Crassulacean acid metabolism in tropical dicotyledonous trees of the genusClusia

Summary The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genusClusia sampled at three sites in the state of Falcon in northern Venezuela is characterized.Clusia leaves have a somewhat succulent appearance. Unlike leaves of many other CAM plants, which are uniformly built up of very large isodiametric cells, there are distinct layers of palisade and spongy mesophyll, with individual cells being smaller. There is no specialized water storage tissue. ¹³C values indicate thatC. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at 800 m altitude, performs C₃ photosynthesis (¹³ –27.1). However,C. rosea in the tall cloud forest on Cerro Santa Ana (600m altitude), andC. rosea andC. alata in the dry forest on Serrania San Luis (900 m altitude) perform CAM (¹³C –14.1 to –19.2). InC. alta andC. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240 mmol 1^–1 for malid acid and from 35 to 112 mmol 1^–1 for citric acid. The sum of the changes in malate and citrate levels accounts for the changes of titratable protons measured. With a day-night change of titratable protons of 768 mmol 1^–1 in one of the analyses,C. rosea showed the highest value yet encountered in a CAM plant. Oscillations of free sugars (fructose, glucose, sucrose) and of starch were also analysed in the CAM performingClusia species. Carbon skeletons of the precursors involved in nocturnal malate and citrate synthesis largely derive from free sugars and not from polyglucan. Unlike some other CAM plants, there is no clear and quantitative correlation between day-night changes of organic acid levels and cell sap osmolality.Dedicated to Professor Dr. Otto L. Lange on the occasion of his 60th birthday.     

Light and dark CO2 fixation in Clusia uvitana and the effects of plant water status and CO2 availability

Summary In well-watered plants of Clusia uvitana, a species capable of carbon fixation by crassulacean acid metabolism (CAM), recently expanded leaves gained 5 to 13-fold more carbon during 12 h light than during 12 h dark periods. When water was withheld from the plants, daytime net CO₂ uptake strongly decreased over a period of several days, whereas there was a marked increase in nocturnal carbon gain. Photosynthetic rates in the chloroplasts were hardly affected by the water stress treatment, as demonstrated by measurements of chlorophyll a fluorescence of intact leaves, indicating efficient decarboxylation of organic acids and refixation of carbon in the light. Within a few days after rewatering, plants reverted to the original gas exchange pattern with net CO₂ uptake predominantly occurring during daytime. The reversible increase in dark CO₂ fixation was paralleled by a reversible increase in the content of phosphoenolpyruvate (PEP) carboxylase protein. In wellwatered plants, short-term changes in the degree of dark CO₂ fixation were induced by alterations in CO₂ partial pressure during light periods: a decrease from 350 to 170 bar CO₂ caused nocturnal carbon gain, measured in normal air (350 bar), to increase, whereas an increase to 700 bar CO₂, during the day, caused net dark CO₂ fixation to cease. The increased CAM activity in response to water shortage may, at least to some extent, be directly related to the reduced carbon gain during daytime.     

Expression of crassulacean acid metabolism in <Emphasis Type="Italic">Clusia hilariana</Emphasis> Schlechtendal in different stages of development in the field

Expression of crassulacean acid metabolism (CAM) in the obligate CAM-tree Clusia hilariana SCHLTDL. was studied in the restinga of Jurubatiba National Park, on the Atlantic coast of Rio de Janeiro state, Brazil, comparing plants at different developmental stages. Between young and mature plants there were trends of differences in six parameters, which are all related to CAM expression. From young to mature plants there were tendencies for a decrease of (1) the degree of succulence, (2) the degree of day/night changes of malic acid levels, (3) titratable acidity with nocturnal acid accumulation, (4) the degree of day/night changes of free hexoses with nocturnal break down, (5) effective quantum use efficiency of photosystem II at high photosynthetic photon flux density, and (6) protection from photoinhibition. These tendencies form a clear pattern which suggests that CAM was somewhat more pronounced in leaves of young plants than in leaves of mature plants. A developmental regulation may be involved. However, the observations are probably best explained by stress, since in the dry soils of the restinga young plants have no access to the ground water table while adult trees develop extensive root systems.     

Correlation between photorespiration,CO<Subscript>2</Subscript>-assimilation and spatiotemporal dynamics of photosynthesis in leaves of the C<Subscript>3</Subscript>-photosynthesis/crassulacean acid metabolism-intermediate species <Emphasis Type="Italic">Clusia minor</Emphasis> L. (Clusiaceae)

The tree Clusia minor L. (Clusiaceae) operates with different modes of photosynthesis in response to different combinations of environmental parameters. Here plants were subjected to experimental conditions eliciting performance of C₃-photosynthesis and crassulacean acid metabolism (CAM), respectively. A combination of instruments was used to determine CO₂ and water vapour gas exchange, relative quantum use efficiency of photosynthesis (Φ_PSII) and for the first time in such studies also photorespiration simultaneously with the other parameters. In the C₃-mode photorespiration was constant during the light period, where oxygenase activity of ribulose-bis-phosphate carboxylase/oxygenase (RubisCO) was ranging between 32.1 and 35.7% of total RubisCO activity. In the CAM-mode photorespiration depended on the CAM phases. In phase II in the morning was 15.6%. In phase IV in the afternoon initially it was 37.9% and then declined to 17.6% of total RubisCO activity towards the evening. Anatomically leaves of C. minor are differentiated in palisade and spongy parenchyma with an internal air space of 9.3% of the total volume and therefore could be structurally homobaric. However, heterogeneity of Φ_PSII under both non-photorespiratory and photorespiratory conditions in the C₃- and CAM-mode indicated that lateral diffusion of CO₂ and O₂ were subject to limitations showing that leaves are functionally heterobaric.     

Short-term changes in carbon-isotope discrimination in the C3-CAM intermediate Clusia minor L. growing in Trinidad

On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO₂ in leaves of exposed and shaded plants of the C₃-CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C₃ photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO₂ at night for the synthesis of malic acid. Citric acid was the major organic acid accumulated by exposed leaves at this time with a citric: malic acid ratio of 11:1. Values of instantaneous discrimination () in exposed leaves during the wet season rose from 17.1 shortly after dawn to 22.7 around mid-day just before stomata closed, suggesting that most CO₂ was fixed by Rubisco at this time. During the late afternoon, instantaneous declined from 22.2 to 17, probably reflecting the limited contribution from PEPc activity and an increase in diffusional resistance to CO₂ in exposed leaves. Shaded leaves showed no CAM activity and CO₂ uptake proceeded throughout the day in the wet season. The decrease in instantaneous from 27 in the morning to 19.2 in the late afternoon was therefore entirely due to diffusional limitation. Leaves sampled in the dry season (mid-March, 1992) had by now induced full CAM activity with both malic and citric acids accumulated overnight and stomata closed for 4–5 h over the middle of the day. Values of instantaneous measured over the first 3 h after dawn (6.4–9.1) indicated that C₄ carboxylation dominated CO₂ uptake for most of the morning when rates of photosynthesis were maximal, implying that under natural conditions, the down regulation of PEPc in phase II occurs much more slowly than laboratory-based studies have suggested. The contribution from C₃ carboxylation to CO₂ uptake during phase II was most marked in leaves which accumulated lower quantities of organic acids overnight. In exposed leaves, measurements of instantaneous during the late afternoon illustrated the transition from C₃ to C₄ carboxylation with stomata remaining open during the transition from dusk into the dark period. Uptake of CO₂ by shaded leaves during the late afternoon however appeared to be predominantly limited by decreased stomatal conductance. The short-term measurements of instantaneous were subsequently integrated over 24 h in order to predict the leaf carbon isotope ratios (_p) and to compare this with the _p measured for leaf organic material. Whilst there was close agreement between predicted and measured _p for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5–9 higher than the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C₃ pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.     

Phosotynthesis in hemiepiphytic species of Clusia and Ficus

Summary Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C₃. This conclusion is based on diurnal CO₂ fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and ¹³C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C₃-type gas exchange with all CO₂ uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29 typical of C₃ plants.     

The correlation between crassulacean acid metabolism and water uptake in Senecio medley-woodii

The combination of a chamber for CO₂ gas exchange with a potometric measuring arrangement allowed concomitant investigations into CO₂ gas exchange, transpiration and water uptake by the roots of whole plants of Senecio medley-woodii, a species which exhibits Crassulacean acid metabolism. The water-uptake rate showed the same daily pattern as malate concentration and osmotic potential. The accumulation of organic acids resulting from nocturnal CO₂ fixation enhanced the water-uptake rate from dusk to dawn. During the day the water-uptake rates decreased with decreasing organic-acid concentration. With gradually increasing water stress, CO₂ dark fixation of S. medley-woodii was increased as long as water could be taken up by the roots. It was also shown that a reestablished water supply after drought caused a similar increase which in both cases ameliorated the water uptake in order to conserve a positive water balance for as long as possible. This water-uptake pattern shows that Crassulacean acid metabolism is not only a water-saving adaptation but also enhances water uptake and is directly correlated with the amelioration of the plant water status.Abbreviation CAM Crassulacean acid metabolism     

Seasonal patterns of acid fluctuations and resource storage in the arborescent cactus Opuntia excelsa in relation to light availability and size

Summary We investigated relationships between light availability, diel acid fluctuation, and resource storage in the arborescent cactus Opuntia excelsa growing in western Mexico. We compared canopy and understory individuals from a deciduous forest as well as open-grown plants of the same approximate size as those in the understory. During the wet season light availability and daily fluctuations in titratable acidity (an index of carbon uptake) were lower in the understory than in unshaded habitats. In the dry season all plants had reduced levels of acid fluctuation, with the smallest individuals, regardless of habitat, showing the greatest reduction. These data suggest that light availability in the forest understory constrains carbon assimilation during the wet season, but that a factor associated with plant size, possibly water status, limits carbon gain during the dry season. Plants in all habitats remained physiologically active for at least five months into the dry season. We suggest that this was possible due to the maintenance of constant concentrations of water and nitrogen in the photosynthetically active chlorenchyma. Parenchyma in terminal cladodes showed a different seasonal pattern of resource storage; water content and nitrogen concentration were reduced from the wet to the dry season in the parenchyma. Using the parenchyma to supply photosynthetic tissues during times of reduced resource availability allows O. excelsa to assimilate carbon during times of the year when most other trees in the forest are leafless.     

<Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C values and crassulacean acid metabolism in <Emphasis Type="Italic">Clusia</Emphasis> species from Panama

The genus Clusia is notable in that it contains arborescent crassulacean acid metabolism (CAM) plants. As part of a study of CAM in Clusia, titratable acidities were measured in 25 species and ¹³C values were measured for 38 species from Panamá, including seven undescribed species, and 11 species from Colombia, Costa Rica and Honduras. CAM was detected in 12 species. Clusia flava, C. rosea and C. uvitana exhibited ¹³C values or diurnal fluctuations in acidity indicative of strong CAM. In C. croatii, C. cylindrica, C. fructiangusta, C. lineata, C. odorata, C. pratensis, C. quadrangula, C. valerioi and C. sp. D diurnal fluctuations in acidity were consistent with weak CAM but the ¹³C values were C₃-like. All of the species that exhibited strong or weak CAM were in the C. flava or C. minor species groups. CAM was not detected in any member of the C. multiflora species group. Strong CAM species were not collected at altitudes above 680 m a.s.l. On the basis of ¹³C values, the expression of CAM was similar in terrestrial, hemi-epiphytic and epiphytic species and did not differ between individuals of the same species that exhibited different life-forms. This study indicates that phylogenetic affiliation may be a predictor of an ability to exhibit CAM in Clusia species from the Panamanian region, and that weak CAM is probably a common photosynthetic option in many Clusia species. ¹³C value is not a particularly good indicator of a potential of Clusia species growing in the field to exhibit CAM because it appears that the contribution in most species of CAM to carbon gain is generally rather small when integrated over the life-time of leaves.     

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     

Cool-night temperature induces spike emergence and affects photosynthetic efficiency and metabolizable carbohydrate and organic acid pools in <Emphasis Type="Italic">Phalaenopsis aphrodite</Emphasis>

After being acclimated to constant warm (28 degrees C day/28 degrees C night) and cool-night temperature (28 degrees C day/20 degrees C night) regimes in growth chambers for 2 weeks, the two groups of mature Phalaenopsis aphrodite subsp. formosana plants both clearly exhibited a diurnal oscillation of stomatal conductance, net CO(2) uptake rate, malate and starch levels, and the phosphoenolpyruvate carboxylase (EC 4.1.1.31) and NAD(+)-malic enzyme (EC 1.1.1.39) activities. Hence, P. aphrodite is an obligate crassulacean acid metabolism plant. Nevertheless, different night temperature greatly affected both the stomatal conductance and the contribution of ambient and respiratory CO(2) to the nocturnal accumulation of malate. However, the amounts of nocturnal accumulated malate and daily deposited starch appeared to have no significant difference between the two groups. These results demonstrate that P. ahrodite is congruent with the characteristics of CAM plants having great flexibility and plasticity in response to changes in environmental conditions. In addition, the formation of reproductive stem, viz. spike, was noticeably inhibited by a constant warm temperature, but induced by a fluctuating warm day and cool night condition. The relationship between the metabolic pool variation and spike induction of Phalaenopsis is also discussed.     

Photosynthetic changes and oxidative stress caused by iron ore dust deposition in the tropical CAM tree Clusia hilariana

The effect of iron solid particulate matter (SPM_Fe) deposited onto soil and leaves on photosynthesis and oxidative stress was evaluated in Clusia hilariana, a CAM tropical tree of high occurrence in Brazilian restingas. Significant increases in iron content were found in plants exposed to SPM_Fe applied onto leaf and soil surfaces. However, only the application of SPM_Fe on leaves of C. hilariana caused significant reductions in some evaluated characteristics such as photosynthetic rate, stomatal conductance, transpiration, organic acid accumulation, potential quantum yield of PSII, and changes in daily CAM photosynthesis pattern. Increase in relative membrane permeability and reduction in catalase and superoxide dismutase activities in the leaves of plants exposed to SPM_Fe also were observed; however, lipid peroxidation did not change. These responses seem to be due to the combination of physical effects such as increase of leaf temperature, reduction in light absorption, obstruction of stomatal pores, and biochemical effects triggered by oxidative stress.     

Properties of phosphoenolpyruvate carboxylase in rapidly prepared,desalted leaf extracts of the Crassulacean acid metabolism plant Mesembryanthemum crystallinum L.

Properties of phosphoenolpyruvate (PEP) carboxylase, obtained from leaves of Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism (CAM), were determined at frequent time points during a 12-h light/12-h dark cycle. Leaf extracts were rapidly desalted and PEP carboxylase activity as a function of PEP concentration, malate concentration, and pH was measured within 2 min after homogenization of the tissue. Maximum velocity of PEP carboxylase was similar in the light and dark at pH 7.5 and pH 8.0. However, PEP carboxylase had as much as a 12-fold lower K _m for PEP and as much as a 20-fold higher K _i for malate during the dark than during the light periods, the magnitude of these differences being dependent on the assay pH. Assuming that enzyme properties immediately after isolation reflect the approximate state of the enzyme in vivo, these differences in enzyme properties reduce the potential for CO₂ fixation via PEP carboxylase in the light. A small decrease in cytoplasmic pH in the light would greatly magnify the above differences in day/night properties of PEP carboxylase, because the sensitivity of PEP carboxylase to inhibition by malate increased with decreasing pH. Properties of PEP carboxylase were also studied in plants exposed to short-term perturbations of the normal 12-h light/12-h dark cycle (e.g., prolonged light period, prolonged dark period). Under all light/dark regimes, there was a close correlation between change in properties of PEP carboxylase and changes of the tissue from acidification to deacidification, and vice versa. Changes in properties of PEP carboxylase were not merely light/dark phenomena because they were also observed in plants exposed to continuous light or dark. the data indicate that, during CAM, PEP carboxylase exists in two stages which differ in their capacity for net malate synthesis. The physiologically-active state is distinguished by a low K _m for PEP and a high K _i for malate and favors malate synthesis. The physiologically-inactive state has a high K _m for PEP and a low K _i for malate and exists during periods of deacidification and other periods lacking synthesis of malic acid.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPC PEP carboxylase - RuBP ribulose 1,5-bisphosphate - RH relative humidity     

Siderophore and organic acid production in root nodule bacteria

Nineteen strains of root nodule bacteria were grown under various iron regimes (0.1, 1.0 and 20 M added iron) and tested for catechol and hydroxamate siderophore production and the excretion of malate and citrate. The growth response of the strains to iron differed markedly. For 12 strains (Bradyrhizobium strains NC92B and 32H1, B. japonicum USDA110 and CB1809, B. lupini WU8, cowpea Rhizobium NGR234, Rhizobium meliloti strains U45 and CC169, Rhizobium leguminosarum bv viciae WU235 and Rhizobium leguminosarum bv trifolii strains TA1, T1 and WU95) the mean generation time showed no variation with the 200-fold increase in iron concentration. In contrast, in Bradyrhizobium strains NC921, CB756 and TAL1000, B. japonicum strain 61A76 and R. leguminosarum bv viciae MNF300 there was a 2–5 fold decrease in growth rate at low iron. R. meliloti strains WSM419 and WSM540 showed decreased growth at high iron.All strains of root nodule bacteria tested gave a positive CAS (chrome azurol S) assay for siderophore production. No catechol-type siderophores were found in any strain, and only R. leguminosarum bv trifolii T1 and bv viciae WU235 produced hydroxamate under low iron (0.1 and 1.0 M added iron).Malate was excreted by all strains grown under all iron regimes. Citrate was excreted by B. japonicum USDA110 and B. lupini WU8 in all iron concentrations, while Bradyrhizobium TAL1000, R. leguminosarum bv viciae MNF300 and B. japonicum 61A76 only produced citrate under low iron (0.1 and/or 1.0 M added iron) during the stationary phase of growth.Abbreviations CAS chrome azurol S - HDTMA hexadecyltrime-thylammonium bromide     

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     

Environmental sensitivity of gas exchange in different-sized trees

The carbon isotope signature (δ¹³C) of foliar cellulose from sunlit tops of trees typically becomes enriched as trees of the same species in similar environments grow taller, indicative of size-related changes in leaf gas exchange. However, direct measurements of gas exchange in common environmental conditions do not always reveal size-related differences, even when there is a distinct size-related trend in δ¹³C of the very foliage used for the gas exchange measurements. Since δ¹³C of foliage predominately reflects gas exchange during spring when carbon is incorporated into leaf cellulose, this implies that gas exchange differences in different-sized trees are most likely to occur in favorable environmental conditions during spring. If gas exchange differs with tree size during wet but not dry conditions, then this further implies that environmental sensitivity of leaf gas exchange varies as a function of tree size. These implications are consistent with theoretical relationships among height, hydraulic conductance and gas exchange. We investigated the environmental sensitivity of gas exchange in different-sized Douglas-fir (Pseudotsuga menziesii) via a detailed process model that specifically incorporates size-related hydraulic conductance [soil–plant–atmosphere (SPA)], and empirical measurements from both wet and dry periods. SPA predicted, and the empirical measurements verified, that differences in gas exchange associated with tree size are greatest in wet and mild environmental conditions and minimal during drought. The results support the hypothesis that annual net carbon assimilation and transpiration of trees are limited by hydraulic capacity as tree size increases, even though at particular points in time there may be no difference in gas exchange between different-sized trees. Maximum net ecosystem exchange occurs in spring in Pacific Northwest forests; therefore, the presence of hydraulic limitations during this period may play a large role in carbon uptake differences with stand-age. The results also imply that the impacts of climate change on the growth and physiology of forest trees will vary depending on the age and size of the forest.     

Characterization of carbon metabolism in Opuntia ficus-indica Mill. exhibiting the idling mode of Crassulacean acid metabolism

Upon transfer from well-watered conditions to total drought, long-day-grown cladodes of Opuntia ficus-indica Mill. shift from full Crassulacean acid metabolism (CAM) to CAM-idling. Experiments using ¹⁴C-tracers were conducted in order to characterize the carbon-flow pattern in cladodes under both physiological situations. Tracer was applied by ¹⁴CO₂ fumigations and NaH¹⁴CO₃ injections during the day-night cycle. The results showed that behind the closed stomata, mesophyll cells of CAM-idling plants retained their full capacity to metabolize CO₂ in light and in darkness. Upon the induction of CAM-idling the level of the capacity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) was maintained. By contrast, malate pools decreased, displaying finally only a small or no day-night oscillation. The capacity of NADP-malic enzyme (EC 1.1.1.40) decreased in parallel with the reduction in malate pools. Differences in the labelling patterns, as influenced by the mode of tracer application, are discussed.Abbreviations CAM Crassulacean acid metabolism - PEP-Case phosphoenolpyruvate carboxylase     

Leaf age and the timing of leaf abscission in two tropical dry forest trees

We used experimental defoliations to examine the effect of leaf age on the timing of leaf shedding in two tropical dry forest trees. Trees of the deciduous Bombacopsis quinata (bombacaceae, a.k.a. Pachira quinata) and the brevi-deciduous Astronium graveolens (anacardiaceae) were manually defoliated for three times during the rainy season. All trees started to produce a new crown of leaves 2 weeks after defoliation, and continued expanding leaves throughout the rainy season. At the transition to the dry season, the experimental groups consisted of trees with known differences in maximum leaf age. Defoliations resulted in declines in stem growth but did not affect the mineral content or water relations of the leaves subsequently produced. There was no effect of leaf age on the timing of leaf abscission in B. quinata. In A. graveolens, the initiation of leaf shedding followed in rank order, the maximum leaf age of the four treatments, but there was substantial coherence among treatments in the major period of leaf abscission such that trees completed leaf shedding at the same time. In the two species, leaf water potential (Ψ_L) and stomatal conducantce (g _S) declined with the onset of the dry season, reaching minimum values of –0.9 MPa in P. quinata and <–2.0 MPa in A. graveolens. Within each species, leaves of different age exhibited similar Ψ_L and g _S at the onset of drought, and then decreased at a similar rate as the dry season progressed. Overall, our study suggests that the environmental factors were more important than leaf age in controlling the timing of leaf shedding.