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Genetic variation in soybean plant morphology is of interest beause of its possible utility in developing cultivars adapted to unique environments. Growth analysis techniques were used to quantify and compare soybean [Glycine max(L.) Merr.] growth in genotypes differing in pubescence density. The experiment was conducted in 1980 and 1981 at Mead, NE on a Typic Argiudoll soil. Data from frequent samplings of phytomass dry weight (W), leaflet dry weight (LW), leaf area (A) and number of leaflets (N) were collected during two crop seasons from near‐isogenic lines of dense and normal pubescent types of ‘Clark’ and ‘Harosoy’ cultivars. Statistical curve‐fitting tests identified the most plausible mathematical models for describing the change of these four variables over time. Ten growth analysis parameters based on these four model equations were then computed and included crop growth rate (CGR), net assimilation rate (NAR), leaf area ratio (LAR), relative growth rate (RGR), leaf weight ratio (LWR), specific leaf area (SLA), relative leaf growth rate (RLGR), relative leaf area expansion rate (RLAER), leaf area partitioning factor (LAPF), and area per leaflet (APL). Dense pubescence affected the growth parameters differently in the two cultivars. There was no difference in any parameter between dense and normal pubescence lines in Clark. The W, LW, A, N, and CGR were, however, affected by pubescence density in Harosoy. The CGR was about 20% greater at maximum growth in the dense Harosoy isoline. The NAR was unaffected, however, indicating that the higher photosynthetic rate in the Harosoy dense pubescent isoline was due to an increase in leaf area and not an increase in photosynthetic efficiency. The fact that the gene giving rise to dense pubescence had differential effects on growth in the two cultivars suggests that the genetic background into which the gene is introduced influences the effect of the gene on growth.

Rainout shelters provide a method for developing drought stress under field conditions in years when rainfall is adequate. The primary objective of this study was to determine the feasibility of using portable rainout shelters in water stress studies. The study was conducted at Mead, NE in 1981 with three near‐isogenic lines of soybeans [Glycine max(L.) Merr. cv. Harosoy] differing in pubescence density. The soil at this site is a Typic Argiudoll (Sharpsburg silty clay loam). Soybean isolines were planted in small plots, 46 m2in size and were either rainfed or rainout sheltered. The shelters covered an area of 18 m2. The plots were instrumented with leaf thermocouples, strip net radiometers, and neutron access tubes. Phytomass and leaf area index (LAI) measurements were made in the 12 small plots and from four adjacent large plots maintained under rainfed conditions. Leaf temperature was 8 °C warmer and net radiation was 40% lower at midday in the sheltered plots compared to the unsheltered plots. The shelters induced water stress as indicated by a rise in leaf temperature of about 2 °C. However, significant lateral and/or upward movement of soil water into the sheltered plots greatly reduced the effectiveness of the shelters. Phytomass accumulation and LAI from the small and large plots did not agree. The errors of the estimates in the small plots were much greater than those from the large plots. Because of our results, we do not recommend the use of small, portable rainout shelters.

Daily trends of water use efficiency (WUE) throughout a growing season are seldom, if ever, reported. In this paper a method for calculating daily WUE from growth analysis data coupled with weekly estimates of water use from soil moisture measurements are made for near‐isogenic dense and normal pubescent lines of 'Harosoy' and 'Clark' soybean [Glycine mar (L.) Merr.]. Data were collected during a 2‐yr field study at Mead, NE. The soil at this site is classified as a Typic Argiudoll. Evapotranspiration rates (ET) were lower in both dense pubescent isolines compared with the normal isolines, but differences were significant only for the Harosoy cultivar. Peak ET rates in 1980 were 4.7, 4.3, 5.8, and 4.8 mm day−1for the Clark normal, Clark dense, Harosoy normal, and Harosoy dense isolines, respectively. The highest WUE per unit ground area (WUEgand per unit leaf area (WUEg) was observed in the dense pubescent Harosoy isoline. This was the result of a higher crop growth rate and a lower ET in this isoline. Average mid‐season WUEgand WUEgvalues were, respectively, 1.4 and 0.25 g dry matter (kg H2O)−1day−1greater for the dense Harosoy isoline than for the normal isoline. No effect of pubescent density on WUE in the Clark isoline was observed. The interaction of dense pubescence as indicated by results of our study illustrate the need to establish the effect of the dense pubescent gene in cultivars into which it is introduced.

Increasing the density of pubescence on the leaves and stems of soybeans (Glycine maxL.) should influence the radiation balance of the soybean canopy and affect the evapotranspiration and photosynthetic rates. This study was undertaken to evaluate the influence of increased pubescence density on various components of the radiation balance. Near‐isogenic lines of two soybean cultivars (Clark and Harosoy) were grown in four adjacent small plots (18 ✕ 18 m) during the 1980, 1981, and 1982 growing seasons near Mead, Nebr. The soil at this site is classified as a Typic Argiudoll. The isolines of each cultivar varied only in the amount of pubescence (dense vs. normal pubescence). Measurements of albedo, reflected photosynthetically active radiation (PAR), emitted longwave radiation, and net radiation were made over the crop surfaces with instruments mounted on a rotating boom located at the intersection of the four plots. Radiative canopy temperatures were measured with a handheld infrared thermometer (IRT). Results show that dense pubescence increased reflection of shortwave radiation and PAR by 3 to 5% and 8 to 11%, respectively. Emitted longwave radiation and radiative canopy temperature were not significantly affected by increased pubescence, although there was a slight tendency for the dense pubescent canopy to be cooler. Increased pubescence decreased net radiation over the canopy by 0.5 to 1.5%. These results suggest that soybeans with dense pubescence may be slightly better adapted to the high radiation, high temperature, and limited moisture conditions of the eastern Great Plains than are those with normal pubescence.

Previous research has shown that irrigation timing during the reproductive development of soybean [Glycine max(L.) Merr.] J genotypes is a crucial determinant in ultimate yield response. To obtain more detailed information on this relationship we evaluated the agronomic responses of 16 soybean cultivars to nine irrigation treatments in a 3‐year field experiment. The experiments were conducted in 1980, 1981, and 1982 near Mead, NE on a Sharpsburg silty clay loam (fine, montmorillitic, mesic Typic Arguidoll). In treatments T1thru T7, only a single irrigation was applied, which coincided with the average R stages 1.1, 2.5, 3.7, 4.7, 5.6, 5.9, and 6.4, respectively. “Check” treatments included T0, which was not irrigated, and T8, which received a total of seven irrigations coincident with those for T1thru T7. Averaged over years and cultivars, the seed yield responses (kg/ha) were: T0: 3470c, T1: 3595abc, T2: 3530bc, T3: 3740a, T4: 3670ab, T5: 3630abc, T6: 3575bc, T7, 3560bc, and T8: 3540bc. The irrigation treatment X cultivar interaction was significant, primarily because T3(R3.7) or T4(R4.7) generally resulted in the highest yields for indeterminate cultivars, whereas T8resulted in the highest yields for determinate cultivars. Averaged over irrigation treatments and years, the determinate cv. Hobbit was the highest yielding, both with and without irrigation. Significant increases in plant height, nodes/plant, and lodging occurred with the T1, T2, T3, and T8treatments, but only with the indeterminate cultivars. Precise irrigation timing and proper cultivar choice seem to be crucial if high yields with minimum lodging are to be attained.

Soybean [Glycine max(L.) Merr.] seed yield components are established and fixed in a temporally ordered sequence during stages R1 to R6. To determine the effect of irrigation timing during R1 to R6 on these traits, we examined the seed yield component responses of 16 soybean cultivars to nine irrigation treatments in a 3‐year experiment. Treatments T1thru T7consisted of only a single irrigation coinciding with the average R stages 1.1, 2.5, 3.7, 4.7, 5.6, 5.9, and 6.4, respectively. ‘Check’ treatments included T0, which was not irrigated, and T8which received seven total irrigations coincident with those for T0and T7. Averaged over cultivars and years, the mean number of seeds/plant were: T0: 87f, T1: 121a, T2: 111bc, T3: 115ab, T4: 105cd, T5: 99def, T6: 102de, T7: 95ef, and T8: 122a. T0and T8thus established the minimum and maximum ‘limits’ in the seeds/plant response, respectively, with declining responses as irrigation was delayed from T1to T7. The 100‐seed weight (g) means were: T0: 19.1b, T1: 17.8c, T2: 17.8c, T3: 18.3c, T4: 20.3a, T5: 20.4a, T6: 19.8ab, T7: 20.3a, and T8: 18.3c. Seed size for T8was thus smaller than T0. The T4thru T7responses were significantly greater than T0, while the T1thru T3responses were significantly less than T0. Average seed yields were greatest for the T3and T4treatments, although for determinate cultivars, T8also resulted in high yields. Only minimal changes in seed number and size were affected by irrigation in the determinate cultivars (in contrast to indeterminates), which may have accounted for the greater yield responsiveness of determinate cultivars to irrigation. Based on both the seed yield and yield component data, it would appear that stage R3.5 to R4.5 represents the reproductive period in soybeans most responsive to irrigation.

A micrometeorological‐physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well‐watered and water‐stressed soybean (Glycine max(L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully‐developed. Measurements of CO2, latent heat and sensible heat flux were made using the Bowen‐ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well‐watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two‐thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water‐stressed, stomatal closure appears to have been the primary cause of the reduction in LE. Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO2exchange appears to have been caused by a combination of high stomatal resistance limiting CO2diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO2‐water flux ratio) was greater when the crop was well‐watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO2‐water flux ratio.

Mass and energy exchanges with the atmosphere were compared in two soybean (Glycine maxL. Merr. cv. Harosoy) isolines differing in pubescence density. The study was conducted in a field with a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll) during the summer of 1980 at Mead, Nehr. Mass and energy exchanges were determined by means of micrometeorological techniques. Evapotranspiration (reported in terms of latent heat flux) was reduced in the densely pubescent isoline. Canopy CO2exchange was unchanged on a per unit land area basis. Water use efficiency (reported in terms of the CO2‐water flux ratio) was, accordingly, greater in the densely pubescent isoline. The increase in pubescence did not significantly alter the net radiation balance, turbulent mixing, canopy CO2exchange, or plant water status. Observed differences in the partitioning of net radiation into latent and sensible heat can be explained by greater penetration of solar radiation into the densely pubescent canopy. Leaf pubescence appears to alter the spectral characteristics of the leaf and, thus, to facilitate the penetration of solar radiation into the canopy.