In situ studies on crassulacean acid metabolism in Sedum acre L. and Sedum mite Gil

Abstract

CO exchange, the diurnal variations in the levels of malic, citric and isocitric acid, and the labelling pattern after CO fixation were measured in Sedum acre and Sedum mite growing in situ. As predicted from laboratory experiments, drought changed the gas exchange pattern from a C type to a crassulacean acid metabolism (CAM) type. This shift correlated with the development of a diurnal rhythm in the malic acid content. The results of CO pulse-chase experiments suggest that in well-watered plants a CAM pattern of carbon flow already exists; hence water stress might enhance latent CAM rather than induce it. The in situ CAM performance by the Sedum species appeared to be highly susceptible to modulation by season and external factors, particularly light and temperature.CAM did not substantially contribute to total carbon gain in S. acre and S. mite. During most of their lifecycles the plants grow under conditions that favour CO uptake by the C pathway rather than by CAM. Hence, despite a capability to feature CAM, the δC values found in S. acre and S. mite are those of C plants.

C3 Photosynthesis and Crassulacean Acid Metabolism in a Kansas Rock Outcrop Succulent, Talinum calycinum Engelm. (Portulacaceae)

“…2 Although lower than many C3 plants, maximal CO2 uptake rates in T. calycinum were similar to those of other succulents (9,18,27) when compared on the same basis (for T. calycinum, 100 ,umol C02 mg ' Chl h-' is approximately equal to 150 ,mol CO2 g-' DW h-'). The mid-day depression in CO2 uptake observed under well-watered conditions and after 3 d of drought in T. calycinum was also seen in the facultative CAM succulents Sedum acre (18) and Sempervivum montanum (27). Water use efficiencies (calculated as g CO2 uptake [g H20 loss]-') under these two hydration regimes ranged from 0.005 to 0.008 (Table II), slightly lower than those of other succulents ( 12,14,24) and other xerophytic C3 plants (21 (Figs.…”

“…Reasons for the larger variability in acid fluctuations observed under identical conditions ( Fig. 2 Although lower than many C3 plants, maximal CO2 uptake rates in T. calycinum were similar to those of other succulents (9,18,27) when compared on the same basis (for T. calycinum, 100 ,umol C02 mg ' Chl h-' is approximately equal to 150 ,mol CO2 g-' DW h-'). The mid-day depression in CO2 uptake observed under well-watered conditions and after 3 d of drought in T. calycinum was also seen in the facultative CAM succulents Sedum acre (18) and Sempervivum montanum (27).…”

“…Within the past several years, it has become increasingly clear that many plants do not fit neatly into the three categories of photosynthetic carbon reduction pathways -C3, C4, and CAM (3, 5, 7, 11, 12, 16,18,20,26,27,30). This is particularly true of plants showing characteristics of the C3 and CAM pathways.…”

“…Mesembryanthemum species, water stress induces a shift from C3 to CAM photosynthesis (3,5,18,30). In others, temperature appears to be a more important factor regulating the mode of photosynthesis (27).…”

Evolution along the crassulacean acid metabolism continuum

“…Similarly, the evolutionary importance of weak CAM might be that it serves as a genetic reservoir for CAM radiations in the context of changing environmental conditions or habitat exploitation, such as epiphytism (Silvera et al 2005(Silvera et al , 2009). Finally, the term 'latent CAM' has been used to describe an intermediate form of CAM wherein organic acid concentrations remain high but constant throughout the diel cycle (Schuber and Kluge 1981). As with CAM cycling or weak CAM, latent CAM might be regarded as a step along the progression from C 3 to CAM (Lee and Griffiths 1987).…”