Leaf Water 18O Research Articles

Disentangling the impact of photosynthesis and stomatal conductance on rising water-use efficiency at different altitudes on the Tibetan plateau

Tree intrinsic water-use efficiency (iWUE) has dramatically increased in recent decades in global forests. The rising iWUE can be a result of either enhanced photosynthesis rate (A), or decreased stomatal conductance (gs) or both. The underlying physiological mechanisms are still not well understood. Here, we investigated tree-ring isotopes δ13C and δ18O from two tree species in three altitudinal transects on the southeast Tibetan plateau. We found that the relationship between iWUE and ∆18Olw was negative at lower-altitude forests, whereas it was positive at the treeline, indicating that enhanced photosynthesis was the main driver of the increasing iWUE at the treeline, whereas reduction in stomatal conductance was the prime regulator at lower-altitude forests. Furthermore, vapor pressure deficit (VPD) and precipitation during the main growing season showed the highest and significant correlations with leaf water 18O (Δ18Olw) at lower-altitude forests, suggesting that gs was strongly controlled by moisture conditions in the growing season particularly in the lower-altitude forests. These findings shed light to a better understanding of the regulatory mechanism of iWUE changes under the background of rising atmospheric CO2 and climate change and serve to improve the reliability of ecophysiological modeling.

Do 2 H and 18 O in leaf water reflect environmental drivers differently?

SummaryWe compiled hydrogen and oxygen stable isotope compositions (δ2H and δ18O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2H was more closely correlated with δ2H of xylem water or atmospheric vapour, whereas leaf water δ18O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2H and Δ18O) to remove the impact of xylem water isotopic variation. For Δ2H, leaf water still correlated with atmospheric vapour, whereas Δ18O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2H and 18O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Role of photosynthesis and stomatal conductance on the long-term rising of intrinsic water use efficiency in dominant trees in three old-growth forests in Bosnia-Herzegovina and Montenegro

Old-growth forests have an important role in maintaining animal and plant diversity, are important carbon (C) reservoirs and are privileged sites to study long-term plant physiological responses, long-term forest dynamics and climate change impact on forest ecosystems. Several studies have highlighted how old-living trees undergo age-related declines with hydraulic limitations and reduction in photosynthesis, though some recent works have suggested that such a decline is not always observed. Our study aims at understanding the role of atmospheric CO2 increase on tree C uptake and stomatal conductance (gs) in old-living trees by analysing the long-term patterns of tree growth and intrinsic water use efficiency (iWUE) in three old-growth forests in the Balkans (Bosnia-Herzegovina and Montenegro), using dendrochronology and isotopic analysis. We hypothesised a long-term increase in iWUE in the studied old-growth forests, mostly related to enhanced photosynthesis rather than reduced stomatal conductance. Tree cores were sampled from dominant silver fir (Abies alba Mill.) trees in each forest. Tree-ring widths were measured and basal area increments (BAI) were assessed for each sampled tree and, from the six longest chronologies, five decades were chosen for cellulose extraction, its isotopic analysis (δ13C, δ18O), iWUE and leaf water 18O evaporative enrichment above the source water (Δ18OL) determination. We observed a continuous and significant increase in iWUE from 1800 to 2010 in the sampled dominant trees at all the three old-growth forests. Our BAI data and our estimates of Δ18OL across the study period support the idea that enhanced photosynthesis rather than reduced stomatal conductance is the major driver of the measured iWUE increase. Thus, our results support some recent findings challenging the hypothesis that iWUE in forests is primarily the result of a CO2-induced reduction in stomatal conductance as well as the so called hydraulic limitation hypothesis.

Leaf transition from heterotrophy to autotrophy is recorded in the intraleaf C, H and O isotope patterns of leaf organic matter

Quantitatively relating 13 C/12 C, 2 H/1 H and 18 O/16 O ratios of plant α-cellulose and 2 H/1 H of n-alkanes to environmental conditions and metabolic status should ideally be based on the leaf, the plant organ most sensitive to environmental change. The fact that leaf organic matter is composed of isotopically different heterotrophic and autotrophic components means that it is imperative that one be able to disentangle the relative heterotrophic and autotrophic contributions to leaf organic matter. We tackled this issue by two-dimensional sampling of leaf water and α-cellulose, and specific n-alkanes from greenhouse-grown immature and mature and field-grown mature banana leaves, taking advantage of their large areas and thick waxy layers. Leaf water, α-cellulose and n-alkane isotope ratios were then characterized using elemental analysis isotope ratio mass spectrometry (IRMS) or gas chromatography IRMS. A three-member (heterotrophy, autotrophy and photoheterotrophy) conceptual linear mixing model was then proposed for disentangling the relative contributions of the three trophic modes. We discovered distinct spatial leaf water, α-cellulose and n-alkane isotope ratio patterns that varied with leaf developmental stages. We inferred from the conceptual model that, averaged over the leaf blade, only 20% of α-cellulose in banana leaf is autotrophically laid down in both greenhouse-grown and field-grown banana leaves, while approximately 60% and 100% of n-alkanes are produced autotrophically in greenhouse-grown and field-grown banana leaves, respectively. There exist distinct lateral (edge to midrib) gradients in autotrophic contributions of α-cellulose and n-alkanes. Efforts to establish quantitative isotope-environment relationships should take into account the fact that the evaporative leaf water 18 O and 2 H enrichment signal recorded in autotrophically laid down α-cellulose is significantly diluted by the heterotrophically formed α-cellulose. The δ2 H value of field-grown mature banana leaf n-alkanes is much more sensitive than α-cellulose as a recorder of the growth environment. Quantitative isotope-environment relationship based on greenhouse-grown n-alkane δ2 H values may not be reliable.

Simulation of δ18O in Platycladus orientalis leaf water with different kinetic fractionation coefficients

Clarifying 18O isotope composition of leaf water (δL,b) would provide theoretical refe-rence for the study of leaf physiology and forest hydrology. We continuously monitored the concentration of atmospheric water vapor (Wa) and 18O isotope composition of atmospheric water vapor (δv) at the canopy of Platycladus lateralis plantation in the mountain area of Beijing. We analyzed the effects of kinetic fractionation coefficients Δk1(32%) and Δk2(28%) on the prediction of δL,b by combining the measured leaf water 18O isotope (δx) and δL,b of P. lateralis. The results showed that the diurnal variation of Wa was irregular. Atmospheric relative humidity (RH) showed a "V" shape of diurnal variation, and stomatal conductance (gs) increased first and then decreased at the diurnal scale. Wa, RH, and gs showed a significant negative correlation with δL,b when isotopes approached a steady-state equilibrium around noon. The kinetic fractionation coefficient Δk1 and Δk2 were applied to the Craig-Gordon model to predict δL,b under the isotopic quasi-steady-state condition. The results showed that the predicted values of Δk2 approached the observed values of δL,b. This result indicated that the application of Δk2 to the model was more consistent with the change of water isotope concentration in the leaves of P. lateralis in the mountain area of Beijing. These results would improve our understanding of water isotope enrichment model and evapotranspiration resolution model in leaves.

Partitioning oak woodland evapotranspiration in the rocky mountainous area of North China was disturbed by foreign vapor, as estimated based on non-steady-state 18O isotopic composition

In terrestrial ecosystems, partitioning ecosystem-scale evapotranspiration (ET) between plant transpiration and soil evaporation remains a technical challenge. In this paper, we used a newly-developed laser-based isotope analyzer (OA-ICOS) and the Keeling plot approach to partition ET components of a Quercus variabilis plantation in a lithoid hilly area of north China. The results showed that, on day-of-year (DOY) 254, 257, and 263, the modeled leaf water 18O composition (δsL,b) and observed leaf water 18O composition (δL,b) were in substantial agreement and had a significant linear correlation with coefficient of 0.96, indicating that Keeling plot approach and Graig–Gorden model can be used in portioning ET between plant transpiration and soil evaporation in a terrestrial forest ecosystem. Isotopic partition revealed that the percent contribution of transpiration to total ET increased from the morning, reached maximum values at noon, with maximum values of 91.19%, 86.30%, and 85.37% for DOY 254, 257, and 263, respectively, indicating the transpiration from Q. variabilis Blum contributed the most to the total ET in this forest ecosystem. On DOY 260, the stability stratification was unstable, which resulted from the foreign vapor concentration. The increased vapor concentration led to a 80.83% difference between δsL,b and δL,b. The correlation coefficients between δsL,b and δL,b decreased from 0.96 to 0.43 when dataset on DOY 260 was included, indicating foreign vapor increased the uncertainty in the estimation of δ18O of ET (δET) and δ18O of transpiration (δT) in the forest ecosystem. Path analysis results suggested that water vapor concentration was the major factor influencing the partitioning of ET with isotopic approach in the forest ecosystem. Since the increased water vapor concentration and decreased atmosphere δ18O disturbed the estimation of δE, δT, δET, the isotopic approach cannot be used in partitioning ET under this condition on DOY 260. Therefore, under most circumstances the isotopic approach can be used to partition ET for forest ecosystem in a non-steady state (NSS), while water vapor concentration may cause uncertainties.

Modeling biophysical controls on canopy foliage water 18O enrichment in wheat and corn

AbstractLeaf water 18O enrichment is an important factor controlling the H218O, C18OO, and O18O exchanges between the biosphere and the atmosphere. At present, there is limited capacity to explain the enrichment mechanisms in field conditions. In this study, three models of varying complexity were used to simulate the leaf water 18O enrichment at the canopy scale. Comparisons were made among the models and with high‐frequency isotopic measurements of ecosystem water pools in wheat and corn. The results show that the steady state assumption was a better approximation for ecosystems with lower canopy resistance, that it is important to consider the effect of leaf water turnover in modeling the enrichment and not necessary to deal with time changes in leaf water content, and that the leaf‐scale Péclet effect was incompatible with the big‐leaf modeling framework for canopy‐air interactions. After turbulent diffusion has been accounted for in an apparent kinetic factor parameterization, the mean 18O composition of the canopy foliage water was a well‐behaved property predictable according to the principles established by leaf‐scale studies, despite substantial variations in the leaf water enrichment with leaf and canopy positions. In the online supplement we provided a discussion on the observed variability of leaf water 18O composition with leaf and canopy positions and on the procedure for correcting isotopic measurements for organic contamination.

Oxygen isotope composition of evapotranspiration and its relation to C4photosynthetic discrimination

[1] The oxygen isotope of water (18O-H2O) and carbon dioxide (18O-CO2) is an important signal of global change and can provide constraints on the coupled carbon-water cycle. Here, simultaneous observations of 18O-H2O (liquid and vapor phases) and 18O-CO2 were used to investigate the relation between canopy leaf water 18O enrichment, 18O-CO2 photosynthetic discrimination (18Δ), isotope disequilibrium (Deq), and the biophysical factors that control their temporal variability in a C4 (Zea mays L.) ecosystem. Data and analyses are presented from a 74 day experiment conducted in Minnesota during summer 2009. Eddy covariance observations indicate that the oxygen isotope composition of C4 evapotranspiration (δE) ranged from about −20‰ (VSMOW scale) in the early morning to −5‰ after midday. These values were used to estimate the isotope composition at the sites of leaf water evaporation (δL,e) assuming non-steady-state conditions and revealed a strong diurnal pattern ranging from about −5‰ in the early morning to +10‰ after midday. With the addition of net ecosystem CO2 exchange measurements and carbonic anhydrase (CA) assays, we derived canopy-scale 18Δ. These estimates typically varied from 11.3 to 27.5‰ (VPDB scale) and were shown to vary significantly depending on the steady state or non-steady-state assumptions related to leaf water enrichment. We demonstrate that the impact of turbulence on kinetic fractionation and steady state assumptions result in larger estimates of 18Δ and Deq. Further, the results indicate that both leaf-scale and canopy-scale CO2 hydration efficiency may be substantially lower than that previously reported for laboratory conditions. These results may have important implications for interpreting variations in atmospheric 18O-CO2 and constraining regional carbon budgets based on the oxygen isotope tracer approach.

Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus

We measured oxygen isotope ratios (delta18O) of xylem sap, phloem sap, leaves, wood and bark of Eucalyptus globulus Labill. growing in southwestern Australia. Carbon isotope ratios (delta13C) were measured in the dry matter of phloem sap, leaves and wood. Results were used to test several aspects of a mechanistic model of 18O enrichment and provided insights into post-photosynthetic variations in dry matter delta13C. Xylem water delta18O varied little within the tree crown, whereas variation at the landscape-level was more pronounced, with plantations near the coast being enriched by up to 3 per thousand compared with plantations less than 100 km inland. Phloem water was significantly enriched in 18O compared with xylem water in two of three sampling campaigns; mean enrichments were 0.5 and 0.8 per thousand. Phloem sap sugars exported from E. globulus leaves closely reflected observed leaf water enrichment when diurnal variation in photosynthesis was taken into account. Photosynthetic rates were higher in the morning than in the afternoon, whereas leaf water 18O enrichment increased to maximum values in the afternoon. A non-steady-state model of leaf water 18O enrichment accurately predicted observed values through a full diel cycle. Mean estimates of the proportion of organic oxygen effectively exchanging with xylem water during cellulose synthesis were close to 0.40 for both leaves and wood. Carbon isotope ratios of nascent xylem tissues did not differ from those of phloem sap sugars collected concurrently, whereas nascent leaf tissues were depleted in 13C by 2 per thousand compared with phloem sap sugars, suggesting that, in E. globulus, 13C enrichment of sink tissues compared with source leaves does not result from an enriching process within the sink tissue.

Separating soil and leaf water 18O isotopic signals in plant stem cellulose

The oxygen-18 signal of soil and leaf water are both recorded in heterotrophically synthesized plant stem cellulose. Presently, these signals can only be teased apart with modeling and assumptions on the nature of the isotopic enrichment of leaf water. A method by which these two signals are chemically separated and analyzed is tested here. Heterotrophically synthesized cellulose from germinating seeds having a mixture of isotopic signals from the reserve carbohydrate (starch) and that of the water during cellulose synthesis was hydrolyzed and the resulting glucose converted to glucose phenylosazone. The analysis of the 18O/ 16O ratios of cellulose and of glucose phenylosazone were used to calculate the oxygen isotope ratio of the oxygen attached to the second carbon of the glucose moieties of the cellulose molecule. The calculated δ 18O value of this oxygen was highly correlated with that of the water available for cellulose synthesis showing a nearly one-to-one relationship (slope = 1.027) and leading to the conclusion that it completely exchanges with water during heterotrophic cellulose synthesis. Once this method is refined so as to increase precision, it will be possible to derive the δ 18O values of soil water available to plants from the oxygen isotope analysis of stem cellulose and its derivative.

Oxygen isotope composition of phloem sap in relation to leaf water in Ricinus communis.

We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The 18O / 16O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of αo=1.027, where αo=Ro / Rw , and Ro and Rw are 18O / 16O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14-22‰ compared with source water under our experimental conditions, and depleted by 4-7‰, compared with evaporative site water. This showed that it is the average lamina leaf water 18O / 16O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the 18O / 16O signatures of plant organic material and plant cellulose.

Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis

ABSTRACTThe 18O content of CO2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO2 and 18O‐enriched leaf water and retrodiffusion of most of this CO2 back to the atmosphere, leaves effectively discriminate against 18O during photosynthesis. Discrimination against 18O (Δ18O) is expected to be lower in C4 plants because of low ci and hence low retrodiffusing CO2 flux. C4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C3 plants. Low CA may limit the extent of 18O exchange and further reduce Δ18O. We investigated CO2–H2O isotopic equilibrium in plants with naturally low CA activity, including two C4 (Zea mays, Sorghum bicolor) and one C3 (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ18O in C4, as well as in some C3, plants. Variations in CA activity and in the extent of CO2–H2O isotopic equilibrium (θeq) estimated from on‐line measurements of Δ18O showed large range of 0–100% isotopic equilibrium (θeq= 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ18O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θeq< 1 (incomplete isotopic equilibrium) on 18O of atmospheric CO2 can be similar for C3 and C4 plants and in both cases it increases with natural enrichment of 18O in leaf water.

The Fractionation of 2H and 18O in Leaf Water of Barley

Small changes in the natural abundance of hydrogen and oxygen isotopes in water occur as a result of transpiration. We considered whether these may be sufficient to study the differences in water relations of barley cultivars having different water-use efficiency (W). A range of cultivars of varying W were grown in the glasshouse and in the field in breeders' plots. Foliage water samples were taken from a specific leaf stage over the course of a day. Certain field-grown cultivars had consistently higher 2H and 18O compositions in foliage water than others at almost all sampling times, resulting in a consistent ranking of the cultivars. Some evidence of a consistent ranking of isotopic compositions was also present in the glasshouse experiment. Concurrent variations in 2H and 18O compositions of the stem and leaf water reflect the conditions under which transpiration is occurring. The composition of stem water remains steady over the course of such experiments. The slope of the line connecting the isotopic compositions of the stem water and leaf water varied with changes in leaf water composition. This variation in slope was more pronounced under the more humid conditions of the glasshouse experiment, as is predicted from theory. The 2H and 18O compositions of water in the other plant parts, atmosphere and soil were also examined and porometry measurements were made. These indicated that daily variations in the contributions of soil and leaf water to the atmosphere occurred. This involves a maximum in the proportion of soil evaporation in the middle of the day, and a converse maximum in flag leaf evaporation early and late in the solar day.