Vapor Conductance Research Articles

Changes in photon flux can induce stomatal patchiness

ABSTRACTImages of chlorophyll fluorescence were used to detect the occurrence of stomatal patchiness in leaves from eight species under variable photon flux conditions. Pronounced stomatal patchiness was induced within 5–10 min after PFD was changed from intermediate (∼450 μmol quanta m−2 s−1) to low (∼150 μmol quanta m−2 s−1) levels. This effect was completely reversible by returning PFD to intermediate levels. The pattern of heterogeneous fluorescence for each leaf was usually similar during repeated applications of medium and low PFD. In three species, stomatal patchiness could only be induced in slightly water‐stressed plants. Leaves of more severely water‐stressed Xanthium strumarium plants in low air humidity exhibited oscillations in fluorescence that corresponded with oscillatory changes in leaf diffusion conductance for water vapour. Stomatal patchiness was also induced by illuminating dark‐adapted leaves with low PFD (below 200–300 μmol quanta m−2 s−1). Infiltration of leaves with distilled water showed that heterogeneous chlorophyll fluorescence was caused by changes in stomatal apertures.

Infrared Raman scattering as a sensitive probe for the thermal conductivity of chemical vapor deposited diamond films

The infrared Raman spectrum of chemical vapor deposited (CVD) diamond films has been correlated with the in-plane thermal conductivity of the films. The scattering strength of the 1332 cm−1 zone-center phonon line of diamond, measured relative to the intensity of the nondiamond carbon phase, was found to increase strongly with increasing thermal conductivity. A good correlation between these two properties was found even for the highest quality CVD diamond films with peak thermal conductivities up to 54 W/cm K. The dependence of the peak thermal conductivity on the intensity of the 1332 cm−1 phonon line normalized to the scattering strength of the nondiamond carbon phase can be described by a power law with an exponent of 0.5.

Influence of an overstorey tree (Prosopis glandulosa) on associated shrubs in a savanna parkland: implications for patch dynamics.

The arborescent legume, honey mesquite (Prosopis glandulosa), appears to play a central role in patch dynamics of southern Texas savannas by modifying soils and microclimate and by facilitating the ingress, establishment and/or growth of shrubs in its understorey. As an indirect test for the occurrence and persistence of facilitation in mature shrub clusters (patches), we examined the gas exchange, water relations and production of associated shrubs growing in patches where a Prosopis overstorey was present and in patches where Prosopis had succumbed to natural mortality. Surface (0-10 cm) soils associated with shrub patches were enriched in total [N] and [C] compared to soils of neighboring herbaceous zones. However, there were no detectable differences in soil [N] or [C] in patches with and without Prosopis. Foliar [N] and biomass of various shrub species were also statistically comparable for patches with and without Prosopis. These results are in accordance with other studies that indicate the nutrient legacy associated with Prosopis occupation of a patch may persist for decades after its demise. In comparison to plants growing in the absence of Prosopis, leaf water potentials (predawn and midday), and net photosynthesis and water vapor conductance (morning and midday) of outer-canopy sunlit leaves over an annual growth cycle were comparable for two common evergreen shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing in patches with a live Prosopis. These findings indicate that the presence of Prosopis was not enhancing the growth or activity of mature understorey shrubs; facilitation may, therefore, be important only during early stages of cluster development. In addition, we found no indication that the loss of Prosopis has initiated a downward phase in a cyclic succession of patch initiation, growth and death. Rather, the understorey shrubs appear to be able to maintain growth and productivity in the absence of a Prosopis overstorey, and may, therefore, represent persistent components of woody patches on these savanna landscapes.

Effects of crop residue on soil and plant water evaporation in a dryland cotton system

Dryland agricultural cropping systems emphasize sustaining crop yields with limited use of fertilizer while conserving both rain water and the soil. Conservation of these resources may be achieved with management systems that retain residues at the soil surface simultaneously modifying both its energy and water balance. A conservation practice used with cotton grown on erodible soils of the Texas High Plains is to plant cotton into chemically terminated wheat residues. In this study, the partitioning of daily and seasonal evapotranspiration (Et) into soil and plant water evaporation was compared for a conventional and a terminated-wheat cotton crop using the numerical model ENWATBAL. The model was configured to account for the effects of residue on the radiative fluxes and by introducing an additional resistance to latent and sensible heat fluxes derived from measurements of wind speed and vapor conductance from a soil covered with wheat-stubble. Our results showed that seasonalEt was similar in both systems and that cumulative soil water evaporation was 50% ofEt in conventional cotton and 31% ofEt in the wheat-stubble cotton. Calculated values ofEt were in agreement with measured values. The main benefit of the wheat residues was to suppress soil water evaporation by intercepting irradiance early in the growing season when the crop leaf area index (LAI) was low. In semiarid regions LAI of dryland cotton seldom exceeds 2 and residues can improve water conservation. Measured soil temperatures showed that early in the season residues reduced temperature at 0.1 m depth by as much as 5°C and that differences between systems diminished with depth and over time. Residues increased lint yield per unit ofEt while not modifying seasonalEt and reducing cumulative soil water evaporation.

Properties and Variability of Soil and Trench Fill at an Arid Waste‐Burial Site

AbstractArid sites commonly are assumed to be ideal for long‐term isolation of wastes. Information on properties and variability of desert soils is limited, however, and little is known about how the natural site environment is altered by installation of a waste facility. During fall construction of two test trenches next to the waste facility on the Amargosa Desert near Beatty, NV, samples were collected to: (i) characterize physical and hydraulic properties of native soil (upper 5 m) and trench fill, (ii) determine effects of trench construction on selected properties and vertical variability of these properties, and (iii) develop conceptual models of vertical variation within the soil profile and trench fill. Water retention was measured to air dryness (ψ = 2 × 106 cm water suction). The 15 300‐cm pressure‐plate data were omitted from the analysis because water‐activity measurements showed the actual suction values were significantly less than the expected 15 300‐cm value (avg. difference = 8550 ± 2460 cm water). Trench construction significantly altered properties and variability of the natural site environment. For example, water content ranged from 0.029 to 0.041 m3 m‐3 for fill vs. 0.030 to 0.095 m3 m‐3 for soil; saturated hydraulic conductivity was ≈ 10‐4 cm s‐1 for fill vs. 10‐2 to ≈ 10‐4 cm s‐1 for soil. Statistical analyses showed that the native soil may be represented by three major horizontal components and the fill by a single component. Under initial conditions, calculated liquid conductivity (Kl) plus isothermal vapor conductivity (Kv) for the upper two soil layers and the trench fill was ≈ 10‐13 cm s‐1, and Kl was ≤ Kv. For the deeper (2–5 m) soil, total conductivity was ≈ 10‐10 cm s‐1, and Kl was >Kv. This study quantitatively describes hydraulic characteristics of a site using data measured across a water‐content range that is representative of arid conditions, but is seldom studied.

Water Limitations and Plant Community Development in a Polar Desert

Polar deserts of the High Arctic contain vast areas of minimal plant cover and low primary productivity. Significant development of polar desert plant communities is largely restricted to areas with considerable cover of a cryptogamic soil crust, which develops in sites exposed to continued surface runoff from melting snow for some of the short growing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in polar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over three growing seasons in a typically barren ("noncrusted") site and a site with a well—developed cryptogamic crust and vascular plant community ("crusted"). Soil water content was consistently higher in the crusted site than the noncrusted site through all growing seasons. These differences had limited biological relevance because subsurface soils at both sites remained effectively saturated (soil water potential > —0.1 MPa) through each growing season, despite low amounts of precipitation that varied nearly twofold from year to year. However, the surface soils (0.5—2.0 cm in depth), especially in noncrusted sites, dry considerably in some years. There were no significant differences in plant water potential and midday values of whole—plant transpiration and water vapor conductance for adult plants growing in crusted vs. noncrusted sites. Water stress for established plants was minor in both sites. Greater plant community development in crusted areas of this polar desert does not result from a reduction in plant water stress by a greater supply of meltwater through the growing season. Instead, surface meltwaters probably benefit vascular plants indirectly by facilitating the growth, development, and nitrogen fixation activities of cryptogamic organisms in the soil crust. The presence and activity of these cryptogams favor vascular plant success through increased nutrient availability, soil organic matter, surface temperatures, reduced soil cryoturbation, and more favorable sites for germination and seedling establishment.

Estimating canopy conductance to ozone uptake from observations of evapotranspiration at the canopy scale and at the leaf scale*

AbstractStomatal uptake by vegetation is often the major sink for the destruction of tropospheric ozone. Using data obtained during the summer of 1991 at a grape vineyard and a cotton field in the San Joaquin Valley of California, we compare canopy (stomatal) conductances to ozone estimated (1) from eddy covariance ozone flux data (2) from eddy covariance evapotranspiration data and (3) by scaling leaf trans pi rational conductance to the canopy level using a canopy radiative transfer model. These simultaneous data, obtained at two levels of biological organization and for two trace gases, allow us to contrast the pathways for canopy‐atmosphere exchange of water vapour and ozone, to evaluate limitations to scaling from leaf to canopy, and to predict ozone uptake parameters from those governing transpiration. At the vineyard site the eddy covariance ozone results underestimate the ET‐based (eddy covariance and leaf scaling) approaches between 25% and 36%. At the cotton site the ozone‐based results overestimate the ET‐based approaches between 9% and 62%. A number of modelling and measurement uncertainties are of appropriate magnitude to reconcile these estimates. Some of the possible causes for these discrepancies that are discussed include NO effects, mesophyll resistances to ozone uptake and flaws in the K‐theory (first‐order closure) approach on which the canopy‐scale analysis is based. Nevertheless, both canopy and single leaf measurements of conductance for water vapour provide acceptable estimates of conductance for ozone, but further experiments in which all are measured simultaneously are suggested.

Porometric measurements indicate wound severity and tuber maturity affect the early stages of wound-healing

We demonstrated that porometrically derived wound-healing profiles of whole tubers were greatly affected by the depth or severity of the wound. A rapid decrease in water vapor conductance was detected during the first 24 h of healing after mature, whole tubers were wounded by tangentially cutting into the cortex. The rapid decline in vapor conductance, indicating deposition of soluble waxes, was not concurrent with detectable deposition of polymeric phenolic or polymeric aliphatic components of the suberin polyester; instead these polymerized components were detected after vapor conductance decreased by approximately 80% and was nearly stable. The rate of decline of vapor conductance of wound-healing tubers was hormonally hastened with abscisic acid treatments. Although deeper tangential wounds (3.0 mm) had slower initial rates of wound-healing than did shallow wounds (0.75 mm) through tuber growth and maturation, the ability to wound-heal more rapidly increased as the tubers matured in the field. The vapor conductance of wound healing tubers declined in a log-linear fashion during the first 1 to 3 days after wounding depending upon tuber maturity and genotype.

Factors affecting hatchability during commercial incubation of Ostrich (Struthio camelus)eggs

1. A batch of 320 ostrich eggs from 9 different farms in Zimbabwe were incubated in a single stage operation and the fate of each was recorded. 2. Hatchability was only 37.2% and the result of high rates of infertility and contamination (22.2% and 22.8% respectively); it varied between eggs from different farms. 3. Embryonic mortality was high at the start and end of incubation, a pattern similar to that of other domestic birds. 4. Mortality of late stage embryos was related to percentage water loss and mass specific water vapour conductance of the shell, with extremes of the ranges causing the highest mortality. 5. Microbial contamination of the eggs was a significant problem and varied in eggs from different farms indicating that more attention is needed in both breeder bird and nest management.

Gas conductance and metabolism of shorebird eggs: variation within and between species

Fresh egg mass (M 0; g), water vapor conductance of the egg shell (G H 2O ; mg·[Torr·d] −1), and neonate mass (M n; g) were measured in the ruff ( Philomachus pugnax), common redshank ( Tringa totanus), northern lapwing ( Vanellus vanellus), black-tailed godwit ( Limosa limosa), and Eurasian curlew ( Numenius arquata). In addition, the development of embryonic O 2 consumption (Ṁ O 2 ; ml·d −1) and CO 2 production (Ṁ CO 2 ; ml·d −1) were measured in these species, except the ruff. In northern lapwing and black-tailed godwit eggs the coefficients of variation for G H 2O were 3.8 and 2.3 times higher, respectively, than those for M 0. In these two species only about 10% of the variation for G H 2O was attributable to M 0, and about 77% to differences between clutches, suggesting a strong maternal component. In the northern lapwing, embryonic Ṁ O 2 plateaued prior to internal pipping, but not in the common redshank and black-tailed godwit. The latter result is in contrast to embryonic patterns previously described for other precocial species. In shorebirds the occurrence of an embryonic Ṁ O 2 plateau is not related to the neonatal level of cold-induced thermogenesis.

The influence of ozone and nutrition on δ13C in Betula pendula.

In the cellulose of stems and leaves, δ13C was investigated in a birch clone (Betula pendula), which was exposed throughout the growing season to either <3 (control) or 90/40 nl O3 1-1 (day/night). Each regime was split into plants under high or low nutrient supply. δ13C was increased (becoming less negative), in stems rather than leaves, by both high nutrition (+2‰) and O3 stress (+1‰). Whereas high nutrition raised the wateruse efficiency (WUE) while lowering the CO2 concentration in the inner leaf air space (c i), WUE decreased and c i increased under O3 stress. Therefore, only the nutritional effect on the carbon isotope fractionation was reproduced by the model of Farquhar et al. (1982) which estimates WUE by means of δ13C based on c i. c i was not biased by 'patchiness' in respect to stomatal opening. The latter was verified by microscopical analysis and the complete water infiltration of the birch leaves through the stomata, independent of the diurnal course of the leaf conductance for water vapour. Under low nutrient supply, the activity of phosphoenol pyruvate carboxylase (PEPC) was roughly doubled by ozone to about 1.3% of the total carboxylation capacity (by PEPC + rubisco), and was increased to 1.7% under high nutrition. The fractionation model, extended to account for varying activities of the carboxylating enzymes, indicated that stimulated PEPC was the cause of elevated δ13C, although c i was increased under O3 stress. The stimulation of PEPC and, as a consequence, elevated δ13C are discussed as part of a whole-plant acclimation to O3 stress.

Physiological Responses of Leaves of Rumex obtusifolius to Damage by a Leaf Miner

1. Larvae of the leaf-mining fly Pegomya nigritarsis cause blotch mines on leaves of Rumex obtusifolius. 2. Effects of mines on net photosynthesis, water vapour conductance and stomatal functioning were measured. 3. Net photosynthesis of mined tissue was on average 20% of healthy tissue. 4. Water vapour conductance was significantly higher from the abaxial surface than the adaxial. Conductance from the adaxial surface of younger (smaller) mines was significantly lower than from adjacent healthy adaxial epidermis. For older (larger) mines, conductance was higher from the mined area than from adjacent healthy lamina. 5. Water loss from detached leaves with mines occurred at a faster rate than from undamaged leaves. 6. Stomata in the epidermis above mines did not function properly. In smaller mines stomata tended to remain closed whereas in older mines stomata tended to remain open. 7. Consequences arising from insect damage in the field are discussed.

Techniques for detecting and measuring developmental and maturational changes in tuber native periderm

We developed a technique to quantitate native periderm permeability during tuber growth and maturation by porometrically determining vapor conductances. We also describe a simple, inexpensive means of determining the corresponding weight per unit area of suberized phellem cells (skin) in native periderm during tuber growth and maturation as a means of assessing the developmental status of native periderm. The porometric technique is very sensitive and rapid. Porometrically derived vapor conductances of mature native periderms were measured without difficulty, and native periderms were found to release ~0.005 moles of water vapor/meter2 native periderm surface/sec. into a dry test atmosphere. Immature native periderm released up to 28 times more water vapor than mature periderm even though the phellem cells were histochemically determined to possess both major components of suberin. We demonstrated use of the porometric technique by determining maturational changes in periderm permeability of growing/immature tubers and mature tubers at harvest. The technique revealed that the native periderm of Russet Burbank tubers was more permeable and had the potential to lose more water vapor, until becoming permeably mature, than Norchip or Norland tubers harvested on the same dates. The dry weight of the skin (phellem tissue) of native periderm (mg/ cm2) for all three cultivars increased during tuber growth, but plateaued to a relatively constant level, or slightly declined, while plants were still alive during the latter period of tuber growth. During this period, periderm vameable conductances were undergoing substantial declines. Comparison of skin dry weights of immature tubers indicated that stem end regions of tubers were developmentally ahead of equatorial and bud end regions; this difference decreased as the tubers matured at harvest time and was less apparent with some cultivars.

Gas exchange and growth responses to elevated CO2 and light levels in the CAM species Opuntia ficus‐indica

ABSTRACTGas exchange and dry‐weight production in Opuntia ficus‐indica, a CAM species cultivated worldwide for its fruit and cladodes, were studied in 370 and 750 μmol mol−1 CO2 at three photosynthetic photon flux densities (PPFD: 5, 13 and 20 mol m−2 d−1). Elevated CO2 and PPFD enhanced the growth of basal cladodes and roots during the 12‐week study. A rise in the PPFD increased the growth of daughter cladodes; elevated CO2 enhanced the growth of first‐daughter cladodes but decreased the growth of the second‐daughter cladodes produced on them. CO2 enrichment enhanced daily net CO2 uptake during the initial 8 weeks after planting for both basal and first‐daughter cladodes. Water vapour conductance was 9 to 15% lower in 750 than in 370 μmol mol−1 CO2. Cladode chlorophyll content was lower in elevated CO2 and at higher PPFD. Soluble sugar and starch contents increased with time and were higher in elevated CO2 and at higher PPFD. The total plant nitrogen content was lower in elevated CO2. The effect of elevated CO2 on net CO2 uptake disappeared at 12 weeks after planting, possibly due to acclimation or feedback inhibition, which in turn could reflect decreases in the sink strength of roots. Despite this decreased effect on net CO2 uptake, the total plant dry weight at 12 weeks averaged 32% higher in 750 than in 370 μmol mol−1 CO2. Averaged for the two CO2 treatments, the total plant dry weight increased by 66% from low to medium PPFD and by 37% from medium to high PPFD.

Whole‐plant gas exchange responses of Spartina alterniflora (Poaceae) to a range of constant and transient salinities

A two‐chamber‐system was used to study whole‐plant gas exchange responses of Spartina alterniflora to long‐term and transient salinity treatments over the range of 5 to 40 ppt NaCl. Lower photosynthetic rates, leaf water vapor conductances, belowground respiration rates, and higher aboveground respiration rates in plants adapted to 40 ppt NaCl were observed. Area‐specific leaf weight increased with salinity, although the salt content of leaf tissues did not. A reduced rate of gross photosynthesis and higher aboveground respiration rate in 40‐ppt NaCl plants significantly lowered the net whole‐plant CO2 gain below that of 5‐ppt NaCl plants, while the net CO2 gain of 25‐ppt NaCl plants was intermediate. Within 6 hr of increasing the salinity of 5‐ and 25‐ppt NaCl plants by 20 and 15 ppt NaCl, S. alterniflora responded by reducing leaf water vapor conductance, which in turn reduced the photosynthetic rate. This response was reversed by returning the plants to their original salinity, which indicates that S. alterniflora adjusts water loss and gas exchange in response to transient salinity stress by regulating stomatal aperture. On the other hand, decreasing salinity of the growth media of plants cultured at 25 and 40 ppt NaCl had little or no effect on gas exchange characteristics. This suggests that S. alterniflora adapts to constant salinity through fixed, salinity‐dependent structural modifications, such as stomatal density.

NEST MICROCLIMATE, EGG TEMPERATURE, EGG WATER LOSS, AND EGGSHELL CONDUCTANCE IN CAPE WEAVERS PLOCEUS CAPENSIS

Summary Brown, C.R. 1994. Nest microclimate, egg temperature, egg water loss, and eggshell conductance in Cape Weavers Ploceus capensis. Ostrich 65: 26–31. The nest of the Cape Weaver Ploceus capensis is an enclosed, kidney-shaped chamber of closely woven grass. Eggs are incubated by the female alone. Measurements of nest microclimate (temperature and water vapour pressure) were made, and egg water loss and water vapour conductance were measured. Mean nest temperatures and water vapour pressures were generally slightly higher than ambient values and probably related to presence of the female in the nest. Egg temperatures (TE) were relatively constant at night (mean = 36.9°8) when the female incubated continuously. During the da, despite the female's periodic absences from the nest, egg temperature were only slightly lower (35,9°C5 and overall egg temperature averaged 36,4°C. Eggs lost water at a rate of 45 mg day−1, close to that expected for a 3,6 g egg. The nest of Cape Weavers apparently plays little role in the regulation of egg water loss during incubation.

Calcium deficient diet, acetazolamide and gas exchange characteristics of avian eggshells

The effects of calcium deficient diet and acetazolamide on the gas exchange characteristics of avian eggshells were independently investigated in two groups of unmated hens ( Gallus domesticus). In one group, eggs were collected during both a normal diet (3.00% Ca) and a calcium deficient diet (0.34% Ca). In another group, eggs were collected both before and after acetazolamide administration (200 mg/kg) per os. Eggshell water vapor conductance (G H 2O ) increased 30% during the calsium deficient diet and was accompanied by a 21% decrease in eggshell thickness (L). Eggshell G H 2O increased 200% one day after acetazolamide administration and was not only accompanied by a 36% decrease in L, but also by an 89% increase in total functional pore area (A p). We conclude that a calcium deficient die increases G H 2O by eggshell thinning with litle effect on A p. On the other hand, acetazolamide profoundly increases G H 2O , not only by eggshell thinning but also by a remarkable increase in A p.

CO2 Exchange and Growth of the Crassulacean Acid Metabolism Plant Opuntia ficus-indica under Elevated CO2 in Open-Top Chambers.

CO2 uptake, water vapor conductance, and biomass production of Opuntia ficus-indica, a Crassulacean acid metabolism species, were studied at CO2 concentrations of 370, 520, and 720 [mu]L L-1 in open-top chambers during a 23-week period. Nine weeks after planting, daily net CO2 uptake for basal cladodes at 520 and 720 [mu]L L-1 of CO2 was 76 and 98% higher, respectively, than at 370 [mu]L L-1. Eight weeks after daughter cladodes emerged, their daily net CO2 uptake was 35 and 49% higher at 520 and 720 [mu]L L-1 of C02, respectively, than at 370 [mu]L L-1. Daily water-use efficiency was 88% higher under elevated CO2 for basal cladodes and 57% higher for daughter cladodes. The daily net CO2 uptake capacity for basal cladodes increased for 4 weeks after planting and then remained fairly constant, whereas for daughter cladodes, it increased with cladode age, became maximal at 8 to 14 weeks, and then declined. The percentage enhancement in daily net CO2 uptake caused by elevated CO2 was greatest initially for basal cladodes and at 8 to 14 weeks for daughter cladodes. The chlorophyll content per unit fresh weight of chlorenchyma for daughter cladodes at 8 weeks was 19 and 62% lower in 520 and 720 [mu]L L-1 of CO2, respectively, compared with 370 [mu]L L-1. Despite the reduced chlorophyll content, plant biomass production during 23 weeks in 520 and 720 [mu]L L-1 of CO2 was 21 and 55% higher, respectively, than at 370 [mu]L L-1. The root dry weight nearly tripled as the C02 concentration was doubled, causing the root/shoot ratio to increase with CO2 concentration. During the 23-week period, elevated CO2 significantly increased CO2 uptake and biomass production of O. ficus-indica.

Electrical conductivity and thermopower of Cs liquid and dense vapour

The electrical conductivity and thermopower of dense cesium vapour and expanded liquid cesium is calculated within the framework of linear response theory, expressing the transport properties in terms of equilibrium correlation functions. The theoretical values are compared with experimental results obtained recently.

Effects of doubled atmospheric carbon dioxide concentration on the responses of assimilation and conductance to humidity

ABSTRACTExperiments were performed to determine if growth at elevated partial pressure of CO2 altered the sensitivity of leaf water vapour conductance and rate of CO2 assimilation to the leaf‐to‐air difference in the partial pressure of water vapour (Δw). Comparisons were made between plants grown and measured at 350 and 700 μPa Pa−1 partial pressures of CO2 for amaranth, soybean and sunflower grown in controlled environment chambers, soybean grown outdoors in pots, and orchard grass grown in field plots. In amaranth, soybean and orchard grass, both the absolute and the relative sensitivity of conductance to Δw at the leaf surface were less in plants grown and measured at the elevated CO2. In sunflower, there was no change in the sensitivity of conductance to Δw for the two CO2 partial pressures. Tests in soybeans and amaranth showed that the change in sensitivity resulted from elevated CO2 during the measurement of the Δw response. Assimilation rate of CO2 was not altered by Δw in amaranth, which has C4 metabolism. In sunflower, the assimilation rate of plants grown and measured at elevated CO2 was insensitive to Δw, consistent with the response of assimilation rate to intercellular CO2 partial pressure in the prevailing range. In soybean, the sensitivity of assimilation rate to Δw was not different between CO2 treatments, in contrast to what would be expected from the response of assimilation rate to intercellular CO2 partial pressure.