Comparison of CAM expression,photochemistry and antioxidant responses in Sedum album and Portulaca oleracea under combined stress

Previous studies of crassulacean acid metabolism (CAM) pathway during stress have been directed at individual drought and salinity stress, here, we studied the effects of a combination of drought and salt on CAM expression, chlorophyll fluorescence and antioxidant parameters in the C₃-CAM facultative Sedum album and C₄-CAM facultative Portulaca oleracea plants. While salinity alone was not able to induce functional CAM expression in P. oleracea leaves, we showed that salinity induced low level of nocturnal acid accumulation in S. album species. After 20 d of exposure to the combination of simultaneous salt and drought stress, P. oleracea plants exhibited more resistance to photoinhibition as compared to S. album plants. The decrease of maximum quantum yield (F_v/F_m) in S. album leaves under combined stress was in parallel with the largest suppression of CAM expression of >50%, probably displaying the withdrawal of functional CAM back to C₃ pathway. However, under drought treatment alone, S. album plants exhibited higher photosynthetic flexibility, which was associated with the up-regulation of antioxidant enzymes activities and maintenance of glutathione (GSH) pool, and consequently higher photochemical functioning. The levels of nitric oxide (NO) correlated well with CAM expression, which was observed only in S. album, suggesting that NO acts in a different way in C₃ and C₄ species during CAM induction. Additionally, in both species, over the course of CAM induction, the changes in CAM expression parameters exhibited a similar pattern to that of antioxidant capacity and photochemical functioning parameters.     

Antioxidative protection in the inducible CAM plant Sedum album L. following the imposition of severe water stress and recovery

The antioxidative protection during the C₃-CAM shift induced by water stress was investigated in the temperate succulent Sedum album L. The C₃-CAM shift was characterized in terms of CO₂ exchange, titratable acidity and phosphoenolpyruvate carboxylase activity. Well-watered plants displayed C₃-like patterns of gas exchange and exhibited a mild day-night acid fluctuation indicating that those plants were performing CAM-cycling metabolism. Imposed drought highly stimulated CAM cycling, decreasing the net CO₂ uptake during the day, eliminating net CO₂ efflux at night and stimulating tissue acid fluctuations. As water deficit developed, chlorophyll fluorescence measurements showed a decrease in the Fv/Fm ratio, indicating that photoinhibition could follow after severe drought. Protection might be performed by the increased activity of enzymes involved in the destruction of free radicals and oxidants, but their response depended on the water status of the plant. Ascorbate peroxidase and superoxide dismutase activities increased in plants subjected to mild stress but declined during severe water stress. Catalase activity, however, was quite stable under mild water stress and was clearly inhibited under severe water stress. At this stage, glutathione reductase and monodehydroascorbate reductase seemed to be very important in the protection against oxidants, both increasing considerably their activities under severe water stress. Even if recycling has been shown to alleviate photoinhibition, our results clearly demonstrate that antioxidative enzymes play an important role in the protection of plants from oxidants during the C₃-CAM shift induced by water stress.     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

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

Summary CO₂ exchange, the diurnal variations in the levels of malic, citric and isocitric acid, and the labelling pattern after ¹⁴CO₂ 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 C₃ 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 ¹⁴CO₂ 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 CO₂ uptake by the C₃ pathway rather than by CAM. Hence, despite a capability to feature CAM, the ¹³C values found in S. acre and S. mite are those of C₃ plants.Abbreviations CAM Crassulacean Acid Metabolism - PEP-C Phosphoenolpyruvate-Carboxylase - DW Dry weight Dedicated to Prof. Dr. Dr. h.c. M. Evenari on the occasion of his 75th birthday and to Dr. K.F. Springer     

Comparative ecophysiology of five species of Sedum (Crassulaceae) under well-watered and drought-stressed conditions

Summary Gas exchange patterns, diurnal malic acid fluctuations, and stable carbon isotope ratios of five species of Sedum were investigated to assess the ecophysiological characteristics of three different photosynthetic pathways under well-watered and drought-stressed conditions. All five species have succulent leaves and stems and were examined under identical environmental conditions. When well-watered, Sedum integrifolium (Raf.) Nels. and S. ternatum Michx. displayed C₃ photosynthesis, S. telephioides Michx. and S. nuttallianum Raf. exhibited CAM-cycling, and S. wrightii A. Gray showed CAM. When grown under a less frequent watering regime, S. integrifolium and S. ternatum exhibited CAM-cycling, whereas S. telephioides and S. nuttallianum displayed CAM-cycling simultaneously with low-level CAM. Sedum wrightii retained its CAM mode of photosynthesis. In general, leaf ¹³C values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C₃ and C₄ species, no correlation of malic acid accumulation in the CAM and CAM-cycling (including low-level CAM) species with increased WUE was found. Sedum wrightii (CAM) had the highest WUE value at night, yet its 24-h WUE was not different from S. ternatum when the latter was in the C₃ mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.     

Inducibility of crassulacean acid metabolism (CAM) in Clusia species; physiological/biochemical characterisation and intercellular localization of carboxylation and decarboxylation processes in three species which exhibit different degrees of CAM

The biochemical basis for photosynthetic plasticity in tropical trees of the genus Clusia was investigated in three species that were from contrasting habitats and showed marked differences in their capacity for crassulacean acid metabolism (CAM). Physiological, anatomical and biochemical measurements were used to relate changes in the activities/amounts of key enzymes of C₃ and C₄ carboxylation to physiological performance under severe drought stress. On the basis of gas-exchange measurements and day/night patterns of organic acid turnover, the species were categorised as weak CAM-inducible (C.aripoensis Britt.), C₃-CAM intermediate (C. minor L.) and constitutive CAM (C.␣rosea Jacq. 9.). The categories reflect genotypic differences in physiological response to drought stress in terms of net carbon gain; in C. aripoensis net carbon gain was reduced by over 80% in drought-stressed plants whilst carbon gain was relatively unaffected after 10 d without water in C. rosea. In turn, genotypic differences in the capacity for CAM appeared to be directly related to the capacities/amounts of phosphoenolpyruvate carboxylase (PEPCase) and phosphoenolpyruvate carboxykinase (PEPCK) which increased in response to drought in both young and mature leaves. Whilst measured activities of PEPCase and PEPCK in well-watered plants of the C₃-CAM intermediate C. minor were 5–10 times in excess of that required to support the magnitude of organic acid turnover induced by drought, close correlations were observed between malate accumulation/PEPCase capacity and citrate decarboxylation/PEPCK capacity in all the species. Drought stress did not affect the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein in any of the species but Rubisco activity was reduced by 35% in the weak CAM-inducible C. aripoensis. Similar amounts of glycine decarboxylase (GDC) protein were present in all three species regardless of the magnitude of CAM expression. Thus, the constitutive CAM species C. rosea did not appear to show reduced activity of this key enzyme of the photorespiratory pathway, which, in turn, may be related to the low internal conductance to CO₂ in this succulent species. Immuno-histochemical techniques showed that PEPCase, PEPCK and Rubisco were present in cells of the palisade and spongy parenchyma in leaves of species performing CAM. However, in leaves from well-watered plants of C. aripoensis which only performed C₃ photosynthesis, PEPCK was localized around latex-producing ducts. Differences in leaf anatomy between the species suggest that the association between mesophyll succulence and the capacity for CAM in these hemi-epiphytic stranglers has been selected for in arid environments. Received: 4 July 1997 / Accepted: 27 November 1997     

Epidermal structures and composition of epicuticular waxes of Sedum album sensu lato (Crassulaceae) in Balkan Peninsula

Sedum album is a succulent plant, with phenotypic variability resulting in delimitation of a number of taxa with diverse taxonomic recognition. We analyzed the taxa from Sedum album in broad sense (S. album, S. micranthum, S. athoum and S. serpentini) from Balkan Peninsula, to check whether the variability of their epidermal structures and epicuticular wax composition corresponds to their taxonomic reliability. Epidermal structures and epicuticular waxes were analyzed using light and scanning electron microscopy, gas chromatography-mass spectrometry and gas chromatography-flame ionization detector chromatography and multivariate statistical techniques. Analyses of studied taxa have shown that significant systematic characters include adaxial epidermal and guard cell length, stomatal frequency and subsidiary cells area on abaxial epidermis. The content of n-alkanes C₂₉, C₃₀, C₂₇, C₃₂ and C₃₃ was shown to be valuable criterion for their phytochemical delimitation. The comparison of epicuticular wax microstructures has shown no significant differences. Results indicate presence of two (micromorphology) or three (phytochemistry) well-defined groups of populations, with S. serpentini manifesting a considerable level of separation toward the others. Sedum athoum have shown weak to moderate degree of differentiation, while S. micranthum has shown general resemblance to S. album.     

Crassulacean Acid Metabolism in the Succulent C(4) Dicot, Portulaca oleracea L Under Natural Environmental Conditions

Crassulacean acid metabolism (CAM) was examined under natural environmental conditions in the succulent C₄ dicot Portulaca oleracea L. Two groups of plants were monitored; one was watered daily (well watered), while the other received water once every 3 to 4 weeks to produce a ψ of −8 bars (drought stressed). Gas exchange, transpiration rate, and titratable acidity were measured for 24-hour periods during the growing season. CAM activity was greatest in drought-stressed plants during late August which had 13 hour days and day/night temperatures of 35/15°C. Under these conditions net CO₂ uptake occurred slowly throughout the night. Diurnal fluctuations of titratable acidity took place in both leaves and stems with amplitudes of 17 and 47 microequivalents per gram fresh weight, respectively. Transpiration data indicated greater opening of stomata during the night than the day. CAM was less pronounced in drought-stressed P. oleracea plants in July and September; neither dark CO₂ uptake nor positive carbon balance occurred during the July measurements. In contrast, well-watered plants appeared to rely on C₄ photosynthesis throughout the season, although some acid fluctuations occurred in stems of these plants during September.     

Daily malate fluctuations in Sedum dasyphyllum L.

Abstract

Sedum dasyphyllum L. leaves of well-watered plants kept outdoors and under controlled conditions show diurnal malic acid fluctuations. In well-watered plants growing outdoors the malate accumulation undergoes seasonal variations and seems to be inhibited by short photoperiod and/or by low temperature. The seasonal variations of CAM activity correspond to seasonal variations of mesophyll succulence. Water stress markedly depressed CAM activity. In fact, plants of S. dasyphyllum show, under controlled conditions, a decrease of malate accumulation as relative water content decreases. Recovery from water stress is fairly slow. Water potential quickly increases during rewatering and exceeds the original value after few days, suggesting a consumption of osmotic compounds during the water stress period.     

德国景天扦插苗对干旱胁迫的生理响应

以耐旱性较强的德国景天(Sedum hybridum)扦插苗为材料进行干旱胁迫处理(60d),并测定其叶片相对含水量、丙二醛含量、细胞伤害率、光合参数(净光合速率、气孔导度、蒸腾速率)、荧光参数(F_v/F_m、q~P、PHiPS2),以及苹果酸含量、磷酸烯醇式丙酮酸羧化酶(PEPCase)活性等生理指标。结果显示,干旱胁迫导致德国景天叶片相对含水量、光合参数、荧光参数降低,细胞伤害率、丙二醛含量升高;干旱胁迫30d时,各项生理指标变化幅度较小,但40d后变化幅度显著增加,并且德国景天叶片的苹果酸含量和PEPCase活性显著升高,表明C_3光合途径不断减弱,景天酸代谢(CAM)途径被激活,参与了德国景天对干旱胁迫的响应,提高了其耐旱性;当干旱胁迫达60d时,因超过其耐受范围,德国景天受损严重,逐渐死亡。这说明德国景天在生理上表现出极强的耐旱性,并且具有兼性CAM植物的特征,通过CAM途径活化提高植株耐旱性是德国景天重要的耐旱机理。     

Recycling of CO2 via Crassulacean acid metabolism in the rock outcrop succulent Sedum pulchellum Michx. (Crassulaceae)

Under well-watered conditions in the laboratory, Sedum pulchellum assimilated CO₂ only during the day, yet exhibited small nocturnal increases in tissue acid content followed by deacidification in the light (CAM-cycling). When drought-stressed, little CO₂ was fixed in the day and none at night, yet even greater acid fluctuations were observed (CAM-idling). Calculations indicate that water savings associated with CAM-cycling when water is available are small. Water saving is more likely to be significant during CAM-idling when water supply is limited and stomata are closed day and night. Thus, in this species, CAM-idling may be of greater benefit to the plant, relative to CAM-cycling, in surviving habitats prone to frequent drought stress.Abbreviations A CO₂ exchange rate - CAM Crassulacean acid metabolism - c_i shoot internal CO₂ concentration - g_c shoot conductance to CO₂ - PPFD photosynthetic photon flux density - WUE water-use efficiency Supported by National Science Foundation Grant No. DMB 8506093.     

Is Sedum acre L. a CAM plant?

Summary Sedum acre L. collected from its natural stands south of Darmstadt (Germany) showed ¹³C values typical for C₃ plants. This suggests that in situ at the natural stand CO₂ was fixed mainly via the C₃ mode of photosynthesis rather than via the CAM mode. However, experimental water stress shifts the CO₂ exchange pattern from the C₃ type to CAM type. Simultaneously, a diurnal rhythm of malic acid oscillation, typical for CAM, and increase of PEP-carboxylase and malic enzyme activities developed. Hence, Sedum acre is obviously to be classified as a facultative CAM plant. Because of the temperature characteristics of CO₂ exchange in Sedum acre, in situ CO₂ should be harvested from the atmosphere mainly during the seasons where water stress situations capable of inducing CAM are unlikely to occur.     

Juvenile tank-bromeliads lacking tanks: do they engage in CAM photosynthesis?

In the epiphytic tillandsioids, Guzmania monostachia, Werauhia sanguinolenta, and Guzmania lingulata (Bromeliaceae), juvenile plants exhibit an atmospheric habit, whereas in adult plants the leaf bases overlap and form water-holding tanks. CO₂ gas-exchange measurements of the whole, intact plants and δ¹³C values of mature leaves demonstrated that C₃ photosynthesis was the principal pathway of CO₂ assimilation in juveniles and adults of all three species. Nonetheless, irrespective of plant size, all three species were able to display features of facultative CAM when exposed to drought stress. The capacity for CAM was the greatest in G. monostachia, allowing drought-stressed juvenile and adult plants to exhibit net CO₂ uptake at night. CAM expression was markedly lower in W. sanguinolenta, and minimal in G. lingulata. In both species, low-level CAM merely sufficed to reduce nocturnal respiratory net loss of CO₂. δ¹³C values were generally less negative in juveniles than in adult plants, probably indicating increased diffusional limitation of CO₂ uptake in juveniles.     

Superoxide dismutase activity in C<Subscript>3</Subscript> and C<Subscript>3</Subscript>/CAM intermediate species of <Emphasis Type="Italic">Clusia</Emphasis>

The C₃-CAM intermediate Clusia minor L. and the C₃ obligate Clusia multiflora H.B.K. plants were exposed for 7 d to a combination of drought stress and high irradiance of about 1200 μmol m⁻² s⁻¹ for 12 h per day. In both species under these conditions a strong decrease in stomatal conductance was observed at dawn and dusk. Changes in stomatal behaviour of C. minor were accompanied by only a low nocturnal accumulation of malate and citrate. Thus, in C. minor drought stress applied in combination with high irradiance limited CAM expression, and possibly this is the main reason why C. minor prefers semi-shaded sites in the field. The mitochondrial MnSOD, in both well watered and stressed plants of two species showed strong diurnal oscillations with maximum activity at dusk. These oscillations can be explained by the engagement of mitochondria in dissipation of an excess of reducing equivalents. In plants which are able to carry out CAM metabolism tricarboxylic acid cycle is expected to be down regulated in the dark period to prevent breakdown of the entire malate and citrate.     

Responses of succulents to plant water stress

Experiments were performed to test the hypothesis that succulents “shift” their method of photosynthetic metabolism in response to environmental change. Our data showed that there were at least three different responses of succulents to plant water status. When plant water status of Portulacaria afra (L.) Jacq. was lowered either by withholding water or by irrigating with 2% NaCl, a change from C₃-photosynthesis to Crassulacean acid metabolism (CAM) occurred. Fluctuation of titratable acidity and nocturnal CO₂ uptake was induced in the stressed plants. Stressed Peperomia obtusifolia A. Dietr. plants showed a change from C₃-photosynthesis to internal cycling of CO₂. Acid fluctuation commenced in response to stress but exogenous CO₂ uptake did not occur. Zygocactus truncatus Haworth plants showed a pattern of acid fluctuation and nocturnal CO₂ uptake typical of CAM even when well irrigated. The cacti converted from CAM to an internal CO₂ cycle similar to Peperomia when plants were water-stressed. Reverse phase gas exchange in succulents results in low water loss to carbon gain. Water is conserved and low levels of metabolic activity are maintained during drought periods by complete stomatal closure and continual fluctuation of organic acids.     

Properties of Phosphoenolpyruvate Carboxylase and Carbohydrate Accumulation in the C3-CAM Intermediate Sedum telephium L. Grown Under Different Light and Watering Regimes

A comparison of the activity and properties of the enzyme phosphoenolpyruvatecarboxylase (PEPC) was made for plants of Sedum telephium L.grown under low (70 µmol m^–2 s^–1) or high(500µmol m^–2 s^–1) PPFD and subjected to varyingdegrees of water stress. Under well-watered conditions onlyplants grown under high PPFD accumulated titratable acidityovernight and the extractable activity of PEPC was almost 2-foldhigher in these plants than in plants grown under low PPFD.Increasing drought stress resulted in a substantial increasein the activity of PEPC extracted both during the light anddark periods and a decrease in the sensitivity to inhibitionby malic acid. The magnitude of these changes was determinedby the severity and duration of drought and by light intensity.A comparison of the kinetic properties of PEPC from severelydroughted plants revealed that plants droughted under high PPFDhad a lower K_m for PEP than plants under low PPFD. Additionof 2·0 mol m^–3 malate resulted in an increase inthe K_m for PEP, with plants draughted under low PPFD havinga significantly higher K_m in the presence of malic acid comparedto those under high _PPFD. Response to the activator glc-6-P,which lowered the K_m for PEP, also varied between plants grownunder the two light regimes. Under well-watered conditions PEPCextracted from plants under high PPFD was more sensitive toactivation by glc-6-P than those under low PPFD. After the severedrought treatment, however, the K_m for PEP in the presence ofglc-6-P was similar for enzyme extracted from plants grown underboth light regimes. Soluble sugars and starch were depletedovernight and were both possible sources of substrate for PEPC.With increasing drought, however, the depletion of starch relativeto soluble sugars increased under both light regimes. The propertiesof PEPC and the characteristics of carbohydrate accumulation/depletionare discussed in relation to the regulation of CAM in S. telephiumgrown under different light and watering regimes. Key words: PEP carboxylase, CAM, carbohydrates, Sedum telephium     

Induction by drought of crassulacean acid metabolism in the terrestrial bromeliad, <Emphasis Type="Italic">Puya floccosa</Emphasis>

In the terrestrial bromeliad, Puya floccosa, a value of carbon isotopic composition (δ¹³C) of −22‰ has been previously reported, suggesting the operation of weak and/or intermediate (C₃-CAM) crassulacean acid metabolism (CAM). In order to characterize the operation of CAM in P. floccosa and its possible induction by drought, plants were grown in Caracas and subjected to four independent drought cycles. Additionally, since plants of this species grow in Venezuela in a large range of elevations, leaf samples were collected at elevations ranging from 725 to 2,100 m a.s.l. in the Venezuelan Andes and the Coastal Range, in order to evaluate the effect of elevation on CAM performance. Even though nocturnal acid accumulation occurred in both watered and droughted plants, mean ΔH⁺ was higher in droughted than watered plants [ΔH⁺ = 60.17.5 and 22.9 ± 5.2 μmol g⁻¹(FM), respectively]. The majority of plants from all the natural populations sampled had low values of δ¹³C not differing significantly from those of C₃ plants collected as standards and δ¹³C did not change with elevation. We conclude that P. floccosa is capable of a weak CAM activity, with a large variability among populations and drought experiments probably due to local and temporal differences in microclimatic variables and drought stress; elevation bears no influence on values of δ¹³C in this species.     

Effects of silicon on defense of wheat against oxidative stress under drought at different developmental stages

The effects of silicon application before sowing on the drought-induced oxidative stress and antioxidant defense in wheat (Triticum aestivum L.) were investigated. Drought stress was applied by withholding watering till sampling at booting or filling stage. Application of Si increased the water potential of drought-stressed plants at filling stage, whereas it did not at booting stage. The superoxide dismutase (SOD) activity was inhibited and peroxidase (POD) activity was enhanced by drought at booting stage, and no differences were observed due to the Si treatment. At filling stage, however, application of Si increased the SOD activity and decreased the POD activity of drought-stressed plants. The catalase (CAT) activity was slightly increased by drought only in the absence of Si and at booting stage. The activity of glutathione reductase (GR) was not greatly influenced. Application of Si did not change the contents of H₂O₂, total soluble protein and protein carbonyl of drought-stressed plants at booting stage, whereas at filling stage, it decreased the content of H₂O₂ and protein carbonyl and increased the content of total soluble protein. The content of thiobarbituric acid reactive substances (TBARS) and the activities of acid phospholipase (AP) and lipoxygenase (LOX) in drought-stressed plants were also decreased by application of Si at both stages.     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

The Response of Leaf Water Potential and Crassulacean Acid Metabolism to Prolonged Drought in Sedum rubrotinctum

Plants of Sedum rubrotinctum R. T. Clausen were studied in a green-house over a 2-year period without watering. Only the apical leaves survived and were turgid at the end of the experiment. The midday leaf water potential of these apical leaves was −1.20 megapascals, while the leaf water potential of comparable leaves on well-watered control plants was −0.20 megapascals. The unwatered plants appear to have maintained turgor by means of an osmotic adjustment. After 2 years without water the plants no longer exhibited a nocturnal accumulation of titratable acidity. However, the daytime levels of titratable acidity of the unwatered plants were more than 2-fold greater than the levels in well-watered control plants. Well-watered plants of S. rubrotinctum exhibited seasonal shifts in biomass stble carbon isotope ratios, indicating a greater proportion of day versus night CO₂ uptake in the winter than in the summer. The imposition of water stress prevented the expression of this seasonal rhythm and restricted the plants to dark CO₂ uptake.