Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.     

Contributions of evaporation, isotopic non-steady state transpiration and atmospheric mixing on the delta18O of water vapour in Pacific Northwest coniferous forests

Changes in the ²H and ¹⁸O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (δ¹⁸O) as high as 4‰ were observed for water vapour (δ¹⁸O_vp) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ¹⁸O_vp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the ¹⁸O of canopy water vapour over a 2‐d period by considering the ¹⁸O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ¹⁸O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ¹⁸O_vp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted ¹⁸O‐isoflux in the morning of day 1, causing values of δ¹⁸O_vp to decrease. An isotopically enriched ¹⁸O‐isoflux resulting from transpiration then offset this decreased δ¹⁸O_vp later during the day. Contributions of ¹⁸O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H₂¹⁶O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ¹⁸O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ ¹⁸O_vp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest ¹⁸O water balance.     

Water use efficiency as a function of leaf age and position within the cotton canopy

The gas exchange properties of whole plant canopies are an integral part of crop productivity and have attracted much attention in recent years. However, insufficient information exists on the coordination of transpiration and CO₂ uptake for individual leaves during the growing season. Single-leaf determinations of net photosynthesis (Pn), transpiration (E) and water use efficiency (WUE) for field-grown cotton (Gossypium hirsutum L.) leaves were recorded during a 2-year field study. Measurements were made at 3 to 4 day intervals on the main-stem and first three sympodial leaves at main-stem node 10 from their unfolding through senescence. Results indicated that all gas exchange parameters changed with individual main-stem and sympodial leaf age. Values of Pn, E and WUE followed a rise and fall pattern with maximum rates achieved at a leaf age of 18 to 20 days. While no significant position effects were observed for Pn, main-stem and sympodial leaves did differ in E and WUE particularly as leaves aged beyond 40 days. For a given leaf age, the main-stem leaf had a significantly lower WUE than the three sympodial leaves. WUE's for the main-stem and three sympodial leaves between the ages of 41 to 50 days were 0.85, 1.30, 1.36 and 1.95 μmol CO₂ mmol⁻¹ H₂O, respectively. The mechanisms which mediated leaf positional differences for WUE were not strictly related to changes in stomatal conductance (g_s·H₂O) since decreases in g_s·H₂O with leaf age were similar for the four leaves. However, significantly different radiant environments with distance along the fruiting branch did indicate the possible involvement of mutual leaf shading in determining WUE. The significance of these findings are presented in relation to light competition within the plant canopy during development.     

Modelling the isotope enrichment of leaf water

Farquhar and Gan [10] have proposed a model for the spatial variation in the isotopic enrichment of H₂¹⁸O across a leaf, which is specifically formulated for monocotyledoneous leaves. The model is based on the interaction between mass fluxes longitudinally within the xylem, and fluxes laterally through veinlets into the lamina mesophyll, where moisture leaves the leaf through transpiration. The lighter, more abundant, molecule H₂¹⁶O escapes preferentially with the evaporating water, resulting in the enrichment of H₂¹⁸O at these sites. Enriched water diffuses throughout the leaf, and it is this spatial distribution of enriched water which the model seeks to capture. In this paper we present a general formulation of the model in terms of mass flux, extending it to include variable transpiration rates across the leaf surface, as well as a tapering xylem. Solutions are developed for the general case and, since the solutions present in the form of Kummer functions, properties are established as well as methods for estimating the solutions under certain conditions relevant to the biology. The model output is compared with Gans data ([14, 15]) collected from maize plants.     

Transpiration and Calcium Deposition by Unifoliate Leaves of Phaseolus vulgaris Differing in Maturity

Unifoliate leaves were individually enclosed in clear, plastic chambers for the 24 hour treatment periods and then sacrificed for Ca analysis. Two transpiration rates were obtained by passing dry air through the chambers tising flow rates of 160 and 260 cm³/min. A third rate was obtained by a combination of shade and the lower air flow rate. Neither the transpiration rate nor solution-Ca concentration (0.5mM and 2.5 mM of 0.1, and 0.5 strength Hoagland solution) altered the amount of Ca deposited in the unifoliate leaves of 22 day old bean plants (Phaseolus vulgaris). The transpiration rate per unit area of leaf remained constant for all ages studied (1l–20 days) and was 1.8, 2.7, 3.6 g H₂O per dm² day for the three different imposed conditions. A definite pattern of Ca deposition occurred. With all the transpiration rates there was a maximum rate of calcium deposition at 13 days of growth and a gradual decrease thereafter. When the Ca concentration of the nutrient solution was 20 μg/ml the daily Ca deposition in terms of water transpired by the unifoliate leaves exceeded this amount, except for the oldest leaf tested, and, the maximum Ca to water ratios were 250, 320, and 430 (μg Ca/g) in order of decreasing transpiration rates. The uptake of Ca against a concentration gradient and approximately the same total uptake regardless of transpiration rates and solution concentrations used, firmly suggest that Ca secretion into root-xylem elements from a surrounding low level Ca solution requires energy expenditure by the plant. A possible explanation was proposed for the decreased rate of Ca deposition by the unifoliate leaves subsequent to the 13th day.     

Stomatal control of photosynthesis in detached leaves of woody and herbaceous plants

Photosynthesis and transpiration were assayed in leaves and needles of some woody species (Pinus sibirica Du Tour, P. sylvestris L., Larix sibirica Ledeb., Betula pendula Roth., Quercus robur L.), annual herbaceous plants (Amaranthus cruentus L. cv. Tampala, Celosia argentea L. f. cristata (L.), Gomphrena dispersa Standl., Solanum tuberosum L., Helianthus annuus L., H. tuberosus L.), and perennial herbs (Inula helenium L., Poligonum weyrichii F. Schmidt, Polymnia sonchifolia Poepp. & Endl.). A high-precision portable gas-analyzing system GFS-3000 with a climate-controlled chamber was used for measurements on leaves both before and after leaf detachment from the shoot under conditions optimal for photosynthesis: photosynthetically active radiation of 2000 μE/(m² s), 22–25°C, and a relative humidity of 65–70%. The steady-state gas exchange in illuminated leaves of all plant species examined was characterized by a directly proportional relationship between photosynthesis and transpiration (R ₂ = 0.87). This means that the temporal course of H₂O and CO₂ gas exchange in detached leaves suffices to characterize the status of stomatal control of photosynthesis. The general trend in the effect of leaf detachment, observed in herbaceous and woody plants, is that the stomatal control of photosynthesis was retained within first 3–5 min after leaf excision. By contrast, the increase in transpiration after leaf detachment was species-specific. Because of this circumstance, the measurements of transpiration by rapid weighing method may result in overestimation of transpiration rates by 10–15% for some plant species, compared to steady-state rates of gas exchange in undetached leaves.     

Strong seasonal disequilibrium measured between the oxygen isotope signals of leaf and soil CO2 exchange

The oxygen isotope composition (δ¹⁸O) of atmospheric CO₂ is among a very limited number of tools available to constrain estimates of the biospheric gross CO₂ fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO₂ and CO¹⁸O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ¹⁸O signals of foliage and soil CO₂ exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ¹⁸O signatures of net CO₂ fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ¹⁸O of CO₂ exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.     

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa^-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa^-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa^-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO₂ partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.     

Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ¹⁸O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸O relationship between leaf water and cellulose. We measured δ¹⁸O values of bulk leaf water (δ¹⁸O_LW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio) of more than 27‰ relative to δ¹⁸O_LW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸O_LW and δ¹⁸O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio = 25.1‰). Interestingly, damping factor p_exp_x, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸O_LW but not if modelled directly from δ¹⁸O of individual carbohydrates. We conclude that δ¹⁸O_LW is not always a good substitute for δ¹⁸O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ¹⁸O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.     

Two Steps of Gas Exchange in Leaf Photosynthesis

Photosynthesis and transpiration of excised leaves of Taraxacum officinale L. and a few other species of plants were measured, using an open gas analysis system. The rates of CO₂ uptake and transpiration increased in two steps upon illumination of stomata-bearing epidermis of these leaves at a light intensity of 50 mW × cm⁻². Abscisic acid inhibited only the second step of gas exchange. Illumination of the astomatous epidermis of hypostomatous leaves caused only the first step of gas exchange. These data indicate that the first and second steps arise from cuticular and stomatal gas exchange, respectively. The rate of the cuticular photosynthesis in a Taraxacum leaf reached saturation at a light intensity of 5 mW × cm⁻², and the rates of the stomatal photosynthesis and transpiration reached saturation at a higher intensity of 35 mW × cm⁻². The cuticular photosynthesis of a Taraxacum leaf was 18% of the stomatal photosynthesis at 50 mW × cm⁻² and 270% at 5 mW × cm⁻². The other species of leaves showed the same trend. The importance of cuticular CO₂ uptake in leaf photosynthesis, especially under low light intensity was stressed from these data.     

The After-effect of Wind on the Photosynthesis and Transpiration of Festuca arundinacea

The after-effect of wind on photosynthesis and transpiration of Festuca arundinacea Schreb, was determined. Following a period of exposure In a controlled environment wind tunnel the wind-treated plants showed reduced rates of photosynthesis when compared with the controls under standard conditions. Evaporation from paper model tillers was measured and the boundary layer resistance was shown to be low in all but very low wind speeds. Analysis of CO₂ and H₂O diffusion pathways indicated that mesophyll resistance in wind-treated plants was higher whilst leaf surface resistance was lower than in the controls. The high mesophyll resistance in the wind-treated plants was attributed to reduced water content.     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

Effects of an increase in summer precipitation on leaf, soil, and ecosystem fluxes of CO2 and H2O in a sotol grassland in Big Bend National Park, Texas

Global climate models predict that in the next century precipitation in desert regions of the USA will increase, which is anticipated to affect biosphere/atmosphere exchanges of both CO₂ and H₂O. In a sotol grassland ecosystem in the Chihuahuan Desert at Big Bend National Park, we measured the response of leaf-level fluxes of CO₂ and H₂O 1 day before and up to 7 days after three supplemental precipitation pulses in the summer (June, July, and August 2004). In addition, the responses of leaf, soil, and ecosystem fluxes of CO₂ and H₂O to these precipitation pulses were also evaluated in September, 1 month after the final seasonal supplemental watering event. We found that plant carbon fixation responded positively to supplemental precipitation throughout the summer. Both shrubs and grasses in watered plots had increased rates of photosynthesis following pulses in June and July. In September, only grasses in watered plots had higher rates of photosynthesis than plants in the control plots. Soil respiration decreased in supplementally watered plots at the end of the summer. Due to these increased rates of photosynthesis in grasses and decreased rates of daytime soil respiration, watered ecosystems were a sink for carbon in September, assimilating on average 31 mmol CO₂ m⁻² s⁻¹ ground area day⁻¹. As a result of a 25% increase in summer precipitation, watered plots fixed eightfold more CO₂ during a 24-h period than control plots. In June and July, there were greater rates of transpiration for both grasses and shrubs in the watered plots. In September, similar rates of transpiration and soil water evaporation led to no observed treatment differences in ecosystem evapotranspiration, even though grasses transpired significantly more than shrubs. In summary, greater amounts of summer precipitation may lead to short-term increased carbon uptake by this sotol grassland ecosystem.     

Water Balance of Birch and Larch Leaves and Their Resistance to Short and Progressive Soil Drought

In a greenhouse experiment, which imitated a short (4-day-long) and progressive (3-week-long) soil drought, an infrared gas analyzer was employed to assess transpiration (mol/(m² s)) and leaf transpiration conductivity (g _w, mol H₂O/(m² s)) in intact one-year-old plants of Betula platyphylla Suk. and Larix gmelini (Rupr.) Rupr. grown at the saturation levels of photosynthetically active radiation. The drought duration did not affect the adaptation of leaf water balance, it was determined only by leaf temperature and water supply. At ample water supply and with all other conditions being equal, the transpiration of birch exceeded that of larch by the factor of 3; however, birch was less tolerant to soil drought. The authors conclude that water supply and leaf temperature determine plant resistance to soil drought. The species-specific drought resistance depends on the g _w value.     

Variation in leaf physiology of <Emphasis Type="Italic">Salix arctica</Emphasis> within and across ecosystems in the High Arctic: test of a dual isotope (Δ<Superscript>13</Superscript>C and Δ<Superscript>18</Superscript>O) conceptual model

Leaf carbon isotope discrimination (Δ¹³C) varies with the balance between net photosynthesis (A) and stomatal conductance (g _s ). Inferences that can be made with Δ¹³C are limited, as changes could reflect variation in A and/or g _s . Investigators have suggested that leaf δ¹⁸O enrichment above source water (Δ¹⁸O) may enable differentiation between sources of variation in Δ¹³C, as leaf Δ¹⁸O varies with transpiration rate (E), which is closely correlated with g _s when leaves experience similar leaf to air vapor pressure differences. We examined leaf gas exchange of Salix arctica at eight sites with similar air temperatures and relative humidities but divergent soil temperatures and soil water contents near Pituffik, Greenland (76°N, 38°W). We found negative correlations at the site level between g _s and Δ¹⁸O in bulk leaf tissue (r ² = 0.62, slope = −17.9‰/mol H₂O m⁻² s⁻¹, P = 0.02) and leaf α-cellulose (r ² = 0.83, slope = −11.5‰ mol H₂O m⁻² s⁻¹, P < 0.01), consistent with the notion that leaf water enrichment declines with increasing E. We also found negative correlations at the site-level between intrinsic water-use efficiency (iWUE) and Δ¹³C in bulk leaf tissue (r ² = 0.65, slope = −0.08‰/μmol CO₂ /mol H₂O, P = 0.02) and leaf α-cellulose (r ² = 0.50, slope = −0.05 ‰/[μmol CO₂ /mol H₂O], P = 0.05). When increasing Δ¹³C was driven by increasing g _s alone, we found negative slopes between Δ¹³C and Δ¹⁸O for bulk leaf tissue (−0.664) and leaf α-cellulose (−1.135). When both g _s and A _max increased, we found steeper negative slopes between Δ¹³C and Δ¹⁸O for bulk leaf tissue (−2.307) and leaf α-cellulose (−1.296). Our results suggest that the dual isotope approach is capable of revealing the qualitative contributions of g _s and A _max to Δ¹³C at the site level. In our study, bulk leaf tissue was a better medium than leaf α-cellulose for application of the dual isotope approach.     

The role of leaf and canopy-level gas exchange in the replacement of Quercus virginiana (Fagaceae) by Juniperus ashei (Cupressaceae) in semiarid savannas

Photosynthesis, transpiration, and leaf area distribution were sampled in mature Quercus virginiana and Juniperus ashei trees to determine the impact of leaf position on canopy-level gas exchange, and how gas exchange patterns may affect the successful invasion of Quercus communities by J. ashei. Sampling was conducted monthly over a 2-yr period in 12 canopy locations (three canopy layers and four cardinal directions). Photosynthetic and transpiration rates of both species were greatest in the upper canopy and decreased with canopy depth. Leaf photosynthetic and transpiration rates were significantly higher for Q. virginiana (4.1–6.7 μmol CO₂·m⁻²·s⁻¹ and 1.1–2.1 mmol H₂O·m⁻²·s⁻¹) than for J. ashei (2.1–2.8 μmol CO₂·m⁻²·s⁻¹ and 0.7–1.0 mmol H₂O·m⁻²·s⁻¹) in every canopy level and direction. Leaves on the south and east sides of both species had higher gas exchange rates than leaves on the north and west sides. Although Quercus had a greater mean canopy diameter than Juniperus (31.3 vs. 27.7 m²), J. ashei had significantly greater leaf area (142 vs. 58 m²/tree). A simple model combining leaf area and gas exchange rates for different leaf positions demonstrated a significantly greater total canopy carbon dioxide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g CO₂·tree⁻¹·d⁻¹, respectively). Total daily water loss was also greater for Juniperus (125 vs. 73 Ltree⁻¹·d⁻¹). Differences in leaf gas exchange rates were poor predictors of the relationship between the invasive J. ashei and the codominant Q. virginiana. Leaf area and leaf area distribution coupled with leaf gas exchange rates were necessary to demonstrate the higher overall competitive potential of J. ashei.     

Water relations of stem succulent trees in north-central Baja California

Summary Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H₂O/m³ of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H₂O/m³. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m^–2 s^–1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Transition of stable isotope ratios of leaf water under simulated dew formation

Dew formation, a common meteorological phenomenon, is expected to intensify in the future. Dew can influence the H₂¹⁸O and HDO isotopic compositions of leaf water (δ_L), but the phenomenon has been neglected in many experimental and modelling studies. In this study, the dew effect on δ_L was investigated with a dark plant chamber in which dew formation was introduced. The H₂¹⁸O and HDO compositions of water vapour, dew water and leaf water of five species were measured for up to 48 h of dew exposure. Our results show that the exchanges of H₂¹⁸O and HDO in leaf water with the air continued in the darkness when the net H₂¹⁶O flux was zero. Our estimates of the leaf conductance using the isotopic mass balance method ranged from 0.035 to 0.087 mol m^?2 s^?1, in broad agreement of the night‐time stomatal conductance reported in the literature. In our experiments, the conductance of the C₄ species was 0.04 ± 0.01 mol m^?2 s^?1 and that of the C₃ plants was 0.10 ± 0.04 mol m^?2 s^?1. At the end of 16 h dew exposure, 72 (±17) and 94 (±11)% of the leaf water came from dew according to the ¹⁸O and D tracer, respectively.     

Indirect effects of insect herbivory on leaf gas exchange in soybean

Herbivory can affect plant carbon gain directly by removing photosynthetic leaf tissue and indirectly by inducing the production of costly defensive compounds or disrupting the movement of water and nutrients. The indirect effects of herbivory on carbon and water fluxes of soybean leaves were investigated using gas exchange, chlorophyll fluorescence and thermal imaging. Herbivory by Popillia japonica and Helicoverpa zea (Boddie) caused a 20–90% increase in transpiration from soybean leaflets without affecting carbon assimilation rates or photosynthetic efficiency (Φ_PSII). Mechanical damage to interveinal tissue increased transpiration up to 150%. The spatial pattern of leaf temperature indicated that water loss occurred from injuries to the cuticle as well as from cut edges. A fluorescent tracer (sulforhodamine G) indicated that water evaporated from the apoplast approximately 100 µm away from the cut edges of damaged leaves. The rate of water loss from damaged leaves remained significantly higher than from control leaves for 6 d, during which time they lost 45% more water than control leaves (0.72 mol H₂O per cm of damaged perimeter). Profligate water loss through the perimeter of damaged tissue indicates that herbivory may exacerbate water stress of soybeans under field conditions.