The effect of temperature on yield of soybeans (Glycine max (L.) Merr.), is often underestimated despite reports of a significant relationship between yield and growing season temperatures. The identification of genotypes having heat tolerance appears to be warranted, but a simple, rapid technique for measuring tolerance to high temperature is needed. A technique previously used for assessing genotypic differences in membrane thermostability (heat tolerance) in sorghum (Sorghum bicolor (L.) Moench) was evaluated for use in soybeans. The technique involves the measurement by electrical conductance of the amount of electrolyte leakage from heat‐damaged leaf tissue cells after exposure to elevated temperatures. The relationship between the degree of injury and the temperature at which that injury was induced was observed to be a sigmoidal response. Genotypic differences in heat tolerance were associated with differences in the relative position of the response curve with respect to the treatment temperature. Greatest sensitivity in detecting genotypic differences occurred at temperatures inducing about 50% injury. Genotypic differences were greatest in newly developed leaf tissue. Consequently, only the most recently developed leaves should be used in the assay. Plant‐to‐plant variation was appreciable and necessitated the use of bulked leaflets from several plants as samples. Genotypic differences were consistent across sampling dates, indicating that the assay can be conducted during any phase of vegetative growth. Results obtained from cultivar trials over several years show significant differences among genotypes and consistent relative ranking of genotypes in different environments. Although requiring replication to achieve a sufficiently small standard error, the technique shows promise as a screening method.

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A method has been developed to measure concentratons of CO2 in gases rapidy. A gas sample is injected into a flowing carrier gas that passes through an Inrared CO2 analyzer. A strip chart recorded peak respome is obtained which is proportional to the CO2 concentration. A resolution of better than 2 microliters of CO2 per liter of gas was obtained.Seven to 10 seconds were required for sample analysis once the sample was obtained. So bum iclor plant respirto was deterned at different temperatures by measuring CO2 using this system ad by using a conventonl system. The correlaton between tehqs was 0.996, and about the same variation occurred within each method. This te ue greatly increased the efficiency of the infrared CO2 analyr in our laboratory for use in plant resprato d photosynthetic studies.The concentration of CO2 in gases is frequently measured in environmental and biological studies. Both respiration and photosynthesis can be monitored by measuring CO2 exchange. Because analysis of a large number of samples is often necessary, the method for measurement should be rapid and sensitive. The IR gas analyzer is the instrument most commonly used to measure CO2 concentrations. Flow-through systems which monitor CO2 exchange of plants or plant parts enclosed in a chamber usually depend on the detection of a 10 to 30 pl C02/1 concentration differential between input and exhaust air (3,(7)(8)(9)12). The CO2 differentials can be related to net photosynthesis or respiration. CO2 concentrations of the air surrounding photosynthesizing plants can also be monitored and a concentration level maintained by automatically injecting CO2. The amount of CO2 added is equal to net photosynthesis (5,11). In the field, photosynthesis may be determined aerodynamically by determining the CO2 gradients within the crop canopy (4, 6, 10).In many photosynthetic studies determination of CO2 concentrations requires an IR gas analyzer to monitor continuously a single CO2 exchange chamber, or several chambers may be monitored consecutively with a sample switching device at the analyzer while continually purging the sampling lines. After switching, I to several min equilibration time is usually necessary to stabilize the analyzer and obtain a reliable reading (13).A method for measuring CO2 in small volumes of gas has been described which was used in a working range generally between 1,000 and 15,000 1s1/1 CO2 (1 MATERIALS AND METHODSThe basic system consisted of an IR gas analyzer, mv recorder, flow meter, and drying columns (Fig. 1). Tygon2 tubing was used to connect the system. The tubing lengths from the tee connector to the IR gas analyzer were the same to balance line resistance. The flow rate of the carrier gas (usually N2) was adjusted to approximately 0.6 I/min. At this flow, a narrow based peak was recorded on the strip chart recorder and only measurement of peak heights was necessary to obtain the CO2 concentration ofthe injected gas sample. A 10-ml or less sample was injected with a syringe through a short section of sur...

External visualization of the apparent dark closing layer in the placental area near the sorghum [Sorghum bicolor (L.) Moench] kernel attachment point coincides closely with the cutoff of radioactive assimilate translocation to the kernel. Dark layer determination permits identification of physiologic maturity or date of maximum dry weight accumulation. This judgment is meaningful because yield is a function of both time and metabolic efficiency. Genotypes can be characterized in terms of grain filling‐period duration by noting the dates of bloom and dark layer formation. Yield data plus physiologic maturity data permit direct field quantitation of the time and metabolic efficiency components of Grain dry weight accumulation.