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The purpose of this experiment was to examine the interrelationships between CO2‐exchange rate (CER) and certain growth analysis variables in soybean [Glycine max(L.) Merr.] plants. Soybean plants representing 16 cultivars were grown in a controlled environment chamber. At 22 and 29 days after planting, leaf area and dry weights of the leaves, supporting tissue, and roots were measured for each plant. From these data, the rate of vegetative growth (GR) was determined for each cultivar as well as leaf area growth rate (LAGR), specific leaf weight (SLW), and other growth analysis variables. At 28 days after planting, the CER per unit leaf area of the terminal leaflet of the second trifoliolate leaf was measured. The resistance to CO2diffusion was partitioned into boundary layer, stomatal, and mesophyll components. Gas exchange was measured independently for each leaf surface. Growth rate was negatively related to CER and SLW and positively related to LAGR. Furthermore, CER was positively and negatively related to SLW and LAGR, respectively. As the leaves increased in SLW and CER across the cultivars, their rate of area expansion decreased. Since GR was positively correlated with LAGR and negatively correlated with CER, it appears that increases in CER did not compensate for decreases in LAGR; consequently, differences in GR among the cultivars were detected.

An evaluation was made of leaf diffusive resistance and chlorophyll concentrations of dark green (normal) and pale green (chlorophyll deficient) phenotypes of a soybean [Glycine max(L.) Merrill] mutant when grown 10‐ and 30‐klux light intensities. When grown at the 10‐klux intensity the two leaf types had similar mesophyll and total CO2diffusive resistances and similar net photosynthetic rates per unit leaf area at normal ambient CO2concentrations (e.g., 300 µl/liter). Chloropyhll concentration of the pale green leaf was one‐fourth that of the dark green leaf. On a chlorophyll basis, rate of CO2fixation was four times faster in the pale green type than in the dark green type. The similarity in net photosynthetic rates per unit leaf area of the two phenotypes when grown at the 10‐klux light intensity was the result of (1) similar CO2diffusive resistances and (2) the ability of the photosynthetic apparatus of the pale green leaf to fix CO2as rapidly as that of the dark green leaf, in spite of a much lower amount of chlorophyll. When grown at the 30‐klux light intensity, chlorophyll concentration of the pale green phenotype progressively declined, to one‐tenth the amount at the 10‐klux intensity. Chlorophyll concentration of the dark green phenotype was not significantly affected by light intensity. Photosynthesis of the pale green leaf grown at the 30‐klux intensity became CO2saturated at concentrations well below 300 µl/liter. Therefore, light intensity during leaf growth strongly influenced the relative CO2assimilation rates of the dark green and pale green phenotypes.

A nondestructive indicator of instantaneous seed growth rate in soybean [Glycine max(L.) Merr.] and other legumes would useful in studies of the physiology of seed growth. The responsiveness of soybean fruit respiration rate to whole plant environmental treatments known to affect photosynthate supply and seed growth was evaluated. Dark respiration (CO2efflux) rates of individual, intact soybean fruits at the mid‐pod‐filling stage were measured while manipulating photosynthetically active radiation (PAR) or ambient CO2concentration in growth chambers. The PAR levels employed were 450, 110, or 30 μmol m−2s−1and CO2concentrations were 300 or 150 μL L−1. Fruit respiration rate.began to decline within 24 h after plants were subjected to either decreased PAR or CO2concentration. After 2 to 3 days the response was complete and fruit respiration rate remained constant at a level determined by the treatment for an additional 4 days, the longest period evaluated. The respiratory response was rapidly reversible when either PAR or CO2concentration were returned to control levels. As PAR was lowered from 450 to 110 or 30 μmol m−2s−1, seed growth rate declined from 20.3 to 11.0 or 8.7 mg day−1fruit−1, respectively. Fruit respiration and seed growth were apparently responding to changes in photosynthate supply. The results indicate that in situ fruit respiration rate is responsive to changes in the physiological status of the plant and may be useful as an indicator of instantaneous seed growth rate in some situations.

Growth analysis techniques were used to study components of dry matter accumulation in field soybean [Glycine max(L.) Merrill] communities at Lafayette, Ind. in 1968 and 1969. An analysis was made of dry matter accumulation in leaf, supporting, pod wall, and seed fractions, as well as the total aboveground portion of the crop. General approach with the 1968 data was to select mathematical functions which describe the dry weight and leaf area vs. time relationships and then to calculate, for each day during the growing season, instantaneous values of relative growth rate (RGR), crop growth rate (CGR), net assimilation rate (NAR), leaf area ratio (LAR), and the components of LAR. From the 1969 data, weekly mean RGR's and CGR's were calculated, using traditional growth analysis formulae. The RGR of each individual plant fraction steadily decreased, at a decreasing rate, as the season progressed, At any given time, the most recently initiated plant fraction had the greatest RGR. The CGR of each fraction rose to a peak and then declined. Growth rate of the leaf component peaked first, followed in sequence by supporting, pod wall, and seed components. Apparently due to late planting in 1968, there was a greater overlap of vegetative and seed growth than in 1969 when planting was earlier. Total aboveground RGR declined until early August, then rose to a secondary peak in mid‐August. At the same time, there also occurred a peak in total aboveground CGR. The increases in RGR and CGR during August are attributable to a concurrent increase in NAR. The increase in NAR is interpreted as a response of the photosynthetic apparatus to an increased demand for assimilates. The increased demand for assimilates was due to rapid growth of the seed fraction.

Leaf area (A) and leaf dry weight (LW) were determined throughout the growing season for lower, middle, and upper main stem segments and branches of a field soybean [Glycine max(L.) Merrill] community. The A/LW ratios were regressed on number of days after planting, using polynomial regression models. For the middle main‐stem segment, A was also regressed on LW. Mean seasonal A/LW decreased from the base to the top of the canopy. Mean A/LW of branches was higher than that of any main‐stem segment. The A/LW ralio of each main‐stem segment continually declined until just prior to leaf senescence. The trend subsequently reversed and A/LW increased rapidly during leaf abscission. The trend in A/LW of the branches differed in that there was an initial increase in A/LW, followed by a decline and then a second increase during leaf abscission. Due to time related changes in A/LW, models for the prediction of soybean leaf area from leaf weight should also include time as a variable if observations are made at different growth stages.