Abstract
Portulacaria afra L. is a dominant facultative CAM species growing in the Southeastern Cape of South Africa. P. afra is well adapted to regions of the Spekboom thicket in areas of limited and sporadic rainfall. P. afra populations occur in isolated drainages. We hypothesized the utilization of CAM would vary in the different populations in response to rainfall and temperature gradients. Carbon isotope composition can be used to determine the contribution of CAM in leaf tissue. P. afra leaves of populations were analyzed in transects running south to north and east to west in locations from the coast to elevations of 1400 m. Carbon isotope values ranged from −16.1‰ in Plutosvale to −21.0‰ to −22.7‰ in Port Alfred and Grahamstown populations respectively with some values reaching −25.2‰. These values indicated an estimated variable contribution of the CAM pathway ranging from 23% to almost 60%. The results indicate a much greater range of variability than previously reported. The carbon isotope values showed no direct correlation with rainfall or maximum or minimum day/night temperatures in the summer or winter for the different locations. The results indicated the microclimate may play a more significant role in determining CAM utilization. We present evidence that CAM is a continuous trait in P. afra and CAM is operating continuously at low levels during C3 photosynthesis which may explain the high variability in its carbon isotope composition. P. afra populations illustrate a large phenotypic plasticity and further studies may indicate genotypic differences between populations. This may be valuable in ascertaining the genetic contribution to its water use efficiency and possible use in engineering higher water use efficiency in C3 plants. The results revealed here may explain P. afra’s ability to sequester carbon at high rates compared to more mesic species.
Highlights
Crassulacean acid metabolism (CAM) is a metabolic and anatomical adaptation that is characterized by net nocturnal carbon dioxide uptake with a temporal separation of the C4 and C3 pathway (Osmond, 1978; Ting, 1985)
The results reported for populations of P. afra are similar to the range of values detected in three species of Clusia from Panama (Holtum et al, 2004)
We provide evidence which indicated a much greater range of variability in the contribution of CAM than has been previously identified in areas where P. afra is found
Summary
Crassulacean acid metabolism (CAM) is a metabolic and anatomical adaptation that is characterized by net nocturnal carbon dioxide uptake with a temporal separation of the C4 and C3 pathway (Osmond, 1978; Ting, 1985). The CO2 is fixed by Phosphoenolpyruvate carboxylase (PEPCase), converted to malate and stored as malic acid in the vacuole during the night. In the subsequent light period, the malate is decarboxylated to release CO2 for utilization by Rubisco in the C3 cycle. CAM plants typically have a leaf mesophyll anatomy with primarily spongy parenchyma cells with a large central vacuole (Gibson, 1982). The uptake of CO2 at night with the stomata closed during the day results in an increased water use efficiency when compared to C3 and C4 plants (Black, 1973). CAM has evolved in at least 35 different plant families including six aquatic families and over 343 genera (Borland et al, 2011)
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