon from direct Rubisco fixation was preferentially used for leaf export.

Diel leaf growth cycles in Clusia spp. are related to changes between C3 photosynthesis and crassulacean acid metabolism during development and during water stress

This study reports evidence that the timing of leaf growth responds to developmental and environmental constraints in Clusia spp. We monitored diel patterns of leaf growth in the facultative C(3)-crassulacean acid metabolism (CAM) species Clusia minor and in the supposedly obligate CAM species Clusia alata using imaging methods and followed diel patterns of CO2 exchange and acidification. Developing leaves of well-watered C. minor showed a C3-like diel pattern of gas exchange and growth, with maximum relative growth rate (RGR) in the early night period. Growth slowed when water was withheld, accompanied by nocturnal CO2 exchange and the diel acid change characteristic of CAM. Maximum leaf RGR shifted from early night to early in the day when water was withheld. In well-watered C. alata, similar changes in the diel pattern of leaf growth occurred with the development of CAM during leaf ontogeny. We hypothesize that the shift in leaf growth cycle that accompanies the switch from C3 photosynthesis to CAM is mainly caused by the primary demand of CAM for substrates for nocturnal CO2 fixation and acid synthesis, thus reducing the availability of carbohydrates for leaf growth at night. Although the shift to leaf growth early in the light is presumably associated with the availability of carbohydrates, source-sink relationships and sustained diurnal acid levels in young leaves of Clusia spp. need further evaluation in relation to growth processes.

Intracellular transport and pathways of carbon flow in plants with crassulacean acid metabolism

The massive daily reciprocal transfer of carbon between acids and carbohydrates that is unique to crassulacean acid metabolism (CAM) involves extensive and regulated transport of metabolites between chloroplasts, vacuoles, the cytosol and mitochondria. In this review of the CAM pathways of carbon flow and intracellular transport, we highlight what is known and what has been postulated. For three of the four CAM pathway variants currently known (malic enzyme- or PEP carboxykinase-type decarboxylase, and starch- or soluble sugar-type carbohydrate storage), the mechanisms of intracellular transport are still hypothetical and have yet to be demonstrated experimentally. Even in malic enzyme starch-storing species such as Kalanchoë daigremontiana Hamet et Perr. and Mesembryanthemum crystallinum L., the best-described variants of plants with the second-most common mode of photosynthetic carbon metabolism known, no tonoplast or mitochondrial transporter has been functionally described at a molecular level.

Are polyamines involved in the induction and regulation of the Crassulacean acid metabolism?

Leaves of plants with Crassulacean acid metabolism (CAM) were analyzed for variation in the content of polyamines in connection with the metabolism of malic acid in the dark and in the light, and with the induction of full-CAM activity. Under conditions (long days) resulting in extremely low CAM activity, young leaves of K. blossfeldiana have very low content in the polyamine-precursor arginine and in putrescine. The content in these two substances was increased dramatically by full-CAM induction with short days. During the course of the night/day cycle two peaks of putrescine content were observed in leaves of Kalanchoe blossfeldiana Poelln. Tom Thumb performing full-CAM operation: a large increase occurs toward the end of the day and the first half of the night, and its kinetics corresponds to the increase in the rate of malic acid synthesis; another peak, very sharp, appears during the first hours of the day, concomitant with the time of release of malic acid from the vacuole into the cytoplasm. In the case of Bryophyllum daigremontianum Berger similar variations were observed for the content in spermidine. These results support the hypothesis that polyamines could be involved in countering the tendency toward acidification of the cytoplasm at those moments of CAM operation at which the local concentration of malic acid is increased (i.e., during active synthesis in the dark and during the efflux from the vacuole in the light).

Changes in the isozymic pattern of phosphoenolpyruvate : An early step in photoperiodic control of crassulacean acid metabolism level

Two major isofunctional forms of phosphoenolpyruvate carboxylase (EC 4.1.1.31) have been separated from the leaves of Kalanchoe blossfeldiana Poelln. Tom Thumb by acrylamide gel electrophoresis and diethylaminoethyl cellulose techniques: one of the forms prevails under long-day treatment (low crassulacean acid metabolism level), the other develops under short-day treatment (high Crassulacean acid metabolism level). Molecular weights are significantly different: 175·10(3) and 186·10(3), respectively. These results indicate that two populations of phosphoenolyruvate carboxylase are present in the plant, one of which is responsible for Crassulacean acid metabolism activity under the control of photoperiod.The Crassulacean acid metabolism appears to depend on the same endogenous clock that governs other photoperiodically controlled events (e.g. flowering). The metabolic and energetic significance of this feature is discussed. It is suggested that modification in isozymic composition could be an early step in the response to photoperiodism at the metabolic level.