Crassulacean Acid Metabolism in the Strangler Clusia rosea Jacq.

Observations of malic acid fluctuation, leaf anatomy, and stable carbon isotopic composition showed that the epiphytic strangler Clusia rosea, growing on Saint John, U.S. Virgin Islands, has crassulacean acid metabolism. This hemiepiphyte may be the only woody dicotyledonous tree species among the many thousands of flowering species in the 30 or more plant families that shows this type of metabolism. The finding has implications with respect to water balance during the process whereby Clusia rosea establishes itself as a tree, since crassulacean acid metabolism is a photosynthetic adaptation to water-stressed environments.

The use of stable isotopes to study ecosystem gas exchange.

Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of "Keeling plots." This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO2 exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limited arsenal of ecosystem-scale research tools.

Photosynthesis in epiphytic and rooted Clusia rosea Jacq.

Clusia rosea Jacq. is a hemiepiphyte having Crassulacean Acid Metabolism (CAM). In its natural habitat Clusia begins its life cycle as an epiphyte and eventually becomes a rooted tree. These two stages of the life cycle of Clusia represent markedly different water regimes. Our CO2 exchange, stomatal conductance, titratable acidity, and stable carbon isotope ratio measurements indicate that Clusia has a flexible photosynthetic mode, where CO2 is fixed mostly via CAM during its epiphytic stage, when water availability is low, and via both CAM and C3 during its rooted stage.

Phosotynthesis in hemiepiphytic species of Clusia and Ficus.

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 δ13C 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.

Developmental Control of CAM in Peperomia scandens.

Experiments were conducted to examine the development of photosynthetic carbon metabolism in Peperomia scandens, a tropical epiphyte. Leaves were sampled during a 10-day period when they were between 30 to 165 days old. P. scandens exhibits a C(3) to CAM-cycling to CAM shift during maturation with the magnitude of CAM increasing with age. Initially, during both day and night, no significant CO(2) uptake or diurnal acid flux was evident. C(3) gas exchange was detected at 41 days of age with a gradual shift towards CAM gas exchange maximized thereafter. An acidity flux of 130 to 150 microequivalents per gram fresh weight was evident by 41 days. Between 40 and 90 days, the leaves shifted their CO(2) uptake pattern from a daytime to a nighttime peak. After 90 days, the leaves remained in CAM. The delta(13)C values became progressively less negative as the leaves matured. In the 30-day-old leaves, the delta(13)C value was -21.1% while in the 165-day-old leaves the delta(13)C value was -18.3%. The time-dependent shift from C(3) to CAM-cycling to CAM in P. scandens does not appear to result from changes in water, light, or temperature regimes since these variables were constant for all leaves sampled.

Compensation point and isotopic characteristics of c(3)/c(4) intermediates and hybrids in panicum.

Leaf CO(2) compensation points and stable hydrogen, oxygen and carbon isotope ratios were determined for Panicum species including C(3)/C(4) intermediate photosynthesis plants, hybrids between C(3)/C(4) intermediates and C(3) plants, C(3) and C(4) plants in the Panicum genus as well as several other C(3) and C(4) plants. C(3) plants had the highest compensation points, followed by hybrids, C(3)/C(4) intermediates, and C(4) plants. delta(13)C values of cellulose nitrate and saponifiable lipids from C(4) plants were about 10 per thousand higher than those observed for cellulose nitrate and saponifiable lipids of C(3)/C(4) intermediates, hybrids, and C(3) plants. Oxygen isotope ratios of cellulose as well as those of leaf water were similar for all plants. There was substantial variability in the deltaD values of cellulose nitrate among the plants studied. In contrast, such variability was not observed in deltaD values of water distilled from the leaves, nor in the deltaD values of the saponifiable lipids. Variability in deltaD values of cellulose nitrate from C(3)/C(4) intermediates, hybrids, C(3), and C(4) plants is due to fractionations occurring during biochemical reactions specific to leaf carbohydrate metabolism.

Oxygen Isotope Exchange between Metabolites and Water during Biochemical Reactions Leading to Cellulose Synthesis.

Cellulose was produced heterotrophically from different carbon substrates by carrot tissue cultures and Acetobacter xylinum (a cellulose-producing bacterium) and by castor bean seeds germinated in the dark, in each case in the presence of water having known concentration of oxygen-18 ((18)O). We used the relationship between the amount of (18)O in the water and in the cellulose that was synthesized to determine the number and (18)O content of the substrate oxygens that exchanged with water during the reactions leading to cellulose synthesis. Our observations support the hypothesis that oxygen isotope ratios of plant cellulose are determined by isotopic exchange occurring during hydration of carbonyl groups of the intermediates of cellulose synthesis.

Oxygen and Hydrogen Isotope Ratios of Water from Photosynthetic Tissues of CAM and C(3) Plants.

Water samples from photosynthetic tissues of C(3) and Crassulacean acid metabolism (CAM) plants that grew together in the field were extracted and the stable oxygen and hydrogen isotope ratios determined. During the day, (18)O/(16)O and deuterium/hydrogen (D/H) ratios of water from CAM plants were lower than those observed in water from C(3) plants. The patterns of diurnal variation (or lack thereof) in isotope ratios of plant water are consistent with the gross anatomical and physiological characteristics of the plants studied here. Our observations support the previously advanced hypothesis that high D/H ratios in cellulose nitrate prepared from CAM plants relative to those for C(3) plants are not caused by greater deuterium enrichment in the water in CAM plants, but rather by isotopic fractionations associated with different biochemical reactions in the two types of plants.