Microsatellite mapping of genes for branched spike and soft glumes in Triticum monococcum L.

The locus responsible for branched spike in a branched spike mutant of Triticum monococcum L. (2n = 2x = 14, A^mA^m genome) and soft glume in Triticum sinskajae Filat. et Kurkiev (2n = 2x = 14, A^mA^m genome) were mapped by genotyping F₂ populations using microsatellite markers. Phenotypic analysis in the cross T. sinskajae PI 418587/a branched spike mutant KT3-24 confirmed that both characters were under control of a recessive allele at a single locus, and they were linked with 26.6 cM. The branched head in T. monococcum (bh ^m ) locus was located on chromosome 2A^mS and the marker Xgwm122 flanked the bh ^m gene distally. Soft glume locus in T. sinskajae was allelic to the soft glume (sog) locus in mm09, a soft glume mutant of T. monococcum. The sog locus was linked with Xwmc644 distally. In the F₂ hybrids of T. monococcum #252/PI 418587 and T. monococcum KT 3-21/PI 418587, sog was linked with Xgwm71. The gene fg which determines a false glume was also located on chromosome 2A^mS and the recombination between sog and fg (1.6 cM) was obtained in F₂ hybrid of KT 3-21/PI 418587.     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

Changes of the displacement height and roughness length of maize during a growing season

Variations in the displacement height (d) and roughness length (z₀) of a maize crop were investigated through a growing season. A programme of measurement from which the wind profile, the Richardson gradient number and the turbulent fluxes of heat and momentum were estimated, was carried out.Two techniques were combined to obtain unique values for d and z₀: the log-profile fitting technique and the eddy correlation technique. Throughout the growing season, the displacement height appeared to correlate very well (r = 0.98) with the height (H) of the canopy. A mean value for d/H was 0.75. The roughness length was strongly correlated with the difference between the canopy height and the displacement height. A mean value of 0.26 for z₀/(H − d) was found (r = 0.86). If the ratio d/H was fixed, the roughness length did not show any clear dependence on wind speed or thermal stability.     

Long-term (13 years) measurements of SO2 fluxes over a forest and their control by surface chemistry

Long-term fluxes of sulphur dioxide (SO₂) have been measured over a mixed suburban forest subjected to elevated SO₂ concentrations. The net exchange was shown to be highly dynamic with substantial periods of both upward and downward fluxes observed in excellent conditions for flux measurement. Upward fluxes constituted 30% of selected fluxes and appeared more frequently when the canopy was acidic. Upward fluxes were shown to be due to desorption from a drying surface or when ambient levels declined after periods of increased SO₂ exposure.The long term average SO₂ flux (F) was −59 ng SO₂ m⁻² s⁻¹ for the period 1997-2009 corresponding to an average SO₂ concentration of 12.3 μg SO₂ m⁻³ and a deposition velocity υ_d of 5 mm s⁻¹. The smallest deposition fluxes and υ_d were measured in dry conditions (−42 ng m⁻² s⁻¹ and 3.5 mm s⁻¹, resp.), which represented 57% of all cases. Wet canopies were more efficient sinks for SO₂ and a dew-wetted canopy had a smaller υ_d (6 mm s⁻¹) than a rain-wetted canopy (ca 10 mm s⁻¹). Seasonal variability reflected differences in chemical climate or canopy buffering properties. During the summer half-year when surface acidity was low due to higher NH₃/SO₂ ratios, a higher deposition efficiency (υ_d/υ_dmax) and lower non-stomatal resistance (R_w) were observed compared to winter conditions. Comparisons of R_c for different combinations of canopy wetness and surface acidity categories emphasized the importance of both factors in regulating the non-stomatal sinks of SO₂. Increased surface water acidity gradually led to a lower υ_d/υ_dmax and an increased R_c for all considered canopy wetness categories. The smallest υ_d/υ_dmax ratio and highest R_c were obtained for a dry canopy with high surface acidity. Conversely, a rain-wetted canopy was the most efficient sink for SO₂. The canopy sink strength was further enhanced by high friction velocities (u_*), optimizing the mechanical mixing into the canopy. Long-term trends were strongly coupled to changes in the NH₃/SO₂ ratio, which has clearly enhanced the deposition efficiency of SO₂ in recent years.     

Use of Fast Repetition Rate Fluorometry on Detection and Assessment of PAH Toxicity on Microalgae

Cultures of the flagellate Isochrysis galbana were used to carry out the ecotoxicological evaluation of four PAHs [(naphthalene, phenanthrene, pyrene (Pyr) and fluoranthene (Flu)] by monitoring growth rate and the fluorescence variables F ₀, F _m, F _v and F _v/F _m, determined with a fast repetition rate fluorometer. The results presented in this investigation showed that F _v was a suitable endpoint in acute ecotoxicological tests with marine phytoplankton. The derived effective concentrations followed the known narcotic mechanism of toxicity and showed sensitivity levels comparable to marine invertebrate embryo-larval bioassays. Pyr and Flu showed the lowest EC₁₀, which ranged between 168–279 and 189–697 nM, respectively.     

Towards an improved and more flexible representation of water stress in coupled photosynthesis-stomatal conductance models

Coupled photosynthesis-stomatal conductance (A-g_s) models are commonly used in ecosystem models to represent the exchange rate of CO₂ and H₂O between vegetation and the atmosphere. The ways these models account for water stress differ greatly among modelling schemes. This study provides insight into the impact of contrasting model configurations of water stress on the simulated leaf-level values of net photosynthesis (A), stomatal conductance (g_s), the functional relationship among them and their ratio, the intrinsic water use efficiency (A/g_s), as soil dries. A simple, yet versatile, normalized soil moisture dependent function was used to account for the effects of water stress on g_s, on mesophyll conductance (g_m) and on the biochemical capacity. Model output was compared to leaf-level values obtained from the literature. The sensitivity analyses emphasized the necessity to combine both stomatal and non-stomatal limitations of A in coupled A-g_s models to accurately capture the observed functional relationships A vs. g_s and A/g_svs. g_s in response to drought. Accounting for water stress in coupled A-g_s models by imposing either stomatal or biochemical limitations of A, as commonly practiced in most ecosystem models, failed to reproduce the observed functional relationship between key leaf gas exchange attributes. A quantitative limitation analysis revealed that the general pattern of C₃ photosynthetic response to water stress may be well represented in coupled A-g_s models by imposing the highest limitation strength to g_m, then to g_s and finally to the biochemical capacity.     

Relationships between single tree canopy and grass net radiations

It is reported a simple approach to transform daily values of grass net (all-wave) radiation (R_n, MJ m⁻² day⁻¹), as measured over standard grass surface at meteorological stations, into whole tree canopy net radiation (A, MJ tree⁻¹ day⁻¹). The revolving Whirligig device [McNaughton, K.G., Green, S.R., Black, T.A., Tynam, B.R., Edwards, W.R.N., 1992. Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree. Agric. For. Meteorol. 62, 87–107] describing a sphere about the tree measured A in five trees of different species (walnut, dwarf apple, normal apple, olives and citrus), with leaf area L_A varying from 8.65 to 40 m². For each tree, A and R_n were linearly related (A = bR_n), as previously reported elsewhere, but it was found that the slope of such regression was also a linear function of L_A or, b = 0.303 (±0.032) L_A. Consequently, the hypothesis that total daily tree canopy net radiation per unit leaf area is linearly related to grass net radiation could not be rejected after 86 days of measurements in five locations, and the empirical relationship is A = 0.303 (±0.032) R_nL_A (R² = 0.9306).     

Empirical and optimal stomatal controls on leaf and ecosystem level CO2 and H2O exchange rates

Linkage between the leaf-level stomatal conductance (g_s) response to environmental stimuli and canopy-level mass exchange processes remains an important research problem to be confronted. How various formulations of g_s influence canopy-scale mean scalar concentration and flux profiles of CO₂ and H₂O within the canopy and how to derive ‘effective’ properties of a ‘big-leaf’ that represents the eco-system mass exchange rates starting from leaf-level parameters were explored. Four widely used formulations for leaf-level g_s were combined with a leaf-level photosynthetic demand function, a layer-resolving light attenuation model, and a turbulent closure scheme for scalar fluxes within the canopy air space. The four g_s models were the widely used semi-empirical Ball-Berry approach, and its modification, and two solutions to the stomatal optimization theory for autonomous leaves. One of the two solutions to the optimization theory is based on a linearized CO₂-demand function while the other does not invoke such simplification. The four stomatal control models were then parameterized against the same shoot-scale gas exchange data collected in a Scots pine forest located at the SMEAR II-station in Hyytiälä, Southern Finland. The predicted CO₂ (F_c) and H₂O fluxes (F_e) and mean concentration profiles were compared against multi-level eddy-covariance measurements and mean scalar concentration data within and above the canopy. It was shown that F_c comparisons agreed to within 10% and F_e comparisons to within 25%. The optimality approach derived from a linearized photosynthetic demand function predicted the largest CO₂ uptake and transpiration rates when compared to eddy-covariance measurements and the other three models. Moreover, within each g_s model, the CO₂ fluxes were insensitive to g_s model parameter variability whereas the transpiration rate estimates were notably more affected. Vertical integration of the layer-averaged results as derived from each g_s model was carried out. The sensitivities of the up-scaled bulk canopy conductances were compared against the eddy-covariance derived canopy conductance counterpart. It was shown that canopy level g_s appear more sensitive to vapor-pressure deficit than shoot-level g_s.     

Modification of ozone effects on photosynthetic capacity of winter wheat (Triticum aestivum L. CV. perlo) at different levels of water availability

Attached leaves were used for the determination of the photochemical capacity by means of a portable fluorimeter. Repeated fluorescence measurements showed the negative effects of ozone on photochemical capacity and these negative effects increased with increasing ozone doses. But impairments of photochemical capacity were smallest if severe water stress co-occurred with ozone exposures. The upper leaf sides experienced more reduction of photochemical capacity in well-watered plants than the lower leaf sides, possibly by the additional effect of light stress on the upper leaf sides. In diurnal studies, a decline of F _v/F _m was observed at noontime and a recovery at evening in both control and ozone-fumigated leaves at two extreme water capacities (w.c.) (75% and 35% of w.c.). The extent of depression and recovery of F _v/F _m was not significantly varying. The oscillations of F _v/F _m could be due to short-term disturbances in the photosynthetic capacity, due to oxidative stress.     

Microbial growth efficiencies across a soil moisture gradient assessed using C-acetic acid vapor and N-ammonia gas

One integrative measurement of microbial activity in soils is the efficiency by which microbes convert assimilated carbon (C) into biomass C. This efficiency, called the microbial growth efficiency (Y), is a key physiological characteristic that regulates soil carbon sequestration, nutrient immobilization, and greenhouse gas emissions. Changes in rainfall patterns and soil water content as the result of global climate change have the potential to influence microbial activity and lead to changes in Y and thus, nutrient cycling at the ecosystem level. Unfortunately, little information is available on how environmental variables such as soil moisture influence Y. We have developed a new method for injecting ¹³C-labeled carbon as acetic acid vapor into soil that will allow measurement of microbial growth efficiency (as Y_C) without increasing soil moisture content. We compare Y determined with this new approach with an alternate method where injected ¹⁵N-labeled ammonia gas is used to quantify microbial N immobilization, and microbial growth efficiency is calculated based on microbial C:N and respiration rate (as Y_N). We also include injections of a solution containing labeled ammonium and acetate in our experiment to compare the results of our vapor methods with more commonly employed liquid-based methods. The ¹³C-acetic acid vapor, which was supplied to soils with soil moisture content ranging from 0.05 to 0.21 g H₂O g⁻¹ soil, was readily assimilated and respired by microbes. Between 0.10 and 0.21 g H₂O g⁻¹ soil (−0.60 to −0.04 MPa), values of Y_C averaged 0.46, and were significantly lower than values of Y_N, with average values of 0.58. Over this range, soil moisture content had no significant effect on either Y_C or Y_N. However, at the lowest soil moisture content (0.05 g H₂O g⁻¹ soil; <−6.0 MPa), Y_C and Y_N diverged substantially, suggesting that in very dry soils, constraints on microbial growth cause differential uptake of C and N resources.     

The magnitude and variability of soil-surface CO2 efflux increase with mean annual temperature in Hawaiian tropical montane wet forests

Soil-surface CO₂ efflux (F_S; ‘soil respiration’) accounts for ≥50% of the CO₂ released annually by the terrestrial biosphere to the atmosphere, and the magnitude and variability of this flux are likely to be sensitive to climate change. We measured F_S in nine permanent plots along a 5.2 °C mean annual temperature (MAT) gradient (13-18.2 °C) in Hawaiian tropical montane wet forests where substrate type and age, soil type, soil water balance, disturbance history, and canopy vegetation are constant. The objectives of this study were to quantify how the (i) magnitude, (ii) plot-level spatial variability, and (iii) plot-level diel variability of F_S vary with MAT. To address the first objective, annual F_S budgets were constructed by measuring instantaneous F_S monthly in all plots for one year. For the second objective, we compared plot-level mean instantaneous F_S in six plots derived from 8 versus 16 measurements, and conducted a power analysis to determine adequate sample sizes. For the third objective, we measured instantaneous F_S hourly for 24 h in three plots (cool, intermediate and warm MATs). The magnitude of annual F_S and the spatial variability of plot-level instantaneous F_S increased linearly with MAT, likely due to concomitant increases in stand productivity. Mean plot-level instantaneous F_S from 8 versus 16 measurements per plot yielded statistically similar patterns. The number of samples required to estimate plot-level instantaneous F_S within 10% and 20% of the actual mean increased with MAT. In two of three plots examined, diel variability in instantaneous F_S was significantly correlated with soil temperature but minimal diel fluctuations in soil temperature (<0.6 °C) resulted in minimal diel variability in F_S. Our results suggest that as MAT increases in tropical montane wet forests, F_S will increase and become more spatially variable if ecosystem characteristics and functioning undergo concurrent changes as measured along this gradient. However, diel variation in F_S will remain a minor component of overall plot-level variation.     

Physiological Effects of Exposure to Arsenic,Mercury, Antimony and Selenium in the Aquatic Moss Fontinalis antipyretica Hedw.

This study assessed the effect of ambient air pollution on leaf characteristics of white willow, northern red oak, and Scots pine. Willow, oak, and pine saplings were planted at sixteen locations in Belgium, where nitrogen dioxide (NO₂), ozone (O₃), sulfur dioxide (SO₂), and particulate matter (PM₁₀) concentrations were continuously measured. The trees were exposed to ambient air during 6 months (April–September 2010), and, thereafter, specific leaf area (SLA), stomatal resistance (R _s), leaf fluctuating asymmetry (FA), drop contact angle (CA), relative chlorophyll content, and chlorophyll fluorescence (F _v/F _m) were measured. Leaf characteristics of willow, oak, and pine were differently related to the ambient air pollution, indicating a species-dependent response. Willow and pine had a higher SLA at measuring stations with higher NO₂ and lower O₃ concentrations. Willow had a higher R _s and pine had a higher F _v/F _m at measuring stations with a higher NO₂ and lower O₃ concentrations, while oak had a higher F _v/F _m and a lower FA at measuring stations with a higher NO₂ and lower O₃ concentrations. FA and R _s of willow, oak, and pine, SLA of oak, and CA of willow were rather an indicator for local adaptation to the micro-environment than an indicator for the ambient air pollution.     

Using digital repeat photography and eddy covariance data to model grassland phenology and photosynthetic CO2 uptake

The continuous and automated monitoring of canopy phenology is of increasing scientific interest for the multiple implications of vegetation dynamics on ecosystem carbon and energy fluxes. For this purpose we evaluated the applicability of digital camera imagery for monitoring and modeling phenology and physiology of a subalpine grassland over the 2009 and 2010 growing seasons.We tested the relationships between color indices (i.e. the algebraic combinations of RGB brightness levels) tracking canopy greenness extracted from repeated digital images against field measurements of green and total biomass, leaf area index (LAI), greenness visual estimation, vegetation indices computed from continuous spectroradiometric measurements and CO₂ fluxes observed with the eddy covariance technique. A strong relationship was found between canopy greenness and (i) structural parameters (i.e., LAI) and (ii) canopy photosynthesis (i.e. Gross Primary Production; GPP). Color indices were also well correlated with vegetation indices typically used for monitoring landscape phenology from satellite, suggesting that digital repeat photography provides high-quality ground data for evaluation of satellite phenology products.We demonstrate that by using canopy greenness we can refine phenological models (Growing Season Index, GSI) by describing canopy development and considering the role of ecological factors (e.g., snow, temperature and photoperiod) controlling grassland phenology. Moreover, we show that canopy greenness combined with radiation use efficiency (RUE) obtained from spectral indices related to photochemistry (i.e., scaled Photochemical Reflectance Index) or meteorology (i.e., MOD17 RUE) can be used to predict daily GPP.Building on previous work that has demonstrated that seasonal variation in the structure and function of plant canopies can be quantified using digital camera imagery, we have highlighted the potential use of these data for the development and parameterization of phenological and RUE models, and thus point toward an extension of the proposed methodologies to the dataset collected within PhenoCam Network.     

Some Measurement Results on Sulfate Aerosol Concentration in and above a Pine Canopy

Profile of sulfate aerosol (SO₄ ^2?) concentration was measured for four days at six heights in and above a 15m-high canopy of pine plantation during 6th and 10th August. 1999. The concentration was the lowest (about 2 nmol/m³) on 6th, and gradually increased to 9th showing the maximum values of about 13 nmol/m³, and then decreased to 2 nmol/m³ on 10th. The vertical profiles of SO₄ ^2? concentration showed mostly higher in the canopy than above the canopy. As for the vertical profiles above the canopy on 8th and 10th, the minimum was observed just above the canopy (16m), showing SO₄ ^2? transport from the upper air layer to the canopy. While on 9th the profiles that are higher concentration just above the canopy and lower at the upper air layer were observed, suggesting SO₄ ^2? emission from the canopy to the upper air layer.     

Water use efficiency in a soybean field: influence of plant water stress

Measurements were made in 1980 over a fully-developed soybean (Glycine max (L.) Merrill) canopy at Mead, Nebraska to determine how crop water status influences photosynthesis, evapotranspiration and water use efficiency. Water use efficiency was calculated in terms of the CO₂—water flux ratio (CWFR). Micrometeorological techniques were used to measure the exchange rates of CO₂ and water vapor above the crop canopy. Crop water status was evaluated by reference to volumetric soil moisture (θ_v), stomatal resistance (r_s), and leaf water potential (ψ) measurements.Stomatal resistance (r_s) was independent of ψ when the latter was greater than ?1.1 MPa. r_s increased sharply as ψ dropped below this threshold. Canopy CO₂ exchange (F_c) decreased logarithmically with increasing r_s under strong irradiance. Although F_c was found to be strongly correlated with r_s, the influence of low values of ψ and of high air temperature cannot be discounted since these factors affect the enzymatic reactions associated with photosynthesis. Stomatal closure also reduced evapotranspiration and influenced the partitioning of net radiation.Under strong irradiance the CO₂ water flux ratio (CWFR) decreased with increasing stomatal resistance. This observation is at variance with predictions of certain early ‘resistance’ models, but substantiates predictions of some recent models in which leaf energy balance considerations are incorporated.     

Protozoan grazing affects estimates of carbon utilization efficiency of the soil microbial community

Reliable estimates of microbial growth yield efficiency (Y=microbial production/substrate utilization) are needed to quantify and predict soil carbon (C) dynamics. We examined patterns of C utilization in two soils, a Paleustoll (USA) and Rhodoxeralf (Australia), under two levels of protozoan grazing (low vs high) when substrate was not limiting. Soil, either amended with unlabeled or ¹⁴C-labeled glucose was incubated at 25°C and glucose-C concentration, CO₂-C evolution, and microbial biomass-C were determined over a 12–20 h period. Three approaches were used for estimating Y: Y_s=(dS_C−ΣCO₂-C)/dS_C, Y_b=dB_C/(dB_C+ΣCO₂-C), and Y_c=dB_C/dS_c where dS_C is the change in substrate concentration (substrate utilization), ΣCO₂-C the cumulative amount of CO₂-C evolved, and dB_C the change in microbial biomass (biomass production). Calculation of Y_s assumes that all substrate-C utilized, minus that respired, is used for biomass and metabolite production. Calculation of Y_b assumes that substrate use equals biomass-C plus respired-C and does not account for biomass production consumed by grazers. Under low grazing, the three estimates of Y were similar with an average value of 0.58 and 0.55 for the Paleustoll and Rhodoxeralf, respectively. Under high grazing, the value of Y varied depending on the calculation used, with values of Y_b (0.44) and Y_c (0.26) being significantly lower than Y_s (0.67). The total amount of glucose utilized did not vary with protozoan grazing intensity, but a high level of grazing increased the rate of glucose use and significantly reduced the amount of measurable biomass C. Substrate-based yield (Y_s) provided the most reliable C assimilation efficiency estimate under both grazing treatments.     

A GC/MS method for the assessment of N and C incorporation into soil amino acid enantiomers

Identifying the transformation process of amino acid enantiomers was essential to probe into the fate, turnover and aging of soil nitrogen due to their important roles in the biogeochemical cycling. If this can be achieved by differentiating between the newly biosynthesized and the inherent compounds in soil, then the isotope tracer method can be considered most valid. We thereby developed a gas chromatography/mass spectrometry (GC/MS) method to trace the ¹⁵N or ¹³C isotope incorporation into soil amino acid enantiomers after being incubated with ¹⁵NH₄⁺ or U-¹³C-glucose substrates. The most significant fragments (F) as well as the related minor ions were monitored by the full scan mode and the isotope enrichment in amino acids was estimated by calculating the atom percentage excess (APE). ¹⁵NH₄⁺ incorporation was evaluated according to the relative abundance increase of m/z F+1 to F for neutral and acidic amino acids and F+2 to F (mass 439) for lysine. The assessment of ¹³C enrichment in soil amino acids was more complicated than that of ¹⁵N due to multi-carbon atoms in amino acid molecules. The abundance ratio increment of m/z F+n to F (n is the original skeleton carbon number in each fragment) indicated the direct conversion from the added glucose to amino acids, but the total isotope incorporation from the added ¹³C can only be calculated according to all target isotope fragments, i.e. the abundance ratio increment summation from m/z (Fa+1) through m/z (Fa+T) represented the total incorporation of the added ¹³C (Fa is the fragment containing all original skeleton carbons and T is the carbon number in the amino acid molecule). This method has a great advantage especially for the evaluation of high-abundance isotope enrichment in organic compounds compared with GC/C/IRMS. And in principle, this technique is also valid for amino acids besides enantiomers if stereoisomers are not concerned. Our assessment approach could shine a light on investigating the biochemical mechanism of microbial transformation of N and C in soils of terrestrial ecosystem.     

Mechanical and hydraulic resistance relations in crust-topped soils

The formation of soil surface crusts leads to increased mechanical and hydraulic resistances. In this study, changes and relationships of both resistances under simulated sprinkle irrigation (or rainfall), and sprinkle followed by flooding, were examined. Results indicated that a silt-loam soil developed a thicker surface crust than a clay soil for any given kinetic energy (KE). Crusts as thick as 3.9 and 2.6 mm formed on the silt-loam and clay soils, respectively. Mechanical resistance, R_m, increased with increasing KE, where the effect was greater in the silt-loam and was attributed to intrinsic resistance and crust thickness. Steady-state infiltration rate (i) was much lower in crusted clay than crusted silt-loam soil. Changes of both R_m, and i closely followed changes in crust thickness (z_c). Thicker crusts showed more resistance against external force than thinner crusts, due to more extended particle interlocking. Obtained functions indicated that the effect of thickness on strength was more significant in the lower range of crust thickness. The effect of z_c on i strongly followed a negative power function for both soils, with higher i in the silt-loam soil.     

Problems with porometry: measuring stomatal conductances of potentially transpiring plants

Porometer measurements of the stomatal conductances (C_s) of potentially transpiring water hyacinth plants at Phoenix, Arizona in October of 1984, May–June of 1985, and September of 1986 indicate that C_s steadily drops as the vapor pressure deficit (VPD) of the air in the measuring system's cuvette or leaf chamber rises. Utilizing this relationship to calculate the foliage-air temperature differential (T_F−T_A) response of these leaves to leaf-chamber air VPD, as per the basic equations of standard heat and water vapor transport theory; we obtain a leaf-chamber “non-water-stressed baseline” that is consistent with leaf-chamber measurements of T_F−T_A vs. air VPD. Free-air T_F−T_A vs. air VPD data, on the other hand, produce a relationship that is similarly consistent with a plant stomatal conductance which is invariant with respect to the air VPD. Hence, we conclude that the very act of stomatal conductance measurement alters a potentially transpiring plant's evaporative water loss rate in such a way that, for very high air VPD conditions, the directly measured C_s value (although correct for the leaf in the cuvette or leaf chamber) may be much reduced from that characteristic of comparable non-chamber-encumbered plants in the free air. We then demonstrate that this instrument-induced reduction in directly measured C_s values is a unique function of the leaf-chamber IJ index, evaluated with respect to the plant's free-air non-water-stressed baseline. Similar results obtained by others for cotton suggest that this phenomenon may be quite general, and that the C_s vs. air VPD interaction, believed by many to be widely operative throughout the plant kingdom, may not really exist in actual field situations.