The contribution of crassulacean acid metabolism to the annual productivity of two aquatic vascular plants

Seasonal variation in crassulacean acid metabolism by the aquatic isoetid Littorella uniflora

The seasonal temperature acclimation in crassulacean acid metabolism (CAM) and photosynthetic performance were investigated in the aquatic isoetid, Littorella uniflora. Plants were collected monthly from January to September, and CAM capacity and photosynthesis rates were measured at 5, 10, 15, and 20 °C. Seasonal acclimation was observed for CAM (Q(10) range: 0.6-1.8), and CAM was optimised close to ambient temperature throughout the season. Thus, in winter acclimated L. uniflora, the short-term response to raised temperature resulted in a decline in CAM capacity. Even though the ambient CAM increased from winter to spring/summer, CAM was present in cold acclimated plants, thus indicating an ecophysiological role for CAM even in winter. Similar to CAM, seasonal acclimation was observed in the light and carbon-saturated photosynthesis (Q(10) values ranged from 1.4 to 2.3), and the photosynthetic capacity was generally higher during the winter at all temperatures, indicating compensatory investments in the photosynthetic apparatus. Thus, L. uniflora displayed seasonal temperature acclimation with respect to both CAM and photosynthesis. The estimated in situ contribution of CAM to the carbon budget in L. uniflora was independent of season and varied from 23 to 46 %. A positive correlation between photosynthetic capacity and CAM capacity (both measured in the lab at temperature close to ambient temperature) was found, and the ratio of CAM activity to photosynthetic capacity was higher in summer compared with winter plants. Overall, the results from the present study support the suggested role of CAM as a carbon conserving mechanism of importance for survival in a carbon-limited habitat.

Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a review

Inorganic carbon can be in short supply in freshwater relative to that needed by freshwater plants for photosynthesis because of a large external transport limitation coupled with frequent depleted concentrations of CO(2) and elevated concentrations of O(2). Freshwater plants have evolved a host of avoidance, exploitation and amelioration strategies to cope with the low and variable supply of inorganic carbon in water. Avoidance strategies rely on the spatial variation in CO(2) concentrations within and among lakes. Exploitation strategies involve anatomical and morphological features that take advantage of sources of CO(2) outside of the water column such as the atmosphere or sediment. Amelioration strategies involve carbon-concentrating mechanisms based on uptake of bicarbonate, which is widespread, C(4)-fixation, which is infrequent, and crassulacean acid metabolism (CAM), which is of intermediate frequency. CAM enables aquatic plants to take up inorganic carbon in the night. Furthermore, daytime inorganic carbon uptake is generally not inhibited and therefore CAM is considered to be a carbon-conserving mechanism. CAM in aquatic plants is a plastic mechanism regulated by environmental variables and is generally downregulated when inorganic carbon does not limit photosynthesis. CAM is regulated in the long term (acclimation during growth), but is also affected by environmental conditions in the short term (response on a daily basis). In aquatic plants, CAM appears to be an ecologically important mechanism for increasing inorganic carbon uptake, because the in situ contribution from CAM to the C-budget generally is high (18-55%).

The role of CO2 uptake by roots and CAM in acquisition of inorganic C by plants of the isoetid life-form: a review, with new data on Eriocaulon decangulare L

The isoetid life-form was originally defined on morphological grounds; subsequent physiological investigations showed that all of the isoetids examined took up a large fraction of the inorganic C fixed in their leaves from the root medium under natural conditions, and that some of them carried out much of their assimilation of inorganic C via a CAM-like mechanism. Root-dominated uptake of inorganic C appeared to be unique to, and ubiquitous in, the isoetids. I However, a large capacity for CAM-like metabolism in submerged vascular plants is not universal in isoetids, nor is it restricted to this life-form, being also found in Crassulaa aquatica. The work described here shows that submerged specimens of the North American Eriocaulon decangulare have a high fraction of their dry weight in the root system, a trait characteristic of isoetids but uncommon in other submerged vascular plants. E. decangulare has vesicular-arbuscular mycorrhizas, as do other flowering plant isoetids hut not, generally, submerged Isoetes spp. Under conditions of natural supply of inorganic C, E. decangulare, like other isoetids, takes up most of its inorganic C through its roots. Uptake of inorganic C by both roots and shoots involves CO₂ rather than HCO₃ : photosynthesis at high external pH values does not exceed the rate of uncatalysed HCO₃ ^- to CO₂ conversion in the medium and there is no detectable extracellular carbonic anhydrase activity. Measurements of titratable acidity and of malate content of leaves sampled at dawn and at dusk showed that E. decangulare, growing and tested under either emersed or submersed conditions, did not exhibit CAM-like behaviour. CAM was also absent from three non-isoetid aquatic macrophytes (Amphibolic antarctica, Eeklonia radiata and Vallisneria spiralis) which were examined. E. decangulare thus resembles all other isoetids tested in acquiring much of its inorganic C via the root system. E. decangulare also resembles most of the isoetids which are not members of the Isoetaceae (e.g.) E. septangulare, Lobelia dortmanna and Subularia aquatica) but differs from submerged Isoetaceae and Littorella uniflora in lacking CAM. The ecological significance of uptake of CO₂ via the roots and, where it occurs, of CAM in isoetids may be related to either inorganic C or, via improved N use efficiency, inorganic C as a limiting resource. The isoetid life-forms has evolved independently in at last five different families of vascular plants; it probably derived fairly immediately from terrestrial or amphibious ancestors with a similar rosette form. Emergent Isoetaceae with acquisition of CO₂ via roots and CAM probably evolved from submerged isoetids. CONTENTS Summary 123 I. Introduction 126 II. Material and Methods 127 III. Results and Discussion 129 IV. Conclusions 142 Acknowledgements 142 References 143.

CARBON DIOXIDE AND WATER DEMAND: CRASSULACEAN ACID METABOLISM (CAM), A VERSATILE ECOLOGICAL ADAPTATION EXEMPLIFYING THE NEED FOR INTEGRATION IN ECOPHYSIOLOGICAL WORK

Plants having crassulacean acid metabolism (CAM) tend to occupy habitats where the prevailing environmental stress is scarcity of water. These are semi-arid or arid regions, salinas or epiphytic sites. CAM plants manage the dilemma of desiccation or starvation by nocturnal malic acid accumulation in the vacuoles. Malic acid serves as a form of CO₂ storage and as an osmoticum. In this way malic acid accumulation allows, firstly, separation of uptake and assimilation of atmospheric CO₂ with water-saving daytime stomatal closure and, secondly, osmotic acquisition of water. There is no very special trait which is specific for CAM. An array of biophysical and biochemical functional elements, which are also found in other plants, is integrated in CAM performance. This leads to a large diversity of behaviour which makes CAM plants highly versatile in their response to environmental variables. Besides CO₂ dark fixation, transport of malic acid across the tonoplast is one of the key elements in CAM function. This is examined in detail at the level of membrane biophysics and biochemistry. The versatility of CAM is illustrated by examples from field work, with comparisons involving different species, seasons, modes of photosynthesis (CAM vs C₃ ), kinds of stress and ways of stress imposition. Contents Summary 593 I. Studies of CAM: an example for the ecophysiological approach 594 II. Malic acid transport at the tonoplast 602 III. Regulation 605 IV. Desiccation or starvation 610 V. Comparative autecology 614 VI. Ecology: promise of integration 621 Acknowledgements 622 References 622.