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The biochemical reactions of aerobic microbial respiration (AMR) suggest that silage temperature (Tsi) rise, oxygen (O2) consumption and carbon dioxide (CO2) emission may be equally useful as indicators of silage deterioration during feed-out, but only temperature has been used extensively to assess aerobic stability. Here we extend the study of aerobic stability to incorporate AMR of silage by developing a novel experimental cell integrated with multiple sensors. Silage samples, ensiled from a triticale crop, were made in twelve air-tight barrels (60 L), packed to bulk densities of 190 or 250 kg m−3 dry matter (DM). Tsi and O2 measurements were co-located at 15- and 30-cm behind the working face. CO2 was measured as flux across the working face. The experimental period of aerobic exposure was 7 days. We provide the first reports of: (i) distinct aerobic responses of these parameters, showing that Tsi varied with CO2 in phase but with O2 out-of phase; (ii) CO2 flux was dominated initially by anaerobic discharge and subsequently by aerobic products; (iii) linear relationships between aerobic reheating and both O2 consumption (0.994 ≥ R2 ≥ 0.815, P

The microbiome in silage may vary substantially from the onset to the completion of fermentation. Improved additives and inoculants are being developed to accelerate the ensiling process, to enhance fermentation quality, and to delay spoilage during feed-out. However, current methods for preselecting and characterizing these amendments are time-consuming and costly. Here, we have developed a multi-sensor mini-bioreactor (MSMB) to track microbial fermentation in situ and additionally presented a mathematical model for the optimal assessment among candidate inoculants based on the Bolza equation, a fundamental formula in optimal control theory. Three sensors [pH, CO2, and ethanol (EtOH)] provided data for assessment, with four additional sensors (O2, gas pressure, temperature, and atmospheric pressure) to monitor/control the fermentation environment. This advanced MSMB is demonstrated with an experimental method for evaluating three typical species of lactic acid bacteria (LAB), Lentilactobacillus buchneri (LB) alone, and LB mixed with Lactiplantibacillus plantarum (LBLP) or with Enterococcus faecium (LBEF), all cultured in De Man, Rogosa, and Sharpe (MRS) broth. The fermentation process was monitored in situ over 48 h with these candidate microbial strains using the MSMB. The experimental results combine acidification characteristics with production of CO2 and EtOH, optimal assessment of the microbes, analysis of the metabolic sensitivity to pH, and partitioning of the contribution of each species to fermentation. These new data demonstrate that the MSMB associated with the novel rapid data-processing method may expedite development of microbial amendments for silage additives.

Documentation in the field of the longitudinal progression of freezing in woody stems during winter has recently become feasible due to development of advanced instrumentation. Similar techniques to detect the radial progression of freezing fronts in the stem have remained challenging and, so far, unresolved. Here we address this problem with a smart sensor, based on an inner fringing dielectric technique, but combined with a new analytical model of radial freezing-thawing-fronts. These fronts, occurring in night and daytime of winter, respectively, are considered as vectors, both moving radially from the outside of the stem inward. We present laboratory calibration data and a field experiment in a 14-year old apple orchard to demonstrate the novel capability of this noninvasive sensor for in situ determination of ice content. We track the diurnal and seasonal variation of ice content, identify the radial location of the freezing/thawing-front, link this to anatomical features of the stem, and characterize hysteresis in freezing and subsequent thawing. We document homeostasis of total tissue water content during winter which allowed quantification of ice and liquid water content over both diurnal and seasonal cycles. Moreover, the progressive, day-to-day, decrease in liquid water during cold acclimation in autumn and increase during de-acclimation in spring are shown, along with the associated transpirational fluxes. These data contribute to fundamental knowledge of the water relations of trees, demonstrate a practical technique to monitor multiple trees in the field, and thus provide insight into the ecophysiological processes involved as trees overwinter.

Near-surface soil water content (NSWC) is an important parameter for characterizing hydrological processes, water and vapor exchange at the soil-atmosphere boundary, and potential biological productivity. Here we develop a dielectric based frequency-domain (FD) impedance sensor operating at 100 MHz, with a novel coaxial probe design and a temperature sensor embedded in the circuit board. This perforated cylinder coaxial (PCC) sensor provides simultaneous measurements of volumetric soil water content (VSWC) and of temperature. The temperature measurement allows signal correction for the temperature response of the circuit board and of the permittivity of soil water. We characterize the performance of the PCC sensor and compare it with alternative 3-pin- and 2-pin-electrode FD impedance sensors each using a similarly performing circuit board. A series of theoretical analyses and experiments were conducted, including (i) sensors calibration, (ii) temperature effects and their correction, (iii) soil volume sensed by each sensor through simulation analysis and a stepwise soil removal (SSR) test and (iv) accuracy comparison of the three sensors when applied for the NSWC measurements. The calibration results verified that the PCC, 3-pin and 2-pin probes were suitable to measure VSWC (RMSE ≤ 0.0142 cm3 cm−3). The simulation analysis using High Frequency Structure Simulator software (HFSS) indicated that the novel PCC probe had a symmetrical distribution of the E-field, wholly contained within the cylinder, avoiding leakage of the E-Field into air at the soil-atmosphere interface and removing the dependency of sensed volume on VSWC. The results of the SSR test showed that the E-field distributed outside the receiving electrode of the PCC probe could be ignored whereas that of both pin-type electrodes was distributed outside of the receiving electrode to a distance that depended on VSWC (i.e. up to 8 mm for 3-pin and up to 11 mm for 2-pin designs when VSWC > 0.17 cm3 cm−3), which corresponds to the simulation analysis. Moreover, the new PCC probe provided more accurate NSWC measurement than 3-pin or 2-pin probes because the E-field was contained within the perforated cylinder, avoiding E-field leakage and fixing sensed soil volume independent of VSWC. The pin-structure probes appear to perform best when installed horizontally in the soil at least 11 mm below the surface.

Origins of abscisic acid (ABA)-mediated metabolic control of stomatal conductance have been suggested to be recent, based on a gradualistic model of stomatal evolution. In ferns, steady-state stomatal conductance (gs ) was unresponsive to ABA in some studies, supporting this model. Stomatal kinetic responses to ABA have not been considered. We used dynamic gas exchange methods to characterise half times of stomatal opening and closing in response to step changes in light, across a range of ABA exposures in three diverse taxa. All taxa had asymmetric kinetics, with closure slower than opening in fern and cedar, but faster than opening in soybean. Closing was fastest in soybean but opening was slowest. Stomatal kinetics, particularly for closure, responded to ABA in all three taxa. Steady-state gs did not respond significantly to ABA in fern or cedar but responded strongly in soybean. Stomatal kinetics were responsive to ABA in fern. This finding supports a contrasting, single origin model, with ABA-mediated regulation of stomata arising early, in conjunction with stomata themselves. Stomatal kinetics are underutilised. Differential responses of opening and closing rates to environmental and hormonal stimuli may provide insights into phylogeny and stomatal regulatory strategies with potential application to selection for crop improvement.

The HYDRUS-2D simulation software has been used for irrigation management. Its performance under realistic irrigation regimes requires evaluation with new methodologies that integrate larger soil volumes, because soil water content is highly variable in time, space and scale. We compare direct measurements in a sloping field environment under drip irrigation with simulation using HYDRUS-2D. An advanced mobile sensor technology is used to track the dynamics of soil water content, and thus of unsaturated flow, at 0.25 m and 0.50 m depth in a field plot (6 m × 3 m; 4° slope) beneath two parallel (0.5 m separation; 7 emitters per delivery tube) dripper arrays. We document an asynchronous sequence of wetting fronts driven by the sloping surface and capture the field results well using HYDRUS-2D, in the spatial (R2 = 0.935–0.963, p

Biological invasions in established cropping systems impact biodiversity and agricultural sustainability. Under conditions of multi-year drought and elevated tropospheric ozone (O3), several Amaranthus species have recently become problematic weeds in California. We examined responses of common waterhemp (A. tuberculatus) to 12-h mean O3 exposures of 4, 59, or 114 nmol O3 mol air−1, with two levels of irrigation, 33% (deficit irrigation, DI) or 100% (well-watered, WW) of field capacity. Daytime stomatal conductance (gs) was reduced by O3 but not by DI. Nocturnal gs was uncommonly large but unaffected by either treatment. Plant productivity was not affected by O3 or DI. Leaf mass per unit area and root mass per unit leaf area were reduced by O3 but not by DI. Tolerance of this species to O3 and deficit irrigation may increase its competitiveness with crops. High night-time water loss may reduce water availability for competing crops. The competitiveness of common waterhemp may increase, as climate ...

Stem water content (StWC) of plants is an important parameter for assessing plant response to drought stress in arid and semi-arid areas and for irrigation scheduling. The measurement accuracies of in-situ non-invasive approaches are degraded by plant growth and by diurnal changes in stem diameter associated with water content. Here we develop a frequency-domain (FD) dielectric sensor operating at 100 MHz with an innovative interdigitated-electrodes (IE) probe design for measuring StWC. We characterize the performance of the IE sensor and compare it with a previously described pair-strap-ring-electrodes (2RE) FD sensor. Simulations and experimental measurements were conducted to assess the electric field distribution and volume of sensitivity (VOS) of each probe, the sensitivity of each probe to stem diameter and the accuracy of each sensor for determining StWC of three apple trees in a greenhouse environment. The simulation analysis and the measurement showed that the IE probe has a smaller but denser VOS than the 2RE probe. The sensitivity test showed that the new IE probe (0.85 mV mm−1, R2 = 0.7108) was less susceptible to stem diameter variation in comparison with the 2RE probe (32.83 mV mm−1, R2 = 0.9977). The observations in the greenhouse showed that the three apple trees (AT-1, AT-2 and AT-3) experienced a daily dehydration-rehydration cycle and the averaged midnight-to-predawn StWC gradually decreased without irrigation. According to the reference values of the maximum daily trunk shrinkage (MDS) reported by a previous study, both AT-2 and AT-3 may experience water deficit before the irrigation. Besides, the stem diameter variation decreased the measurement accuracy of 2RE sensor to be ~0.0410 cm3 cm−3 mm−1. The stronger electric field intensity of the IE probe and its less susceptible to stem diameter variation make the new IE dielectric sensor an improved method for accurate in-situ measurement of diurnal stem water content.

The degree to which ozone (O3) exposure and drought affect stomatal control of water loss and respond to environmental stimuli such as varying light is poorly characterized. To that end, we exposed Pima cotton to chronic O3 exposure (month-long daytime exposures) with and without sufficient water, as well as short term acute O3 exposure and varying light levels to understand stomatal kinetics. Chronic, month-long exposure to moderately high O3 (~114 ppb) reduced daytime steady state stomatal conductance (gs), as did water deficit. Both stomatal opening and closing displayed dose specific, “sluggish” responses to step-changes in illumination with acute, 1-day, O3 exposures of 0, 50, 100, and 125 ppb. At higher concentration (150 ppb), stomatal control of both opening and closing was degraded. Altered steady state and dynamic stomatal function suggest that elevated ambient O3, expected to increase in the future, may increasingly influence field water management and appropriate crop choices.

Stem water content (StWC) is a key parameter characterizing plant water relations. However, a reliable yet non-invasive method of determining StWC in trees has not been developed. Here we examined an inner fringing frequency domain (FD) sensor in a 12-year old apricot tree. Two strap rings operating at 100 MHz accurately measured StWC in stems 4–12 cm in diameter. Effective sensing depth (ESD) inside the stems, and the allowable maximum stem diameter (AMStD) were systematically evaluated. To determine ESD, a novel experimental method, stepwise stem core removal (SStCR), was developed, and combined with numerical simulation using a High Frequency Structure Simulator (HFSS). The experimental and analytical approaches were in good agreement, demonstrating that the ESD must include substantial sapwood. In apricot, with ring-porous distribution of xylem vessels, performance was excellent up to AMStD ≤ 8 cm following temperature correction. In the current study, we characterize diurnal patterns of water loss and recharge in stems of varying diameter, demonstrate hysteresis in water content between stems of different sizes and heights in the canopy, and evaluate potential interactions with ambient temperature fluctuations. The inner fringing FD sensor can be easily constructed at low cost, suggesting the potential for routine measurement of StWC in diverse woody plants.

Soil water potential is a key variable that determines water flow and indicates water availability and content. Although diverse techniques and methods have been developed for water potential measurement in unfrozen soils, most commercial techniques such as dielectric soil water potential (DSWP) sensors are not applicable for frozen soils due to potential errors of the dielectric measurement caused by ice formation within the sensor probes. Rather than directly recording sensor readings, these commercial DSWP sensors have to indirectly estimate soil water potential under equilibrium conditions by temperature measurement and the Clapeyron equation. Here we develop a mathematical model to correct the ice induced measurement error of the DSWP sensor in frozen soils. The DSWP sensor tested in this research was self-developed with a perforated cylinder coaxial (PCC) probe filled with a porous medium (gypsum). The central electrode was a temperature sensor (Pt100) replacing the usual steel pin. Sensor performance was demonstrated in a non-saline, loamy soil, by installing one sensor (DSWP-1) in a wetted soil sample prior to soil freezing, with two additional sensors (DSWP-2 and DSWP-3) installed later in frozen soil, prior to stepwise thawing. The results show that ice formed in the porous gypsum of the DSWP-1 sensor during soil freezing, as would be expected in the field with liquid water initially present to enter the probe. In comparison, no ice formed in the porous gypsums of the DSWP-2 and DSWP-3 sensors during the stepwise thawing process, allowing directly measurement of soil water potential in frozen soils and acting as a reference to develop an ice correction model based on a dielectric mixing model for the ice-formed DSWP-1 sensor to improve accuracy of soil water potential measurement. With the ice correction model, the RMSE between the readings of the DSWP-1 sensor and the reference values determined by the DSWP-3 sensor was decreased from 1.065 MPa to 0.491 MPa. In general, the ice correction model developed here can be used to improve accuracy of soil water potential measurement during soil freeze-thaw process using the DSWP sensor or other dielectric sensors and may extend application of the DSWP sensor to frozen soils under non-equilibrium conditions in the field.

Previous flux measurements in the perhumid cloud forest of northeastern Taiwan have shown efficient photosynthesis of the endemic tree species Chamaecyparis obtusa var. formosana even under foggy conditions in which leaf surface moisture would be expected. We hypothesized this to be the result of ‘xeromorphic’ traits of the Chamaecyparis leaves (hydrophobicity, stomatal crypts, stomatal clustering), which could prevent coverage of stomata by precipitation, fog, and condensation, thereby maintaining CO2 uptake. Here we studied the amount, distribution, and composition of moisture accumulated on Chamaecyparis leaf surfaces in situ in the cloud forest. We studied the effect of surface tension on gas penetration to stomata using optical O2 microelectrodes in the laboratory. We captured the dynamics of condensation to the leaf surfaces with an environmental scanning electron microscope (ESEM). In spite of substantial surface hydrophobicity, the mean water film thickness on branchlets under foggy conditions was 80 µm (upper surface) and 40 µm (lower surface). This amount of water could cover stomata and prevent CO2 uptake. This is avoided by the clustered arrangement of stomata within narrow clefts and the presence of Florin rings. These features keep stomatal pores free from water due to surface tension and provide efficient separation of plant and atmosphere in this perhumid environment. Air pollutants, particularly hygroscopic aerosol, may disturb this functionality by enhancing condensation and reducing the surface tension of leaf surface water.

Agricultural production in the hot and arid low desert systems of southern California relies heavily on irrigation. A better understanding of how much and to what extent irrigated water is transpired by crops relative to being lost through evaporation would improve the management of increasingly limited water resources. In this study, we examined the partitioning of evapotranspiration (ET) over a field of forage sorghum (Sorghum bicolor), which was under evaluation as a potential biofuel feedstock, based on isotope measurements of three irrigation cycles at the vegetative stage. This study employed customized transparent chambers coupled with a laser-based isotope analyzer to continuously measure near-surface variations in the stable isotopic composition of evaporation (E, δE), transpiration (T, δT) and ET (δET) to partition the total water flux. Due to the extreme heat and aridity, δE and δT were very similar, which makes this system highly unusual. Contrary to an expectation that the isotopic signatures of T, E, and ET would become increasingly enriched as soils became drier, our results showed an interesting pattern that δE, δT, and δET increased initially as soil water was depleted following irrigation, but decreased with further soil drying in mid to late irrigation cycle. These changes are likely caused by root water transport from deeper to shallower soil layers. Results indicate that about 46% of the irrigated water delivered to the crop was used as transpiration, with 54% lost as direct evaporation. This implies that 28 − 39% of the total source water was used by the crop, considering the typical 60 − 85% efficiency of flood irrigation. The stable isotope technique provided an effective means of determining surface partitioning of irrigation water in this unusually harsh production environment. The results suggest the potential to further minimize unproductive water losses in these production systems.

Stomatal pore area is heterogeneous across leaf surfaces. This has been considered as “patchy stomatal conductance,” and may have substantial implications for photosynthetic efficiency. Aerosols have always been important elements of plant environments, but their effects on stomatal control of plant water relations, and stomatal heterogeneity specifically, have not been considered. Here we evaluate the spatial coordination of pore area in the glabrous and homobaric leaves of Vicia faba grown under two aerosol treatments and measured at four levels of VPD. We construct a large dataset (n > 88,000 discrete comparisons) of paired pore areas and distances between the pores. Plants were grown in ambient urban air and in filtered air (FA) to determine the effect of ambient aerosol on stomatal properties. Pore area exhibited spatial organization, as well as considerable variability among closely co-located pores. The difference between pore areas was positively correlated with the distance between the pores, in both aerosol treatments and at all VPDs. However, aerosol deposition reduced both the magnitude of variability between pores and the rate at which this variability increased with pore separation distance. These data support previous conclusions that deposition of hygroscopic aerosol may create a thin aqueous film across the leaf surface that connects neighboring stomata to each other and to the leaf interior. Aerosol impacts on stomatal heterogeneity and gas exchange are not adequately considered in current assessments of stomatal control.

Ozone (O3) is a secondary atmospheric pollutant that causes widespread damage to vegetation, including cotton. The high temperatures and abundant sunlight that lead to good cotton yields also lead to production of O3. The early crop loss experiments of the 1970s identified upland cotton as particularly sensitive to O3. Exposure to ambient concentrations of O3 resulted in a 10 to >15% loss in economic yield. More recent experiments with modern cultivars have confirmed damage to upland and Pima cultivars but have not assessed yield loss under agronomic conditions. It is likely that selection for yield in high O3 environments has led to inadvertent selection for tolerance to O3. Ozone reduces yield and fiber quality, mediated by O3-induced changes in gross photosynthesis, respiration, and translocation of current photosynthate out of source leaves. Ozone typically reduces root development and the root-to-shoot biomass ratio, which has secondary effects on plant growth and development and on water relations. Ozone also reduces stomatal conductance as a primary defense against entry of O3 into the plant, but this also restricts entry of CO2 and therefore photosynthesis. Ozone is clearly phytotoxic and impacts cotton as well as other species. Recent research has led to understanding many aspects of the mechanism of O3 damage and could lead to targeted approaches to improving O3 tolerance, and therefore, yield in O3-impacted production areas.

Evapotranspiration (ET) plays an important role in water and energy balance at the surface-atmosphere interface. It is widely reported that near-surface soil water content (SWC) or soil water potential (SWP) significantly affects ET and this parameter has been incorporated into the FAO Penman-Monteith (FAO-PM) model for prediction of ET during crop growth seasons. However, there is little information on the effect of SWC or SWP on prediction of ET during soil freeze-thaw cycles in winter. We present an experiment conducted at a demonstration farm with a crop of winter wheat, over two winters near Beijing, China. A lysimeter system equipped with a weather station was used to measure the ET flux and meteorological data. Unfrozen soil water content (USWC) and soil temperature (Tsoil) were measured using dielectric tube sensors (DTS) and digital temperature sensors, respectively. SWP was determined by measured USWC and a soil moisture characteristic (SMC) curve derived from the soil freezing characteristic (SFC) curve and the Clapeyron equation in frozen soil. Detailed measurements in year 1 showed that the FAO-PM model exhibited a complex error pattern, underestimating ET from unfrozen soil but overestimating ET from stable frozen soil. To address these errors, we define a freezing stress index (Ksf) as a function of SWP. Incorporation of Ksf as a modifier of the standard crop coefficient in the FAO-PM model improved prediction of ET (RMSE declined from 0.424 to 0.187 mm day−1, in year 1). These data revealed a correlation between SWP near the soil surface (

Plant responses to ozone (O3 ) and water deficit (WD) are commonly observed, although less is known about their interaction. Stomatal conductance (gs ) is both an impact of these stressors and a protective response to them. Stomatal closure reduces inward flux of O3 and outward flux of water. Stomatal measurements are generally obtained at midday when gas exchange is maximal, but these may not be adequate surrogates for stomatal responses observed at other times of day, nor for non-stomatal responses. Here, we find in Pima cotton that stomatal responses to O3 observed at midday do not reflect responses at other times. Stomata were more responsive to O3 and WD near midday, despite being at quasi-steady state, than during periods of active opening or closing in morning or evening. Stomatal responsivity to O3 was not coincident with maximum gas exchange or with periods of active regulation, but coincident with plant sensitivity to O3 previously determined in this cultivar. Responses of pigmentation and shoot productivity were more closely related to stomatal responses at midday than to responses at other times of day under well-watered (WW) conditions, reflecting higher stomatal responsivity, sensitivity to O3 , and magnitude of midday gs . Under WD conditions, shoot responses were more closely related to early morning gs. Root responses were more closely related to early morning gs under both WW and WD. Responses of stomata to O3 at midday were not good surrogates for stomatal responses early or late in the day, and may not adequately predicting O3 flux under WD or when maximum ambient concentrations do not occur near midday.

Aerosols are important components of the global plant environment, with beneficial and deleterious impacts. The direct effects of aerosol deposition on plant-water relationships remain poorly characterized but potentially important. Vicia faba was grown in ambient urban air and in the same air with aerosol excluded, in a moderately polluted environment using two exposure protocols. Simultaneous measurement of gas exchange and stomatal pore aperture was combined with leaf dehydration kinetics and microscopic evaluation of leaf wetness formation and aerosol deposition patterns. The ambient aerosol was shown to be hygroscopic. Aerosol exposure increased minimum leaf conductance, shown by dehydration kinetics, and nocturnal water vapor flux, shown by dark-adapted gas exchange. Aerosol exposure decreased stomatal apertures at each level of vapor pressure deficit (VPD) and increased stomatal conductance at comparable levels of aperture. Overall, these effects were modest, and largest when stomata were wide open. The uncoupling of conductance (flux-based) from aperture (directly measured microscopically) implies that aerosol-induced water loss is not fully under stomatal control. This reduces drought tolerance and may provide a mechanism by which deposited aerosol plays a direct role in stomatal response to VPD.

Silage is a critical global feedstock, but is prone to aerobic deterioration. The dominant mechanism of O2 transport into silage remains unresolved. Here, multiple sensors tracked O2 and CO2, gas pressure (ΔP) between internal silage and ambient air, pH and silage temperature (Tsi) during the ensilage of maize and ryegrass. We report the first observation that CO2 produced from microbial respiration was partially dissolved in silage water, with evidence of negative or positive ΔP depending on the changing balance between CO2 production and dissolution. The ΔP 2 production was much greater in anaerobic fermentation stage than in initial aerobic phase or later aerobic feed-out phase. O2 transport into silage is intimately linked to the dynamics of net CO2, ΔP, microbial activity, pH and Tsi. These results suggested that both gas diffusion (based on Fick’s law) and advective transfer (Darcy’s law) play equally important roles in governing the complex temporal progression of inward and outward gas fluxes to and from the silage interior. Even though low pH suppressed microbial activity and supported aerobic stability, the negative ΔP increased the risk of O2 entry and aerobic deterioration during feed-out phase.

Plant sensitivity to ozone (O 3 ) is critical to modeling impacts of air pollution on vegetation. A diel timecourse of sensitivity ( S ) was recently determined in Pima cotton (Grantz et al., 2013). The sensitivity parameter serves as a weighting factor for stomatal uptake (ozone flux, F ), or cumulative F (dose, D ). Previous approaches used various weighting schemes to modify ozone concentration ([O 3 ]) or cumulative [O 3 ] (exposure, E ). Use of the S parameter allows calculation of effective flux ( F eff ) and effective dose ( D eff ). Though theoretically sound, the practical significance of S has not been evaluated due to the previous lack of available data. Here, the newly available S parameter is used to explore the relationships between exposure- and flux-based O 3 metrics in response to scenarios of contrasting stomatal conductance ( g s ) and ambient [O 3 ]. The O 3 scenarios were similar but differed in timing of peak [O 3 ]. E varied by up to 13.7%, D by up to 15.4%, and D eff , which factors in sensitivity, by up to 19.0%. The g s scenarios differed in midday magnitude and nocturnal closure. Cumulative g s varied by 65.2%, which was attenuated in D to 49.2% and in D eff to 51.1%. A simulation of hourly [O 3 ], F , and F eff was run using Monte Carlo techniques with a full month of ambient [O 3 ] data. Resulting diel timecourses of [O 3 ], F , and F eff were realistic, with the principal sources of uncertainty in the physiological parameters, g s and S . Analysis of hourly values from the scenarios and the simulation output demonstrated significant correlation among the O 3 metrics. However, the uncertainty in both F and F eff predicted from [O 3 ] was large and proportional to [O 3 ], yielding greatest uncertainty under conditions of high [O 3 ] and potential phytotoxicity. In contrast, F eff was significantly correlated with F , with low variability that was not proportional to F . As a result, uncertainty was low and prediction potentially useful under conditions of likely injury. These results suggest that F, which incorporates g s , represents a substantial improvement over ambient [O 3 ], which does not. F eff , which incorporates S, was closely related to F , which does not use S. The substantial effort required to measure or model S and F eff may not be justified under some conditions. Further research to obtain additional timecourses of S and to explore additional [O 3 ] and g s scenarios is urgently required.

Event-driven and diel dynamics of soil respiration (Rs) strongly influence terrestrial carbon (C) emissions and are difficult to predict. Wetting events may cause a large pulse or strong inhibition of Rs. Complex diel dynamics include hysteresis in the relationship between Rs and soil temperature. The mechanistic basis for these dynamics is not well understood, resulting in large discrepancies between predicted and observed Rs. We present a unifying approach for interpreting these phenomena in a hot arid agricultural environment. We performed a whole ecosystem wetting experiment with continuous measurement of Rs to study pulse responses to wetting in a heterotrophic system. We also investigated Rs during cultivation of Sorghum bicolor to evaluate the role of photosynthetic C in the regulation of diel variation in Rs. Finally, we adapted a Rs model with sensitivity to soil O2 and water content by incorporating two soil C pools differing in lability. We observed a large wetting-induced pulse of Rs from the fallow field and were able to accurately simulate the pulse via release of labile soil C. During the exponential phase of plant growth, Rs was inhibited in response to wetting, which was accurately simulated through depletion of soil O2. Without plants, hysteresis was not observed; however, with growing plants, an increasingly significant counterclockwise hysteresis developed. Hysteresis was simulated via a dynamic photosynthetic C pool and was not likely controlled by physical processes. These results help characterize the complex regulation of Rs and improve understanding of these phenomena under warmer and more variable conditions.

Perchlorate (ClO4(-)) interferes with uptake of iodide in humans. Emission inventories do not explain observed distributions. Ozone (O3) is implicated in the natural origin of ClO4(-), and has increased since pre-industrial times. O3 produces ClO4(-)in vitro from Cl(-), and plant tissues contain Cl(-) and redox reactions. We hypothesize that O3 exposure may induce plant synthesis of ClO4(-). We exposed contrasting crop species to environmentally relevant O3 concentrations. In the absence of O3 exposure, species exhibited a large range of ClO4(-) accumulation but there was no relationship between leaf ClO4(-) and O3, whether expressed as exposure or cumulative flux (dose). Older, senescing leaves accumulated more ClO4(-) than younger leaves. O3 exposed vegetation is not a source of environmental ClO4(-). There was evidence of enhanced ClO4(-) content in the soil surface at the highest O3 exposure, which could be a significant contributor to environmental ClO4(-).

Carbon dioxide (CO2) is a principal byproduct of various chemical, biological and biochemical processes. CO2 has been measured using various advanced sensors, but a limiting technical challenge has been resolving independent streams of CO2 produced by processes occurring simultaneously. The magnitude and kinetics of each stream may be chemically, biologically or/and physically informative, but such partitioning has received little attention and successful case studies remain rare. In silage production, CO2 flux, an important indicator of aerobic deterioration, microbial activity or oxidative rate of silage, derives from two different pools: gas accumulated in the pores during the early anaerobic phase, and current real-time production of CO2 as oxygenated air enters the silage across the exposed face after opening for feed-out. The former is regarded as a noise confounding the signal and the latter reflects current degradation of the silage. Using a self-developed automatic sensor system with novel signal decomposition, we successfully partitioned CO2 flux into two independent streams derived from the two distinct pools in maize silage, following two independent processes: a physical venting of stored gas through a tortuous diffusive pathway, and a biochemical process generating gas in real time. Three silage samples, treated with a chemical or a biological additive, or left untreated, were tested. The signal decomposition found two best-fit functions (0.8034 ≤ R2 ≤ 0.9036), a quadratic CO2 discharge function and an exponential CO2 production function, for characterizing these distinct processes. These results demonstrate chemical sensor with powerful data-processing capability to resolve the complexity of dual-pool CO2 emission.

Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O3 dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O3 (15 min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure to O3, and O3 impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the performance of this parameter in prediction of O3 injury.

Natural perchlorate (ClO4−) in soil and groundwater exhibits a wide range in stable isotopic compositions (δ37Cl, δ18O, and Δ17O), indicating that ClO4− may be formed through more than one pathway and/or undergoes post-depositional isotopic alteration. Plants are known to accumulate ClO4−, but little is known about their ability to alter its isotopic composition. We examined the potential for plants to alter the isotopic composition of ClO4− in hydroponic and field experiments conducted with snap beans (Phaseolus vulgaris L.). In hydroponic studies, anion ratios indicated that ClO4− was transported from solutions into plants similarly to NO3− but preferentially to Cl− (4-fold). The ClO4− isotopic compositions of initial ClO4− reagents, final growth solutions, and aqueous extracts from plant tissues were essentially indistinguishable, indicating no significant isotope effects during ClO4− uptake or accumulation. The ClO4− isotopic composition of field-grown snap beans was also consistent with that of ClO4− in varying proportions from irrigation water and precipitation. NO3− uptake had little or no effect on NO3− isotopic compositions in hydroponic solutions. However, a large fractionation effect with an apparent e (15N/18O) ratio of 1.05 was observed between NO3− in hydroponic solutions and leaf extracts, consistent with partial NO3− reduction during assimilation within plant tissue. We also explored the feasibility of evaluating sources of ClO4− in commercial produce, as illustrated by spinach, for which the ClO4− isotopic composition was similar to that of indigenous natural ClO4−. Our results indicate that some types of plants can accumulate and (presumably) release ClO4− to soil and groundwater without altering its isotopic characteristics. Concentrations and isotopic compositions of ClO4− and NO3− in plants may be useful for determining sources of fertilizers and sources of ClO4− in their growth environments and consequently in food supplies.

Atmospheric aerosols are liquid, solid, or mixed suspensions of heterogeneous chemical composition, ranging from a few nanometers to almost 100 μm in diameter. Plants are sources and sinks of these diverse aerosols. Vegetation is influenced by aerosols through the water cycle, radiation balance, and nutrient transport, on global and regional scales, but direct interactions of aerosols with plant ecophysiology have not been considered in sufficient detail. Plant surface characteristics and aerodynamic factors control deposition. These factors may be manipulated in efforts to mitigate aerosol concentrations using urban vegetation as efficient aerosol collectors. Hygroscopic aerosols deposited on leaves generate concentrated solutions that reduce surface tension and generate thin liquid films. These films are shown to enter the stomatal pores, facilitating foliar nutrient uptake and enhancing liquid water loss that is poorly controlled by stomata. Aerosol pollution can reduce plant drought tolerance and alter nutrient balance. Anthropogenic aerosols now exceed natural aerosols, particularly in urban areas. The effects of these aerosols on plants require a focused research effort.

The available literature is conflicting on the potential protection of plants against ozone (O(3)) injury by exogenous jasmonates, including methyl jasmonate (MeJA). Protective antagonistic interactions of O(3) and MeJA have been observed in some systems and purely additive effects in others. Here it is shown that chronic exposure to low to moderate O(3) concentrations (4-114 ppb; 12 h mean) and to MeJA induced additive reductions in carbon assimilation (A (n)) and root respiration (R (r)), and in calculated whole plant carbon balance. Neither this chronic O(3) regime nor MeJA induced emission of ethylene (ET) from the youngest fully expanded leaves. ET emission was induced by acute 3 h pulse exposure to much higher O(3) concentrations (685 ppb). ET emission was further enhanced in plants treated with MeJA. Responses of growth, allocation, photosynthesis, and respiration to moderate O(3) concentrations and to MeJA appear to be independent and additive, and not associated with emission of ET. These results suggest that responses of Pima cotton to environmentally relevant O(3) are not mediated by signalling pathways associated with ET and MeJA, though these pathways are inducible in this species and exhibit a synergistic O(3)×MeJA interaction at very high O(3) concentrations.

Crop production in many irrigated agricultural regions is challenged by both air pollution and weed pressure. Impacts of ozone (O 3 ) on C 4 plants have received little attention. Yet many important crops and weeds utilize the C 4 pathway, including yellow nutsedge (Cyperus esculentus L.). A previous study suggested that this species may respond to O 3 exposure with enhanced allocation of biomass below ground, including potentially to reproductive tubers, an apparent exception to the general weakening of belowground sink strength. This hypothesis was not supported by the current experiments, as exposure to 60 nL L -1 and 115 nL L -1 O 3 (12-h daylight mean O 3 concentration) reduced biomass allocation to tubers, relative to 4 nL L -1 . Chlorophyll content, specific leaf weight, and carbon assimilation were reduced, while intercellular CO 2 concentration increased, reducing water use efficiency. We conclude that (i) this C 4 species exhibits substantial sensitivity to ambient O 3 ; (ii) O 3 -inhibition of allocation belowground is not limited to roots but also applies to tubers; and (iii) the O 3 component of global change will increase the competitiveness of nutsedge with respect to specific crops as shown previously, but will decrease tuber production and degrade water use efficiency of this challenging C 4 weed.

The productivity, product quality and competitive ability of important agricultural and horticultural plants in many regions of the world may be adversely affected by current and anticipated concentrations of ground-level ozone (O3). Exposure to elevated O3 typically results in suppressed photosynthesis, accelerated senescence, decreased growth and lower yields. Various approaches used to evaluate O3 effects generally concur that current yield losses range from 5% to 15% among sensitive plants. There is, however, considerable genetic variability in plant responses to O3. To illustrate this, we show that ambient O3 concentrations in the eastern United States cause substantially different levels of damage to otherwise similar snap bean cultivars. Largely undesirable effects of O3 can also occur in seed and fruit chemistry as well as in forage nutritive value, with consequences for animal production. Ozone may alter herbicide efficacy and foster establishment of some invasive species. We conclude that current and projected levels of O3 in many regions worldwide are toxic to sensitive plants of agricultural and horticultural significance. Plant breeding that incorporates O3 sensitivity into selection strategies will be increasingly necessary to achieve sustainable production with changing atmospheric composition, while reductions in O3 precursor emissions will likely benefit world food production and reduce atmospheric concentrations of an important greenhouse gas.

The C 4 perennial grasses have many potential virtues as bioenergy crops-high productivity and water use efficiency, multiyear crop cycle that minimizes replanting costs, and wide environmental adaptation. In the productive San Joaquin Valley (SJV) of California, these species also confront high levels of ozone (O 3 ) air pollution. It is often assumed that C 4 species will exhibit tolerance to O 3 , relative to C 3 species, based on limited crop yield-loss data and on responses to a contrasting gaseous air pollutant, sulfur dioxide (SO 2 ). Tolerance to O 3 of high-biomass C 4 crops such as sugarcane (Saccharum spp.) has not been demonstrated. Here we tested the hypothesis that a clone of sugarcane currently grown on a small commercial scale in the O 3 -impacted SJV will exhibit tolerance to O 3 . The hypothesis was disproven. Biomass production declined by over one-third and allocation to roots declined by over two-thirds, over a range of O 3 exposures (4, 58, and 114 ppb, 12-h means). These responses are comparable to responses previously reported for O 3 -sensitive C 3 crop species. Greater O 3 tolerance may be required to maximize the potential viability of bioethanol production using tall tropical grasses such as sugarcane (for sucrose) or energy canes (for cellulose) in O 3 -impacted western valleys. In the case of the locally successful sugarcane clone examined here, the C 4 photosynthetic pathway did not confer resistance to O 3 exposure.

Horseweed has become increasingly difficult to control in the San Joaquin Valley (SJV) of California. Resistance to glyphosate may not fully explain the quasi-invasive behavior of this native species. We contrast glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed biotypes for vigor during the vegetative stage and for resistance to ozone (O3). The SJV is impacted by O3 air pollution, which could be a factor in competitiveness of GR vs. GS. Both biotypes were exposed during the seedling and vegetative stages of rosette development to a range of O3 concentrations in greenhouse exposure chambers. Leaf injury was evaluated visually and biomass production and allocation destructively. In O3-free air, the GR biotype exhibited fewer foliar lesions, more vigorous growth, and 40% greater biomass than the GS biotype. The slope of the response to O3 was greater in the GR than in the GS biotype, implying greater relative sensitivity to O3. This was due to greater vigor at low O3, as the bioty...

Fertilized soils have large potential for production of soil nitrogen oxide (NOx=NO+NO2), however these emissions are difficult to predict in high-temperature environments. Understanding these emissions may improve air quality modelling as NOx contributes to formation of tropospheric ozone (O3), a powerful air pollutant. Here we identify the environmental and management factors that regulate soil NOx emissions in a high-temperature agricultural region of California. We also investigate whether soil NOx emissions are capable of influencing regional air quality. We report some of the highest soil NOx emissions ever observed. Emissions vary nonlinearly with fertilization, temperature and soil moisture. We find that a regional air chemistry model often underestimates soil NOx emissions and NOx at the surface and in the troposphere. Adjusting the model to match NOx observations leads to elevated tropospheric O3. Our results suggest management can greatly reduce soil NOx emissions, thereby improving air quality., Soil NOx emissions can significantly impact air quality in agricultural regions, particularly high temperature fertilized systems. Here, the authors investigate NOx emissions in one such system in California and suggest that the NOx emissions are the highest ever observed, with implications for air quality.

Anthropogenic nitrogen (N) deposition represents a significant N input for many terrestrial ecosystems. N deposition can affect plants on scales ranging from photosynthesis to community composition, yet few studies have investigated how changes in N availability affect plant water relations. We tested the effects of N addition on plant water relations, hydraulic traits, functional traits, gas exchange, and leaf chemistry in a semi-arid ecosystem in Southern California using long-term experimental plots fertilized with N for over a decade. The dominant species were Artemisia california and Salvia mellifera at Santa Margarita Ecological Reserve and Adenostoma fasciculatum and Ceanothus greggii at Sky Oaks Field Station. All species, except Ceanothus, showed increased leaf N concentration, decreased foliar carbon to N ratio, and increased foliar N isotopic composition with fertilization, indicating that added N was taken up by study species, yet each species had a differing physiological response to long-term N addition. Dry season predawn water potentials were less negative with N addition for all species except Adenostoma, but there were no differences in midday water potentials, or wet season water potentials. Artemisia was particularly responsive, as N addition increased stem hydraulic conductivity, stomatal conductance, and leaf carbon isotopic composition, and decreased wood density. The alteration of water relations and drought resistance parameters with N addition in Artemisia, as well as Adenostoma, Ceanothus, and Salvia, indicate that N deposition can affect the ability of native Southern California shrubs to respond to drought.

Climate and land use models project increasing occurrence of high temperature and water deficit in both agricultural production systems and terrestrial ecosystems. Episodic soil wetting and subsequent drying may increase the occurrence and magnitude of pulsed biogeochemical activity, affecting carbon (C) and nitrogen (N) cycles and influencing greenhouse gas (GHG) emissions. In this study, we provide the first data to explore the responses of CO2 and N2O fluxes to (1) temperature, (2) soil water content as percent water holding capacity (%WHC), and (3) substrate availability throughout (4) multiple soil drying and rewetting (DW) events. Each of these factors and their interactions exerted effects on GHG emissions over a range of four (CO2) and six (N2O) orders of magnitude. Maximal CO2 and N2O fluxes were observed in environments combining intermediate %WHC, elevated temperature, and sufficient substrate availability. Amendments of C and N and their interactions significantly affected CO2 and N2O fluxes, and altered their temperature sensitivities (Q10) over successive DW cycles. C amendments significantly enhanced CO2 flux, reduced N2O flux, and decreased the Q10 of both. N amendments had no effect on CO2 flux and increased N2O flux, while significantly depressing the Q10 for CO2, and having no effect on the Q10 for N2O. The dynamics across DW cycles could be attributed to changes in soil microbial communities since the different responses to wetting events in specific group of microorganisms, to the altered substrate availabilities, or to both. The complex interactions among parameters influencing trace gas fluxes should be incorporated into next generation earth system models to improve estimation of GHG emissions. This article is protected by copyright. All rights reserved.

Plant competition may be altered by ongoing climate change, including rising tropospheric O 3 . This may depend less on the O 3 tolerance of isolated species than on O 3 effects on the mechanisms of competition. To explore this possibility, we investigate the growth and gas exchange responses of a crop-weed system in partial additive competition in open-top chambers (OTCs). Yellow nutsedge (Cyperus esculentus L.) and Pima cotton (Gossypium barbadense L.) were grown in pots in ratios of 0:1, 1:1, 2:1, and 3:1, plus 1:0 to contrast with 0:1. O 3 concentrations (12-h means) were 12.8 nL L -1 (low O 3 , L03), 79.9 nL L -1 (medium O 3 , MO3), and 122.7 nL L -1 (high O 3 , HO3). Cotton was more sensitive than nutsedge to O 3 (reduction at HO3: 75 vs. 20% in shoot growth, 33 vs. 20% in assimilation). Cotton was inhibited by O 3 and by competition and the interaction was significant for leaf properties of cotton and tuber production in nutsedge. A coefficient of competition (slope of inverse cotton shoot biomass vs. nutsedge density) was significantly increased at HO3. The species were mutually inhibitory to similar extents. Root respiration declined with O 3 in nutsedge but increased in cotton, though both species reduced allocation below-ground. Nutsedge tuber production increased inconsistently with O 3 . Rising tropospheric O 3 may decrease the current C 4 advantage of nutsedge in water use efficiency (WUE) and stomatal avoidance of O 3 , but appears likely to increase the competitiveness of nutsedge with respect to cotton.

The mechanism of O 3 action on plants remains poorly characterized. Symptoms include visible lesions on the leaf surface, reduced growth and a hypothesized reduction in allocation of carbohydrate to roots. The generality of this latter phenomenon has not been demonstrated. Here, a meta-analysis is performed of all available experimental data, to test the hypotheses that O 3 exposure of the shoot inhibits biomass allocation below ground (the root/shoot allometric coefficient, k) and inhibits whole-plant growth rate [relative growth rate (RGR)]. Both k and RGR were significantly reduced by O 3 (5.6 and 8.2%, respectively). Variability in k was greater than in RGR, and both exhibited some positive as well as mostly negative responses. The effects on k were distinct from the effects on RGR. In some cases, k was reduced while RGR was unaffected. Slow-growing plants (small RGR) exhibited the largest declines in k. These observations may have mechanistic implications regarding O 3 phytotoxicity. There were no effects of type of exposure chamber on sensitivity to O 3 . The analyses indicate that the O 3 inhibition of allocation to roots is real and general, but variable. Further experiments are needed for under-represented plant groups, to characterize exceptions to this generalization and to evaluate O 3 -environment interactions.

Tomato (Lycopersicon esculentum L.) production in the San Joaquin Valley (SJV) of California is challenged by air pollution and weeds. Differential ozone (O 3 ) tolerance of tomato cultivars and weed species may alter crop-weed competition. A study was conducted in open top chambers (OTCs) at the Kearney Research and Extension Center, Parlier, CA, to assess O 3 impacts on competition between tomato and a C 4 weed, yellow nutsedge (Cyperus esculentus L.). Processing tomato (cv. HD 8892 and EMP 113) and nutsedge (locally collected biotypes) were grown in pots for 4 to 8 wk. Population ratios ranged from a tomato plant alone (0:1) to a nutsedge plant alone (1:0), and included 1:1, 2:1, and 3:1. Ozone exposures were to 12 h means of 19.8, 78.0, and 142.3 nL/L. Chlorophyll content of leaves of tomato and nutsedge was reduced with increasing O 3 . Carbon assimilation was reduced in nutsedge but not in tomato. Root respiration was not affected in either species. Tomato main stem length, shoot, and root biomass declined at the highest O 3 concentration under all levels of nutsedge competition. Nutsedge was much less affected. In the absence of O 3 exposure, interspecific competition (all population ratios combined) reduced tomato and nutsedge shoot and root biomass. Tomato was more sensitive to O 3 than nutsedge, but nutsedge was more sensitive to competition than was tomato. Nutsedge allocated greater resources to reproductive structures (tubers) at the highest O 3 exposure. As nutsedge reduced tomato productivity under low and moderate O 3 concentrations, it may become even more difficult to control, exert greater competitiveness, and colonize fields more rapidly because of greater tuber production, in projected near-future environments. Under conditions of greatly increasing ambient O 3 concentrations, nutsedge may become less competitive because of its sensitivity to O 3 .

In recent decades, air quality has improved near most cities but not in rural areas such as the San Joaquin Valley. Many studies using diverse exposure techniques have shown that ground-level ozone air pollution reduces plant growth and yield, from negligible impacts in some species to over 30% losses in others. We studied the interaction of ozone with weed competition from yellow nutsedge in Pima cotton and tomato in open-top field-exposure chambers at the UC Kearney Research and Extension Center in Fresno County. Ozone impacts on cotton (which is relatively sensitive) were compounded by weed competition, whereas tomato (which is less sensitive) competed well at all ozone concentrations. Our data suggests that crop-loss estimates obtained in single-factor experiments accurately reflect the serious risk of ozone to agriculture, but that more accurate yield predictions will require the consideration of interactions between the components of complex crop production systems, including weed competition.

It is well established that exposure of plant foliage to tropospheric ozone (O3) inhibits photosynthetic gas exchange in leaves and the translocation of current photosynthate to sink tissues. It is less clear what impact O3-reduced source strength has on the physiological responses of sink tissue such as fine roots. The responses were investigated of carbon acquisition in leaves and carbon utilization in the respiration of fine roots, following chronic (weeks) and acute (hours) exposures to O3 in open top chambers. Previous reports indicate increased, decreased, and unchanged rates of root respiration following exposure to O3. A decline in source activity is confirmed, but an increase in sink respiration is reported in fine roots of Pima cotton (cv. S-6) and muskmelon (cv. Ambrosia hybrid). Leaf source strength and root sink activity changed in opposing directions, thus there was no positive correlation that might indicate direct substrate control of root function. Additional linkages between shoot and root following exposure to O3 may be involved.

O(3)could reduce growth and carbohydrate allocation to roots by direct inhibition of photosynthesis and source strength. Alternatively, O(3) could reduce growth indirectly by inhibition of root hydraulic development through a primary lesion in carbohydrate translocation. Another alternative is that O(3) could slow the rate of plant development, only apparently altering carbohydrate allocation at a given plant age. Pima cotton (Gossypium barbadense L.) is used to address these possibilities, and four hypotheses were tested/ (1) O(3) exposure reduces leaf pools of soluble sugars; (2) pruning leaf area and reducing source strength to match that of O(3)-treated plants reproduces O(3)-effects; (3) pruning lower leaf area more closely reproduces O(3) effects than pruning upper leaf area; and (4) manipulating plant age and thereby plant size to match O(3)-treated plants reproduces O(3)-effects. All were falsified. Soluble sugars did not decline. Pruning upper and lower leaves and manipulating plant age all reduced biomass and leaf area similarly to O(3)-exposure, but neither reproduced O(3) effects on biomass allocation nor root function. It is concluded that O(3) induces an allometric shift in carbohydrate allocation that is not mediated by photosynthetic inhibition nor by alteration of developmental age. Effects of O(3) could be mediated by direct effects on phloem loading, with consequent inhibition of translocation to roots and root system development.

Knowledge of single leaf boundary layer conductance ( g b ) within extensive canopies is required to scale stomatally regulated gas exchange from leaf to canopy. Fluxes of terpenes and heat, formation of dew, and other processes occurring at the leaf surface may be controlled by g b . We develop, test, and deploy at five canopy heights, a set of paired, heated and unheated brass surrogate leaves, with realistic 3-dimensional shape and characteristic dimension ( d ) for cotton ( Gossypium hirsutum L.) in California. An equation describing heat transfer from a flat plate, enhanced by a factor of 1.15 for complex leaf shape, adequately described g b of the surrogate leaves. In the field, values of g b measured with the surrogate leaves near the top of the canopy were closely related to wind speed, as expected from theory. The dependence of g b on leaf size was apparent at all heights in the canopy using surrogate leaves differing in d . The profile of within-canopy wind speed ( u ) was determined using the profile of g b and d of the surrogate leaves. This profile of effective u in the leaf environment was well described by a conventional exponential decay model with an empirically derived wind extinction coefficient ( K = 1.5). This profile of u was combined with the directly measured profile of d of biological leaves to determine the actual profile of g b in this canopy. The increasing leaf size toward the middle of the canopy caused actual g b to decline more steeply than predicted from constant d . With depth in the canopy sunflecks increasingly contributed to unequal radiation interception between paired surrogate leaves and therefore to anomalies in calculated g b . A simple data filtering protocol, rejecting data outside the range observed at the top of the canopy, is introduced. Data rejection increased appropriately with increasing sunfleck frequency and depth in the canopy. These results extend the applicability of the heated, paired surrogate leaf technique to sheltered positions in dense canopies, and to large-leafed species, allowing direct measurement of an important canopy parameter ( g b ) that has typically been estimated or calculated as a residual.

Ozone (O 3 ) inhibits plant gas exchange and productivity. Vapour phase (g s ) and liquid or hydraulic phase (K) conductances to water flux are often correlated as both change with environmental parameters. Exposure of cotton plants to tropospheric O 3 reduces g s through reversible short-term mechanisms and by irreversible long-term disruption of biomass allocation to roots which reduces K. We hypothesize that chronic effects of O 3 on gas exchange can be mediated by effects on K without a direct effect of O 3 on g s or carbon assimilation (A). Experimental observations from diverse field and exposure chamber studies, and simulations with a model of mass and energy transport, support this hypothesis. O 3 inhibition of K leads to realistic simulated diurnal courses of g s that reproduce observations at low ambient O 3 concentration and maintain the positive correlation between midday g s and K observed experimentally at higher O 3 concentrations. Effects mediated by reduced K may interact with more rapid responses of g s and A to yield the observed suite of oxidant impacts on vegetation. The model extends these physiological impacts to the extensive canopy scale. Simulated magnitudes and diurnal time courses of canopy-scale fluxes of H 2 O and O 3 match observations under low ambient concentrations of O 3 . With greater simulated concentrations of O 3 during plant development, the model suggests potential reductions of canopy-scale water fluxes and O 3 deposition. This could represent a potentially unfavourable positive feedback on tropospheric O 3 concentrations associated with biosphere-atmosphere exchange.

Tropospheric ozone (O 3 ) causes significant disruption of growth and yield in upland and Pima cottons. Pima cotton (Gossypium barbadense L.) was exposed to brief pulses (0.75 h) of a range of O 3 concentrations (nominally 0.0, 0.2, 0.5, and 0.8 μl l -1 ) to investigate effects on phloem translocation of 14 C-labelled recent photo-assimilate. The initial phase of rapid efflux from source leaves was monitored with a Geiger-Muller Tube as activity remaining in the leaf as a function of time. Visual inspection of unprocessed efflux curves revealed disruption of efflux by O 3 . Single exponential decay functions were fitted to these efflux curves to extract first order rate constants for phloem loading and longitudinal transport of labelled carbohydrates. A single compartment model was applied, with and without an asymptote of non-transported carbohydrate, to calculate leaf sugar contents. The effect of O 3 in retarding efflux of label, decreasing the rate constant, and increasing calculated soluble sugar pools, was consistent regardless of the method of analysis. Following incorporation of the asymptote, calculated rate constants and sugar pools were similar to values from the literature and to preliminary measurements of sugar contents in O 3 -treated cotton leaves. Total carbohydrate transported from source leaves was reduced both by O 3 effects on assimilation (up to 20%) and by O 3 effects on efflux (up to 70%), but was clearly dominated by the impact on phloem translocation. These rapid efflux kinetics likely reflect oxidant damage at the plasmalemma or plasmodesmata of mesophyll or phloem companion cells. Evaluation of effects of O 3 on tonoplast function and consequences for carbohydrate translocation await a more complete compartmental efflux analysis.

Aerodynamic variables are required to apply Monin–Obukhov similarity theory in the bulk formulation of surface fluxes. In the literature, these aerodynamic variables are commonly misinterpreted. In this paper, we review the concept of the aerodynamic variable, its connection to surface-layer similarity theory and how and why the aerodynamic variable is replaced with other variables. Observed mean variables below the surface layer, such as the surface radiation temperature, or the air temperature at canopy height, are often used in place of the extrapolated aerodynamic variables in the bulk formula, requiring empirical relationships between aerodynamic and observed variables, or requiring empirical adjustments of bulk resistances. The present study examines the validity of these relationshi Experiment (CODE). The results indicate that using a measured substitute for an aerodynamic variable can lead to significant errors in estimates of turbulent surface fluxes.

Windblown fugitive dust contributes to violations of air quality standards for particulate matter