Nocturnal sap flow in the C<Subscript>3</Subscript>-CAM species, <Emphasis Type="Italic">Clusia minor</Emphasis>

Clusia minor L. is a C₃-CAM species in which Crassulacean acid Metabolism (CAM) is induced, among other factors, by water deficit. We propose that CAM induction by natural drought in C. minor shifts the sap flow pattern from daytime to a night-time one, and that the decreased osmotic potential due to increased malate content in droughted plants aids in the increase in nocturnal sap flow. In order to test these hypotheses, we followed for 2 years the seasonal changes in parameters of water relationships and sap flow velocity in one single, freestanding tree growing in Caracas. Leaf water and osmotic potential were measured psychrometrically, nocturnal proton accumulation by titration of aqueous leaf extracts and sap flow density with thermal dissipation probes. Leaf water, osmotic and turgor potential remained relatively high throughout the seasons. Nocturnal proton accumulation was nil under extreme drought or after frequent and heavy rains, and high after moderate rainfall. Estimated malate and citrate concentrations contributed up to 80 and 60%, respectively, of the value of osmotic potential. The shape of the daily courses of sap flow velocity varied seasonally, from mostly diurnal during the dry season to mostly nocturnal after a short dry spell during the rainy season, when nocturnal acid accumulation attained high values. There was a strong positive relationship between the proportion of the integrated sap flow courses corresponding to the night and dawn [H⁺] (r ² = 0.88). Increased nocturnal sap flow in the CAM stage of the tree of C. minor may be explained by a lower osmotic potential due to an increased acid concentration, together with increased stomatal aperture, as suggested by increased nocturnal acid accumulation probably due to nocturnal CO₂ fixation.     

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.     

Crassulacean acid metabolism in the Basellaceae (Caryophyllales)

• C₄ and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C₄ nor CAM have been recorded for the Basellaceae, a small family in the CAM‐rich sub‐order Portulacineae.
• 24 h gas exchange and day–night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet–dry–wet cycles.
• While net CO₂ uptake was restricted to the light period in well‐watered plants, net CO₂ fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C₃ photosynthesis upon rewatering.
• The reversible induction of nocturnal net CO₂ uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.

     

Effects of Drought on CAM and Water Relations in Plants of Peperomia carnevalii

The mechanisms underlying the drought tolerance of Peperomiacarnevalii Steyermark (Piperaceae), a succulent herb growingin the understorey of seasonally dry forests, were examined.Crassulacean acid metabolism (CAM) was studied in the fieldand laboratory, and measurements of water status were made inplants subjected to drought in the greenhouse. Nocturnal acidaccumulation and day and night-time CO₂assimilation rates weregreatest in watered plants and decreased in drought. The proportionof CO₂recycled through CAM in droughted plants, with nocturnalCO₂uptake close to zero, was higher than in watered plants.Maximum quantum yield of chlorophyll fluorescence remained unchangedduring drought, but the PSII quantum yield at the photosyntheticphoton flux density at which the plants were grown was significantlydecreased. Leaf anatomy consists of a chlorophyll-less hydrenchymalocated beneath the upper epidermis, and a two-layered mesophyll.Leaves nearer to the apex are thinner than those nearer to thebase of the shoot. Drought caused a reduction in leaf thicknessdue to shrinkage of the hydrenchyma, but not of the mesophyll.This was associated with the occurrence of a gradient of osmoticpotential between these tissues. Comparison of water loss fromthin leaves of watered and droughted plants, either partly defoliatedat the lower nodes or intact, suggested that water moved fromthe thick to the thin leaves. This process was related to theoccurrence of a gradient of water potential between the thickand the thin leaves. Drought tolerance in P. carnevalii is achievedby the operation of CAM and the occurrence of water movementwithin and between leaves. Copyright 2000 Annals of Botany Company Crassulacean acid metabolism, fluorescence, hydrenchyma, mesophyll, Peperomia carnevalii, water relations     

Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water status

The long-term interaction between elevated CO₂ and soil water deficit was analysed in N₂-fixing alfalfa plants in order to assess the possible drought tolerance effect of CO₂. Elevated CO₂ could delay the onset of drought stress by decreasing transpiration rates, but this effect was avoided by subjecting plants to the same soil water content. Nodulated alfalfa plants subjected to ambient (400 μmol mol^?1) or elevated (700 μmol mol^?1) CO₂ were either well watered or partially watered by restricting water to obtain 30% of the water content at field capacity (ampproximately 0.55 g water cm^?3). The negative effects of soil water deficit on plant growth were counterbalanced by elevated CO₂. In droughted plants, elevated CO₂ stimulated carbon fixation and, as a result, biomass production was even greater than in well-watered plants grown in ambient CO₂. Below-ground production was preferentially stimulated by elevated CO₂ in droughted plants, increasing nodule biomass production and the availability of photosynthates to the nodules. As a result, total nitrogen content in droughted plants was higher than in well-watered plants grown in ambient CO₂. The beneficial effect of elevated CO₂ was not correlated with a better plant water status. It is concluded that elevated CO₂ enhances growth of droughted plants by stimulating carbon fixation, preferentially increasing the availability of photosynthates to below-ground production (roots and nodules) without improving water status. This means that elevated CO₂ enhances the ability to produce more biomass in N₂-fixing alfalfa under given soil water stress, improving drought tolerance.     

Characteristics of Crassulacean Acid Metabolism in the Succulent C(4) Dicot, Portulaca oleracea L

Crassulacean acid metabolism (CAM) was investigated in leaves and stems of the succulent C₄ dicot Portulaca oleracea L. Diurnal acid fluctuations, CO₂ gas exchange, and leaf resistance were monitored under various photoperiod and watering regimes. No CAM activity was seen in well watered plants grown under 16-hour days. Under 8-hour days, however, well watered plants showed a CAM-like pattern of acid fluctuation with amplitudes of 102 and 90 microequivalents per gram fresh weight for leaves and stems, respectively. Similar patterns were also observed in detached leaves and defoliated stems. Leaf resistance values indicated that stomata were open during part of the dark period, but night acidification most likely resulted from refixation of respiratory CO₂. In water-stressed plants maximum acid accumulations were reduced under both long and short photoperiods. At night, these plants showed short periods of net CO₂ uptake and stomatal opening which continued all night long during preliminary studies under natural environmental conditions. Greatest acid fluctuations, in P. oleracea, with amplitudes of 128 microequivalents per gram fresh weight, were observed in water-stressed plants which had been rewatered, especially when grown under short days. No net CO₂ uptake took place, but stomata remained open throughout the night under these conditions. These results indicate that under certain conditions, such as water stress or short photoperiods, P. oleracea is capable of developing an acid metabolism with many similarities to CAM.     

Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia

Night-time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well-watered conditions, substantial night-time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night-time CO₂ uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO₂ was substantial for both well-watered and water-stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO₂ for the formation of malate is respiratory CO₂. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO₂, while the breakdown of malic acid releases only one CO₂ per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO₂ concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H⁺/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well-known buffer, and Clusia spp. show the largest day-night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.     

Comparative measurements of chlorophyll a fluorescence, acid accumulation and gas exchange in exposed and shaded plants of Clusia minor L. and Clusia multiflora H. B. K. in the field

 Changes in chlorophyll a fluorescence during the day and diurnal-changes of net CO₂-exchange and organic acid contents were determined in two species of the genus Clusia during the dry season in Venezuela. The investigations included plants of the C₃/CAM intermediate species Clusia minor and the C₃ species C. multiflora growing at exposed and shaded sites. Both species showed a C₃ pattern of net CO₂-exchange at the exposed site. In the shade under extreme drought stress C. minor showed a weak expression of CAM without CO₂-uptake during the afternoon (phase IV of CAM). C. multiflora growing in the shade exhibited a C₃-pattern of net CO₂-exchange and a small but significant nocturnal accumulation of citrate. Shaded plants of C. minor were able to double their light utilisation for electron transport and to reduce non-photochemical quenching during phase III compared to phase II of CAM. Furthermore, increase of electron transport rate through photosystem II in phase III of CAM is correlated to decarboxylation of malate. At the exposed site C. multiflora was less negatively affected by high PPFD than C. minor. This was shown by a lower reduction of potential electron quantum yield (F_v/F_m) and higher light utilisation of electron transport of C. multiflora compared to C. minor. At the exposed site C. minor did not make use of the CAM option to increase light utilisation of electron transport and to reduce non-photochemical quenching as did the plants growing in the shade. Received: 20 March 1996 / Accepted: 24 June 1996     

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.     

Facultative CAM photosynthesis (crassulacean acid metabolism) in four species of <Emphasis Type="Italic">Calandrinia</Emphasis>, ephemeral succulents of arid Australia

Crassulacean acid metabolism (CAM) was demonstrated in four small endemic Australian terrestrial succulents from the genus Calandrinia (Montiaceae) viz. C. creethiae, C. pentavalvis, C. quadrivalvis and C. reticulata. CAM was substantiated by measurements of CO₂ gas-exchange and nocturnal acidification. In all species, the expression of CAM was overwhelmingly facultative in that nocturnal H⁺ accumulation was greatest in droughted plants and zero, or close to zero, in plants that were well-watered, including plants that had been droughted and were subsequently rewatered, i.e. the inducible component was proven to be reversible. Gas-exchange measurements complemented the determinations of acidity. In all species, net CO₂ uptake was restricted to the light in well-watered plants, and cessation of watering was followed by a progressive reduction of CO₂ uptake in the light and a reduction in nocturnal CO₂ efflux. In C. creethiae, C. pentavalvis and C. reticulata net CO₂ assimilation was eventually observed in the dark, whereas in C. quadrivalvis nocturnal CO₂ exchange approached the compensation point but did not transition to net CO₂ gain. Following rewatering, all species returned to their original well-watered CO₂ exchange pattern of net CO₂ uptake restricted solely to the light. In addition to facultative CAM, C. quadrivalvis and C. reticulata exhibited an extremely small constitutive CAM component as demonstrated by the nocturnal accumulation in well-watered plants of small amounts of acidity and by the curved pattern of the nocturnal course of CO₂ efflux. It is suggested that low-level CAM and facultative CAM are more common within the Australian succulent flora, and perhaps the world succulent flora, than has been previously assumed.     

Effects of water stress on gas exchange and water relations of a succulent epiphyte,Kalanchoë uniflora

Summary Measurements of gas exchange, xylem tension and nocturnal malate synthesis were conducted with well-watered and droughted plants of Kalanchoë uniflora. Corresponding results were obtained with plants grown in 9 h and 12 h photoperiods. In well-watered plants, 50 to 90% of total CO₂-uptake occurred during the light period. Nocturnal CO₂-uptake and malate synthesis were higher and respiration rate was lower in old leaves (leaf pairs 6 to 10) compared to young leaves (leaf pairs 1 to 5). Within four days of drought distinct physiological changes occurred. Gas exchange during the light period decreased and CO₂-uptake during the dark period increased. Nocturnal malate synthesis significantly increased in young leaves.Respiration rate decreased during periods of drought, this decrease being more pronounced in young leaves compared to old leaves. Restriction of gas exchange during the light period resulted in a decrease of transpiration ratio from more than 100 to about 20. The difference between osmotic pressure and xylem tension decreased in young leaves, indicating a reduction in bulk leaf turgor-pressure.We conclude that both the CAM-enhancement in young leaves and the decrease of respiration rate are responsible for the increase of nocturnal CO₂-uptake during water stress. During short drought periods, which frequently occur in humid habitats, the observed physiological changes result in a marked reduction of water loss while net CO₂-uptake is maintained. This might be relevant for plant growth in the natural habitat.Abbreviations LP light period - DP dark period - CAM crassulacean acid metabolism     

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.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

CO2 exchange of CAM exhibiting succulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO₂ exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditions of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO₂ and led to CO₂ release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO₂ uptake while the recovery of the nocturnal CO₂ uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO₂ uptake considerably. Even in well watered plants nocturnal CO₂ uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.     

Stachyose Synthesis in Source Leaf Tissues of the CAM Plant Xerosicyos danguyi H. Humb

Leaf tissues from Xerosicyos danguyi H. Humb., a succulent member of the Cucurbitaceae, were found to possess both galactinol synthase activity and the capacity for photosynthetic production of stachyose, the phloem transport oligosaccharide common to other nonsucculent cucurbits. The amounts of stachyose isolated from leaf tissues, and the extractable activity of galactinol synthase, were somewhat higher in leaf tissues obtained from plants operating in the Crassulacean acid metabolism (CAM) mode (well watered plants) compared to leaf tissues from plants operating in the CAM-idling mode (water-stressed plants). In contrast, in leaf discs, the photosynthetic incorporation of label into stachyose following pulse labeling with ¹⁴CO₂ was similar for stressed and for nonstressed tissues. Stachyose could be extracted from, and was synthesized photosynthetically by, leaf discs which contained no vascular tissues, indicating that synthesis of stachyose can occur in photosynthetic mesophyll cells of Xerosicyos.     

Differential usage of storage carbohydrates in the CAM bromeliad Aechmea 'Maya' during acclimation to drought and recovery from dehydration

CAM requires a substantial investment of resources into storage carbohydrates to account for nocturnal CO₂ uptake, thereby restricting carbohydrate partitioning to other metabolic activities, including dark respiration, growth and acclimation to abiotic stress. Flexible modulation of carbon flow to the different competing sinks under changing environmental conditions is considered a key determinant for the growth, productivity and ecological success of the CAM pathway. The aim of the present study was to examine how shifts in carbohydrate partitioning could assure maintenance of photosynthetic integrity and a positive carbon balance under conditions of increasing water deprivation in CAM species. Measurements of gas exchange, leaf water relations, malate, starch and soluble sugar (glucose, fructose and sucrose) contents were made in leaves of the CAM bromeliad Aechmea ‘Maya’ over a 6‐month period of drought and subsequently over a 2‐month period of recovery from drought. Results indicated that short‐term influences of water stress were minimized by elevating the level of respiratory recycling, and carbohydrate pools were maintained at the expense of export for growth while providing a comparable nocturnal carbon gain to that in well‐watered control plants. Longer term drought resulted in a disproportionate depletion of key carbohydrate reserves. Sucrose, which was of minor importance for providing substrate for the dark reactions under well‐watered conditions, became the major source of carbohydrate for nocturnal carboxylation as drought progressed. Flexibility in terms of the major carbohydrate source used to sustain dark CO₂ uptake is therefore considered a crucial factor in meeting the carbon and energy demands under limiting environmental conditions. Recovery from CAM‐idling was found to be dependent on the restoration of the starch pool, which was used predominantly for provision of substrate for nocturnal carboxylation, while net carbon export was limited. The conservation of starch for the nocturnal reactions might be adaptive with regard to responding efficiently to a return of water stress.     

Photosynthetic down-regulation in Larrea tridentata exposed to elevated atmospheric CO2: interaction with drought under glasshouse and field (FACE) exposure

The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO₂ and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO₂. We hypothesized that photosynthetic down-regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 μmol mol^–1 CO₂ in a glasshouse; and (2) from intact adults exposed to 360 and 550 μmol mol^–1 CO₂ at the Nevada Desert FACE (Free Air CO₂ Enrichment) Facility. FACE and glasshouse well-watered Larrea significantly down-regulated photosynthesis at elevated CO₂, reducing maximum photosynthetic rate (A_max), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. A_max and CE were lower in droughted Larrea compared with well-watered plants, and CO₂ had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO₂. Operating C_i increased at elevated CO₂ in droughted plants, resulting in greater photosynthetic rates at elevated CO₂ as compared with ambient CO₂. In well-watered plants, the changes in operating C_i, CE and A_max resulted in similar photosynthetic rates across CO₂ treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO₂ in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO₂.     

Day-night changes in the levels of adenine nucleotides,phosphoenolpyruvate and inorganic pyrophosphate in leaves of plants having Crassulacean acid metabolism

The levels of phosphorylated compounds studied during the dark period of Crassulacean acid metabolism (CAM) in Kalanchoë leaves showed increases for ATP and pyrophosphate and decreases for ADP, AMP and phosphenolpyruvate; levels of inorganic phosphate remained constant. Changes in adenylate levels and the correlated nocturnal increase in adenylate-energycharge were closely related to changes in malate levels. The increase in ATP levels was much inhibited in CO₂-free air and stimulated after induction of CAM in short-day-treated plants of K. blossfeldiana cv. Tom Thumb. Changes in levels of phosphoenolpyruvate and pyrophosphate were independent of the presence of CO₂. The results show the operation of complex regulatory mechanisms in the energy metabolism of CAM plants during nocturnal malic-acid accumulation.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - OAA oxaloacetic acia - PEP phosphoenol pyruvate - PP_i pyrophosphate     

Effects of elevated CO2, drought and temperature on the water relations and gas exchange of groundnut (Arachis hypogaea) stands grown in controlled environment glasshouses

Stands of groundnut (Arachis hypogaea L. cv. Kadiri‐3) were grown in controlled environment glasshouses at mean atmospheric CO₂ concentrations of 375 or 700 μmol mol^?1 and daily mean air temperatures of 28 or 32°C on irrigated or drying soil profiles. Leaf water (Ψ_l) and solute potential (Ψ_s), relative water content (RWC), stomatal conductance (g_l) and net photosynthesis (P_n) were measured at midday for the youngest mature leaf throughout the growing season. Elevated CO₂ and temperature had no detectable effect on the water relations of irrigated plants, but higher values of RWC, Ψ_l and Ψ_s were maintained for longer under elevated CO₂ during progressive drought. Turgor potential (Ψ_p) reached zero when Ψ_l declined to ?1.6 to ?1.8 MPa in all treatments; turgor was lost sooner when droughted plants were grown under ambient CO₂. A 4°C increase in mean air temperature had no effect on Ψ_s in droughted plants, but elicited a small increase in Ψ_l; midday g_l values were lower under elevated than under ambient CO₂, and Ψ_l and g_l declined below ?1.5 MPa and 0.25 cm s^?1, respectively, as the soil dried. Despite the low g_l values recorded for droughted plants late in the season, P_n was maintained under elevated CO₂, but declined to zero 3 weeks before final harvest under ambient CO₂. Concurrent reductions in g_l and increases in water use efficiency under elevated CO₂ prolonged photosynthetic activity during drought and increased pod yields relative to plants grown under ambient CO₂. The implications of future increases in atmospheric CO₂ for the productivity of indeterminate C₃ crops grown in rainfed subsistence agricultural systems in the semi‐arid tropics are discussed.     

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.