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Cell and leaf elongation have been reported to be highly sensitive to plant water deficit and may represent the primary mechanism of stress and yield reduction. This study was designed to determine if leaf expansion is restricted by the routine daytime water deficit experienced by field plants in moist soil. Leaf elongation in soybean [Glycine max(L.) Merrill ‘Rampage’] was monitored during several 2 to 4‐day periods in the field in conjunction with pressure bomb measurement of water and osmotic and turgor potential. Short‐term (minutes) leaf‐elongation rate was highly responsive to step changes in water deficit but gradually recovered over periods of about an hour. Length that was lost during the transient rate reduction tended to be made up by high rates following improved water status. Mean elongation rates were not limited by normal daytime water deficit (turgor pressure as low as 2 bars), except late in the season when pod‐filling occurred at nodes adjacent to the growing leaves. Elongation rate was always sensitive to temperature. We conclude that within the range of moderate, daytime water deficits we encountered, soybean leaves may adapt to the extent that turgor pressure does not limit growth.

Carbon dioxide gas was released from one northeastsouthwest line in a corn (Zea maysL.) field during several days in August and September, 1969. CO2concentration profiles were measured at 15 and 45 m downwind. The purposes of the releases were: (1) to study the micrometeorological transport processes in plant canopies using CO2, as a tracer, and (2) to determine the micrometeorological feasibility of CO2enrichment under field conditions. The study showed that vertical turbulent diffusion and horizontal mass flow quickly diminished the concentration of CO2, downwind from the line source. High concentrations of CO2could be maintained in the canopy only under a stable temperature stratification of the air which reduced vertical wind fluctuations. Short‐term eddies with CO2concentrations in excess of 1,000 ppm were observed 30 m downwind from the release line at an analyzer height of 530 cm, but these were very infrequent. CO2concentration profiles showed that CO2tended to be turbulently diffused upward and wafted downwind, with some of the CO2being again transported into the canopy. Turbulent diffusion in the canopy was usually too active to maintain a high CO2concentration; however, this same effectiveness of turbulence assures an adequate supply of CO2to plant canopies under typical field conditions. This study showed, from a micrometeorological viewpoint, that CO2enrichment would not be practical under most natural field and crop conditions because of rapid gaseous exchange with the bulk atmosphere.

To fully understand the pollution potential of pesticides more knowledge concerning the mechanisms and rates of pesticide exchange between environmental compartments is needed. The momentum balance method was used in a field study to characterize toxaphene (chlorinated camphene) volatilization from cotton (Gossypium hirsutumL.) plants for 5 days following aerial application at 2.24 kg/ha. The momentum balance method uses accurate measurements of windspeed profiles, temperature gradients, and atmospheric pesticide concentration gradients above the plant canopy to provide data for calculating vertical flux densities of pesticides. The calculated volatile loss for the 5‐day period was 358 g/ha, which represented a loss of 26% of the toxaphene present in the canopy. Although typical volatile loss patterns suggested that flux densities were highest during midafternoon, there was evidence that volatility rates were also high when leaves were drying after heavy dew or light rain. Based on comparisons of the amounts of toxaphene transported from nearby cotton fields via surface runoff in earlier studies and the amounts lost by volatilization in this study, it was concluded that aerial transport is the pathway of greater loss.

The penetration and distribution of light in leaf canopies are essential data to understanding the photosynthetic productivity of a crop. Light sensors which measure photosynthetically active radiation were constructed and traversed through the leaf canopies of six types of corn (Zea maysL.) to observe radiation penetration. Under clear sky conditions two major irradiance levels were observed in the corn canopies. One irradiance level was slightly less than the total radiation above the crop and represented the occurrence of sunflecks penetrating into the canopy. The second irradiance level was much less than the first and was comprised of sky radiation and radiation scattered by leaves. However, a decrease in the amount of direct solar radiation by either an overcast sky or a low sun angle resulted in a single, broad‐band irradiance level. The data on sunfleck penetration also provided a basis for testing two physical models of light penetration in leaf canopies. Data from corn planted in a uniform, hexagonal pattern was in good agreement with the Poisson model which assumes a random leaf distribution. However, the Markov chain model which introduces a frequency gap parameter, λ, to account for nonuniform leaf distribution was required to obtain good agreement with data recorded in the canopy of corn planted in rows 76‐cm apart. This was especially true for the top part of the row‐planted corn where small, short leaves resulted in large gaps for radiation penetration. The Markov model would be necessary, for instance, to determine the penetration of photosynthetically active radiation to the leaves in the middle of the canopy. Nevertheless, the Poisson model gave good agreement with the measured values for radiation intercepted by the entire canopy of the row‐planted crop.

A traversing system with a fast responding, photocell light sensor was used to measure mean irradiance in corn (Zea maysL.) canopies. Data were recorded by digital and analog recorders to analyze the sampling problems with a moving sensor. Sampling rates of once every 4 seconds for a row‐planted crop and once every 8 seconds for a uniformly planted crop were required to yield satisfactory mean irradiance values from a fast responding sensor. In addition, sensors with time constants at least as long as 8 seconds could be used to measure mean irradiance. Therefore, we recommend that the sampling rate for a traversing sensor in a maize leaf canopy be at least as frequent as the time constant of the sensor, but in any case the sampling rate does not need to be any more frequent than once every 4 seconds.

Iron occurs in extremely high concentrations in certain maritime Antarctic freshwater lakes which seasonally develop an anoxic zone. In oligotrophic Sombre Lake the data show that Fe(II) precipitates as Fe(III) oxyhydroxides which bind phosphorus and return it to the sediments. In nutrient-enriched Amos lake, significant quantities of sulphide are also produced and this binds a proportion of the released Fe(II) so reducing the ratio of total iron to phosphorus at the redox boundary where the oxyhydroxides are formed. A proportion of the sediment-released phosphorus therefore reaches the upper waters of this lake (unlike in Sombre Lake) and provides the initial nutrient source for under-ice phytoplankton development in spring. Iron-reducing bacteria have been isolated, from Sombre Lake sediments, which apparently utilise the abundant Fe(III) oxyhydroxides. From thermodynamic considerations (assuming Fe(III) is not limiting) these should outcompete sulphate reducers and methanogens (both previously reported from Sombre and Amos Lakes) and could therefore constitute an important component of the anaerobic mineralisation of organic carbon in such lakes.

Hot-wire anemometers were used to measure air temperature and the three velocity components of the wind within and above a maize canopy. From digitized anemometer outputs, correlation coefficients for vertical heat flux and turbulent momentum transfer were calculated. A comparison of these coefficients with profiles of mean wind speed and mean temperature indicates that the main features of the turbulence may be explained in terms of the usual mixing-length theory. Instantaneous records of heat and momentum flux, however, indicate the existence of other competing turbulent mechanisms due to the unsteady, non-equilibrium nature of the turbulent flow. Regimes of flow dominated by mechanical and/or thermal mixing are indicated. Spectral results show that high shear and turbulent intensity levels as well as the presence of the maize leaves and stalks as vortex-shedding surfaces complicate the energy transfer mechanism. An energy balance between radiation and convection reveals that the energy budget is primarily a balance between solar radiation and the flux of latent heat.

The Bowen-ratio profile method for calculating total carbon dioxide, latent heat, and sensible heat flux density above a corn crop was used by measuring temperature, water vapor, and CO2 concentrations at several heights in the aerodynamic boundary layer of the crop. The ratio (a) of sensible heat flux density to carbon dioxide flux density as well as the Bowen ratio (ß) were used in the computations. The two ratios, a and ß were determined graphically from the slopes of linear plots of temperature vs CO2 concentration and vs water vapor. Each of the energy flux densities was computed from the two ratios and net radiation minus soil heat flux density.

A modified infrared CO2 gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO2 above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated:(1)Sensors with a time constant of 0.5 s appear to be fast enough to detect most of the vertical CO2 transfer as long as the sensors are located at least one meter above the crop surface.(2)The deviation from steady-state conditions for 10-min periods was found to have a significant effect on the eddy flux estimates.(3)Temperature fluctuations of the air sample passing through the CO2 infrared gas analyzer were found to be non-negligible but could be easily corrected.(4)A 1° misalignment of the vertical anemometer affected these eddy fluxes by less than 10% under all circumstances studied.

The safety and efficacy of a new topical antiinfective agent, mupirocin, was compared with that of oral erythromycin ethylsuccinate in the treatment of impetigo in children. Sixty-two children aged 5 months to 13 years with impetigo were assigned to be treated with either mupirocin in three daily applications or erythromycin ethylsuccinate (40 mg/kg of body weight per day divided into four doses) according to a randomized treatment schedule. On the initial visit, exudate or cleansed infected sites or both were cultured and therapy was begun. All patients were treated for 8 days. Patients were seen again on days 4 to 5 of therapy, at the end of therapy, and 7 days after the end of therapy. Sites of infection were comparable between the groups, as were bacteriologic responses. At the first visit, 24 of 30 children in the mupirocin group and 14 of 32 children in the erythromycin group were cured or had at least a 75% reduction in size of the lesions. At the end of the study, all 29 of the children in the mupirocin group who came to follow-up, compared with 27 of 29 in the erythromycin group, were cured. Side effects were few. Five children in the erythromycin group developed mild diarrhea. Thus, mupirocin appears to be safe and effective in treating impetigo in children. Our data show a trend toward more rapid clinical response with mupirocin than with erythromycin.

The pharmacokinetics of piperacillin and tazobactam were assessed after single-dose administration to 47 infants and children. Study subjects ranging in age from 2 months to 12 years were randomized to receive one of two different doses of a piperacillin-tazobactam combination (8:1): a low dose (n = 23) of 50 and 6.25 mg of piperacillin and tazobactam per kg of body weight, respectively, or a high dose (n = 24) of 100 and 12.5 mg, respectively. The pharmacokinetic behavior of tazobactam was very similar to that observed for piperacillin, supporting the use of these two agents in a fixed-dose combination. No differences in the pharmacokinetics of piperacillin or tazobactam were observed between the two doses administered. The elimination parameters half-life and total body clearance decreased and increased, respectively, with increasing age, whereas volume parameters (volume of distribution and steady-state volume of distribution) remained relatively constant for both compounds. The primary metabolite of tazobactam, metabolite M1, was measurable in the plasma of 18 of the 47 study subjects; 17 of these 18 subjects received the high doses. More than 70% of the administered piperacillin and tazobactam doses were excreted unchanged in the urine over a 6-h collection period. These data combined with the known in vitro susceptibilities of a broad range of pediatric bacterial pathogens indicate that a dose of 100 mg of piperacillin and 12.5 of mg tazobactam per kg of body weight administered as a fixed-dose combination every 6 to 8 h would be appropriate to initiate clinical efficacy studies in infants and children for the treatment of systemic infections arising outside of the central nervous system.

The fluctuations of photosynthetically active radiation within different corn (Zea maysL.) varieties were studied in the frequency domain using spectral analysis. This approach which uses a single traversing sensor, provides a simple method for separating the temporal and spatial light fluctuations in a crop. It was found that the light spectra are influenced by the height of the light sensors within crop structure and by wind conditions. The most rapid fluctuations observed were about 10 cycles/second, which tends to discount their importance in the dark reaction of photosynthesis. Finally, these results give a better perspective on fluctuating light experiments.

An evaluation of the amount of radiant energy utilized in evapotranspiration under field conditions briefly considers the radiation balance, the various components of the energy balance and their relative magnitudes, and the crop and soil factors influencing the amount of the net radiation exchange utilized in evapotranspiration. Data presented show that when soil moisture is available and a substantial crop cover shades the ground most of the net radiation is used in the evapotranspiration process.

This paper takes up in detail the various factors that influence the oxygen “demand” characteristics of plant roots. Emphasis is given to those root parameters needed in a theoretical model developed for roots growing in soil.

This paper theoretically approaches the problem of adequacy of oxygen for normal root respiration in the soil environment. First to be considered is the root oxygen “demand” aspect of the problem. This is followed by considering the “supply” characteristics of the soil environment. It is theoretically shown that the two aspects of the problem are interdependent and cannot be divorced.

The results of this study agree with the hypothesis that the electrical conductivity of corn stem tissue is primarily related to hydration of the cell wall. Plant stem resistance measurements taken before 8:00 a.m. show a carry‐over effect of moisture stress experienced by the plant during the preceding 24‐hour period or longer. Relative turgidity measurements of corn leaves taken at the same time did not show a marked carry‐over effect of internal moisture stress.

The effects of air turbulence upon the exchange of gases between a soil and the atmosphere were investigated using a specially designed vapor exchange meter. The device measured the rate of evaporation of liquid heptane from a porous stainless steel plate buried in a porous medium. Natural air turbulence as indicated by both mean windspeed and root mean square air pressure fluctuations significantly affected the heptane evaporation rates in straw and coarse gravel, but had a decreased effect in media of smaller pore size. In silt loam, the regression coefficient for evaporation rate on windspeed was 20% of its value for straw. Increasing depth from 1 to 8 cm in coarse sand caused a 91% decrease. Although the heptane flux values are quite scattered, the data indicate that air turbulence can significantly increase the transport of water vapor through coarse mulches or through very shallow depths of soil, but that soil aeration is mostly a diffusive process.

The current obtained in the electrolysis of an electro‐reducible material, as oxygen, at the surface of a platinum electrode can be used to calculate the rate of diffusion of that material to the surface. Measurements made in the soil, based on this principle, give promise of furnishing a new, simple, rapid, and inexpensive method of determining the rate of oxygen supply to an undisturbed environment similar to that in the liquid film surrounding an actively respiring plant root. The method compared favorably with other methods for measuring soil aeration in a greenhouse experiment. Evidence presented indicates that factors controlling the diffusion rate in the gaseous phase of the soil extend into the liquid phase as well. The data show greater oxygen diffusion rates with larger aggregates and greater soil porosity. The rates decreased with increasing depth below the soil surface. At the lower porosities moisture film thickness has a greater effect on oxygen supply than at the higher porosities. Growth of tomatoes correlates well with oxygen diffusion measurements made with the microelectrode, and a critical rate is suggested for their growth.

This paper has two objectives: (1)to make available to American workers the results of Russian work on soil moisture evaporation suppression; and (2)to lay a background for our present work along these lines; some of which is included. In the evaporation process, the loss of soil moisture can be divided into three stages. The first stage is that of rapid loss of water where capillary flow to the soil surface is sufficient to meet the evaporative demand of the above ground environment. The second stage in the evaporation process is one of rapid decline in rate of loss as the soil reservoir is depleted. In this case above‐ground conditions are no longer as important but intrinsic soil factors govern the rate of moisture flow to the soil surface. Capillary flow, vapor transfer, and the combination of the two in the capillary condensation evaporation process dominate the picture. The third stage of moisture loss is that of extremely slow rates and is governed by adsorptive forces of molecular distances at the soil liquid‐solid interface. Potentialities for suppressing this rate process lie in the first two stages of moisture loss. These potentialities group themselves into three categories: (a)Decreasing turbulent transfer of water vapor to the atmosphere by such procedures as, allowing stubble to stand, adding mulching material, increasing soil surface roughness. (b)Decreasing capillary continuity by tillage methods or chemical additives of the soil stabilizer type; and (c)Decreasing capillary flow and moisture holding capacity of the surface soil layers by chemical additives of the surfactant type. Each of these three catgories is discussed in turn. Recent work from this laboratory is cited, particularly that dealing with surfactants and mulches. Data presented in this regard demonstrate the influence of surfactants on capillary flow in clay‐water systems of the montmorillonitic and kaolinitic types. Net radiation data and temperature data obtained with mulched and bare field soils are presented, suggesting that the increased heat storage under a plant residue mulch may account for the surprising lack of evaporation suppression where mulching of this type has been practiced.

An intensive study was conducted to evaluate the movement of dieldrin and heptachlor vapor from a treated watershed. The successful application of micrometeorological estimates of evapotranspiration using average hourly data indicates that reasonable confidence can be placed on the computation of vertical pesticide flux rates using the equation: $$P = K_Z \left( {\frac{{\partial p}}{{\partial Z}}} \right)$$ where vertical diffusivity coefficient, Ki, for pesticide vapor is assumed equal to Kz for water vapor as measured by micrometeorological methods.

Surface characteristics, photosynthesis, and evapotranspiration of vegetative stands of alfalfa and orchardgrass, grown in flats in a glasshouse, were investigated in a small wind tunnel. The windspeed parameters of the crop surfaces were determined at low radiation intensity at each of two windspeeds, and the rates of net photosynthesis and evapotranspiration were measured at the two windspeeds at each of three radiation intensities. The roughness length (zo) increased with windspeed and was greater in orchardgrass. Rate of net photosynthesis, but not evapotranspiration, also increased with windspeed. In consequence, water use was most efficient under the highest wind regime. The rate of net photosynthesis increased to the intermediate radiation intensity, whereas evapotranspiration increased and water use efficiency decreased to the highest radiation regime. The work indicated that a small wind tunnel of the type described would be useful for further studies of the photosynthesis and evapotranspiration of forage stands.

The vertical distribution of the photosynthetic fixation of carbon dioxide within a crop of corn was calculated from carbon dioxide profile data and transfer coefficients obtained by analysis of windspeed measurements. Infrared analyzers were used to measure the carbon dioxide concentration at several heights within and above the crop. The calculated total fixation for the day was approximately 60 g CO2m−2(equivalent to 470 pounds of sugar per acre per day). The results demonstrated the importance of the upper leaves in the fixation of carbon dioxide and showed the increased fixation by the lower leaves during periods of high light penetration. There was some indication that a coupling existed between the level of windspeed and fixation under conditions of high light and relatively low windspeed. With refinement in technique the method could be used to obtain more quantitative estimates of the distribution of photosynthesis in other crops.

Turbulent transfer within a crop of corn was characterized by an aerodynamic approach. The distributions of windspeed within an immature and a mature crop were measured using both cup and heated thermocouple anemometers. A combination of the statistical and mixing‐ length theories was employed to analyze the wind data for transfer coefficients. Eulerian time scales of turbulence were calculated from the windspeed fluctuations of selected 30‐second periods of semi‐steady wind. Momentum transfer coefficients were determined from the scale of turbulence by equating it to the mixing length and assuming isotropy. The resulting values were as large as 8,000 cm2sec−1. The shearing stress profiles calculated with these values had unrealistically sharp maximums just below the top of the crop. This anomaly was considered an artifact of the method resulting from extreme anisotropy in the turbulent shear flow. The K values obtained by the statistical mixing length method were approximately ten times larger than those determined by logarithmic profile analysis. The latter values were considered more nearly correct and were used as a basis for correcting the other values. The results showed that the magnitude of turbulent transfer is several orders of magnitude greater than molecular diffusion even at levels deep within the crop. The transfer coefficient showed a marked attenuation with depth below the top of the corn but remained a function of windspeed at all heights.

Data are assembled giving sinks of shortwave radiation within a corn crop canopy. From these data, and from net radiation measurements obtained at 225‐, 175‐, 100‐, and 20‐cm. heights within a crop of corn 300 cm. high, net thermal radiation is calculated at those respective heights. Transmission data at those heights, using a Miller field light transmission photometer, indicate that shortwave transmission can be expressed as an exponential function of height as well as of cumulative leaf area index.

A spectrophotometer with a sensitive range lying between 0.3 and 1.0 μ and its calibration and use are described. The spectrophotometer was used to measure the spectral characteristics of 0.3‐ to 10‐μ radiation penetrating a dense stand of corn 300 cm. high (26,000 plants/acre, 29‐inch rows, LA1 = 4.3) planted in north‐south oriented rows. Transmission was quite low for the visible range, but was 30 to 40% from 0.75 to 0.9 μ. Efficiency of photosynthesis was 0.42 times the yield obtained from laboratory‐grown Chlorella.

This paper first takes up the oxygen “supply” characteristics of the soil environment, considering the various soil parameters required in the simple model for oxygen diffusion near plant roots. Finally the “demand” and “supply” aspects of oxygen in the root‐soil system is integrated demonstrating that the two aspects are interdependent. Quantitative data offer evidence of the limits imposed by root respiration and soil aeration in the immediate root environment.

Net radiation measurements were made with a traversing system in a 250 cm tall (LAI = 4.0) corn crop with east-west oriented rows. Frequency distributions of flux density of net radiation were plotted using 0.075 cal cm-2 min-1 class intervals. These distributions showed a shift from predominantly sunlit to predominantly shaded conditions with depth into the plant canopy. The frequency distributions showed that net radiation in sunflecks in a canopy can exceed that above the canopy, and that negative values of net radiation can exist in shadows. A Soil-Plant-Atmosphere Model (SPAM) also predicted that net radiation in sunflecks in the canopy would exceed net radiation above the canopy. The effects of spots with high radiation load within the plant canopy on leaf temperature and convective heat exchange were investigated theoretically.

In a previous communication we described a meteorological method of assessing the vertical distribution of photosynthesis and respiration activity in a corn crop. The method, however, is tedious. Here we describe a much simpler method. Results indicate that all the leaves of a corn crop appear to follow the same near‐linear light response curve, at least above the compensation point. Below the compensation point, all leaves evidently respire very little.

The variation of leaf water potential at different heights in a corn crop canopy (Zea mays) and with time during a period of increasing drought stress were examined along with soil moisture tension and evapotranspiration. These measurements plus growth measurements permitted an evaluation of the soil‐plant‐water transport system and its effects on plant growth during drought. On clear summer days the upper leaves of the corn crop had higher water potentials with a bimodal tendency in contrast to the lower leaves which had lower and almost constant water potentials. On high stress days the magnitude of the maximum afternoon leaf water potential increased by an amount equal to the increase in soil moisture tension. On days of comparable evaporative demand, the gradient of water potential from the upper leaves to the soil increased regularly with an increase in soil moisture tension in the root zone. Since total transpiration was not bimodal even though upper leaves exhibited bimodal variations of water potential, it is concluded that if midday stomatal closure of the upper leaves occurred, it apparently was ineffectual or compensating transpiration must have been carried on by lower leaves. A 2‐week drought period was examined in detail. Computation of the “effective conductance” of liquid water in the plant was compared with the moisture conductivity of the soil. This permitted a calculation of the “effective root length” where the gradient of water potential from plant to soil was assumed to represent the gradient from the root surface to the ambient untapped soil moisture. This calculated length was small by comparison to those calculated by Slatyer (1957), but the computational error is large. Transpiration was not detectably changed by the decrease in effective plant conductance and soil conductivity, so it is. concluded that the plant was still able to keep pace by maintaining sufficient potential gradient to transport the needed amount of water. The growth processes, however, were slowed down with increasing leaf water potential and decreasing turgor of the tissues. This slowdown was evident even at the relatively low soil moisture tension of less than one bar and a leaf water potential of less than 3.5 bars.

The effect of soil moisture level on the uptake of Mo, Cu, and Co by alsike clover was investigated in a greenhouse experiment with two Brown Podzolic soils from New Hampshire and a Weisenboden and a Brown soil from Nevada. A wet soil condition increased the Mo in the soil solution and plant Mo content on all four soils. The Co concentration in the soil solution was higher in pots maintained at the high moisture level than at the low moisture level. This increase in Co concentration with soil wetness was reflected in alsike clover, over four cuttings, on the two Brown Podzolic soils, but not in plants from the two Nevada soils. Copper contents in the plants and in the soil solution did not differ consistently because of soil wetness.

This is a further study of the model of oxygen diffusion through a liquid cylindrical shell to plant roots, and the use of an oxygen cathode made of platinum wire to evaluate the resistance to diffusion. The transient current to the oxygen cathode is here analyzed mathematically, and the results compared to new measurements. Differences are believed due to an inadequate representation of the geometry of the soil‐air interface.

An attempt has been made to bridge the gap between the knowledge of the oxygen requirements of roots which has been gained through laboratory studies and the empiricism which now exists in field studies of soil aeration. This objective has been sought through a theoretical approach which combines field measurements of the oxygen supply in the gaseous and liquid phases of the soil and known quantitative demands by plant roots for oxygen into a single expression derived from Fick's Law for radial diffusion. The concentration of oxygen at the root surface is the soil aeration parameter determined. The application of the theory to the quantitative evaluation of soil aeration conditions in two contrasting soil types, Amarillo fine sandy loam and Miller clay, has led to the following conclusions: 1.At field capacity, the concentration of oxygen at certain root surfaces is suboptimal for normal root respiration in Miller clay, but optimum in Amarillo fine sandy loam.2.The concentration of oxygen at the root surfaces increases linearly with the logarithm of soil moisture tension.3.The apparent diffusion path length in the liquid phase about plant roots is more often a limiting factor in normal root respiration than the gaseous composition, per se, in the soil pores. At field capacity, the concentration of oxygen at certain root surfaces is suboptimal for normal root respiration in Miller clay, but optimum in Amarillo fine sandy loam. The concentration of oxygen at the root surfaces increases linearly with the logarithm of soil moisture tension. The apparent diffusion path length in the liquid phase about plant roots is more often a limiting factor in normal root respiration than the gaseous composition, per se, in the soil pores. The above conclusions, based upon the application of the theory, were tested against plant response. The results obtained indicate that the method shows promise as a tool to estimate the supply of oxygen to roots under field conditions.

It is recognized that there is a need for a simple method of determining when to apply irrigation water to a growing crop. To this end a preliminary study was made, using cotton and grain sorghum plants under field conditions, of the relation between the moisture stress in the plant stems and electrical resistance between two electrodes inserted in their stems. The results suggest that this simple electrical measurement is largely related to hydration in the plant stem tissue. Electrical resistance measurements in grain sorghum and cotton under various soil moisture regimes in the field demonstrated that: (a) resistance in cotton was closely correlated with soil moisture, these relationships were not so clearly demonstrated in sorghum; (b) resistance measurements in cotton underwent diurnal fluctuations, but always continued an upward trend during a drying‐out cycle following an initial lag after irrigation; and (c) the coefficient of variation ranged from about 10% for 8 a.m. measurements to 24% for 2 p.m. measurements, depending upon soil moisture conditions. The results indicate that the method has promise as a tool to indicate moisture stress in certain plants. However, additional research is needed to completely evaluate the method.

Evapotranspiration is a function of soil, plant, and meteorological factors. Attempts to predict evapotranspiration without considering all pertinent factors can meet with only qualified success. It is found that evapotranspiration is controlled by: soil moisture tension, physiological factors, the relation of soil moisture of an irrigated area to that of its surroundings as well as purely meteorological factors of radiation, wind, air temperature, and humidity. Examples are given of the effects of these factors on measured evapotranspiration.

Potential errors in the pressure bomb measurement of water potential are important to a wide range of users. We report here specifically on changes in water potential within the chamber associated with measurement technique. Individual soybean leaves (Glycine maxL..) were measured repetitively while left in the chamber for varying amounts of time, both with and without a plastic covering. Thermocouples were used to monitor leaf and air temperature during compression. For uncovered leaves, initially at −3 to −5 bars, water potential decreased as much as 1 bar during a single measurement due to drying associated with high tissue temperatures during the compression. Passing the entering air through water in the bottom of the chamber did not limit either the temperature increase or desiccation. When the leaves were wrapped in more than one layer of plastic, they were effectively insulated from the temperature increase in the chamber and decreases in water potential were reduced to 0.1 to 0.2 bars per measurement. Changes in water potential with time were negligible when desiccation was prevented. We conclude that the pressure bomb can yield highly reproducible measurements if the leaves are covered to prevent desiccation.

Studies of the vertical distributions of CO2fluctuation in a cornfield were made in the 4‐to 0.25‐cycle/min frequency range. Amplitude of fluctuations decreased with height above the ground. Frequency in this range appeared rather constant, however. Sources and sinks for CO2within the cornfield contribute to the fluctuations; however, eddy structure originating inside and/or outside the cornfield plays an important role too.

A letter to the editor is presented in response to the article "High-Efficiency Photosynthesis," by Olle Björkman and Joseph Berry in the October 1973 issue.