Crassulacean acid metabolism as a continuous trait: variability in the contribution of Crassulacean acid metabolism (CAM) in populations of Portulacaria afra

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\u27s ability to sequester carbon at high rates compared to more mesic species

Crassulacean acid metabolism in the Gesneriaceae

The occurrence of the Crassulacean acid metabolism (CAM) was studied in four epiphytic species of the Gesneriaceae: two neotropical species, Codonanthe crassifolia and Columnea linearis, and two paleotropical species, Aoschynanthus pulcher and Saintpaulia ionantha. Gas exchange parameters, enzymology, and leaf anatomy, including mesophyll succulence and rel­ ative percent of the mesophyll volume occupied by airspace, were studied for each species. Codonanthe crassifolia was the only species to show nocturnal CO2 uptake and a diurnal organic acid fluctuation. According to these results, Codonanthe crassifolia shows CAM-cycling under well-watered conditions and when subjected to drought, it switches to CAM-idling. Other characteristics, such as leaf anatomy, mesophyll succulence, and PEP carboxylase and NADP malic enzyme activity, indicate attributes of the CAM pathway. All other species tested showed C3 photosynthesis. The most C3-like species is Columnea linearis, according to the criteria tested in this investigation. The other two species show mesophyll succulence and relative percent of the leaf volume occupied by airspace within the CAM range, but no other characters of the CAM pathway. The leaf structure of certain genera of the Gesneriaceae and of the genus Peperomia in the Piperaceae are similar, both having an upper succulent, multiple epidermis, a medium palisade of one or a few cell layers, and a lower, succulent spongy parenchyma not too unlike CAM photosynthetic tissue. We report ecophysiological similarities between these two distantly related families. Thus, the occurrence of CAM-cycling may be more common among epiphytic species than is currently known

Evolutionary Physiology: The extent of C4 and CAM photosynthesis in the Genera Anacampseros and Grahamia of the Portulacaceae

The Portulacaceae is one of the few terrestrial plant families known to have both C(4) and Crassulacean acid metabolism (CAM) species. There may be multiple origins of the evolution of CAM within the Portulacaceae but the only clear evidence of C(4) photosynthesis is found in members of the genus Portulaca. In the Portulaca, CAM succulent tissue is overlaid with the C(4) tissue in a unique fashion where both pathways are operating simultaneously. Earlier reports have shown that the clade containing the genera Anacampseros and Grahamia may also contain C(4) photosynthetic species similar to the Portulaca, which would indicate multiple origins of C(4) photosynthesis within the family. The aim of the present study was to ascertain the true photosynthetic nature of these genera. An initial survey of the carbon isotope composition of the Anacampseros ranged from -12.6 per thousand to -24.0 per thousand, indicating very little CAM activity in some species, with other values close to the C(4) range. Anacampseros (=Grahamia) australiana which had been previously identified as a C(4) species had a carbon isotope composition value of -24.0 per thousand, which is more indicative of a C(3) species with a slight contribution of CAM activity. Other Anacampseros species with C(4)-like values have been shown to be CAM plants. The initial isotope analysis of the Grahamia species gave values in the range of -27.1 per thousand to -23.6 per thousand, placing the Grahamia species well towards the C(3) photosynthetic range. Further physiological studies indicated increased night-time CO(2) uptake with imposition of water stress, associated with a large diurnal acid fluctuation and a marked increased phosphoenolpyruvate carboxylase activity. This showed that the Grahamia species are actually facultative CAM plants despite their C(3)-like carbon isotope values. The results indicate that the Grahamia and Anacampseros species do not utilize the C(4) photosynthetic pathway. This is the first to identify that the Grahamia species are facultative CAM plants where CAM can be induced by water stress. This work supports earlier physiological work that indicates that this clade containing Anacampseros and Grahamia species comprises predominantly facultative CAM plants. This report suggests there may be only one clade which contains C(4) photosynthetic members with CAM-like characteristics

Induction of PEP Carboxylase and Crassulacean Acid Metabolism by Gibberellic Acid in Mesembryanthemum crystallinum

The induction of Crassulacean acid metabolism in Mesembryanthemum crystallinum was investigated in response to foliar application of gibberellic acid (GA). After 5 weeks of treatment, GA-treated plants showed 1.7- to almost a 4-fold increase of phosphoenolpyruvate carboxylase (PEPcase) activity with a concomitant increase in acid metabolism when compared to control plants. Immunoblot analysis indicated an increase in the PEPcase protein similar to that of salt treatment while Rubisco did not show a similar rise. The results indicate that exogenously applied GA accelerates plant developmental expression of PEPcase and Crassulacean acid metabolism in M. crystallinum

Physiological Changes in Portulacaria afra (L.) Jacq. during a Summer Drought and Rewatering

The changes of titratable acidity, enzyme activity, water status, and pigment composition were studied in Portulacaria afra (L.) Jacq. during a normal summer drought and rewatering. Two groups of plants were grown outside under a clear plastic canopy with water stress initiated at 2-week intervals in May 1986. Drought resulted in a linear decrease of fresh weight for 80 days and there was no further fresh weight change for the next 65 days. Nocturnal CO(2) uptake remained measurable for 83 days. Cessation of exogenous CO(2) uptake corresponded to the point where the pressure potential (Ψ(p)) became zero. Ribulose-1, 5-bisphosphate (RuBP) and phosphoenolpyruvate carboxylase were reduced to 50% of this activity by the end of the drought period. Phosphoenolpyruvate carboxykinase activity was undetectable after 120 to 140 days of drought. Chlorophyll (Chl) levels decreased with a preferential loss of Chl a over Chl b. Carotenoid content was relatively constant over the course of the drought period. After 145 days of drought, plants responded to rewatering within 24 hours; Ψ(p) became positive and daytime CO(2) uptake resumed after 24 hours. After 3 days, RuBP carboxylase activity reached control levels. Activity of the CAM pathway recovered after 5 days, as noted by increased diurnal acid fluctuations. Phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase activity fully recovered within 6 days. Chl levels were greater than control levels within 5 days. Chl a/b ratios took 27 days to return to control levels. The results indicated that P. afra can withstand a normal summer drought by utilizing the CAM and CAM-idling pathway for 130 to 140 days. The plants respond rapidly to rewatering because of the conservation of enzyme activity and the quick recovery of Ψ(p)

Seasonal Patterns of water relations and enzyme activity of the facultative CAM plant Portulacaria afra (L.) Jacq

Portulacaria afra (L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C3 to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from -0.1 to -0.5 MPa, predawn and midday, when the C3 pathway of photosynthesis was utilized. Osmotic potentials were constant, from -0.7 to - 0.8 MPa, indicating very little change in turgor. P. afra shifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m−2) despite C3-like predawn water potentials. Midday water potentials usually decreased 0.2-0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three-to five-fold over C3 levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other inducible CAM species