Nodulated and non-nodulated (not inoculated) soybeans (Glyjine max IL.I Merr. cv Wells) were grown in controlled environments with N2 or nonlimiting levels of N03-, respectively, serving as sole source of nitrogen.The efficiency of the N2-fixing plants was compared with that of the nitrate-supplied plants on the basis of both plant age and plant size. Efficiency evaluations of the plants were expressed as the ratio of moles of carbon respired by the whole plant to the moles of nitrogen incorporated into plant material.Continuous To clarify the discrepancy in the efficiency values reported for soybeans, experiments were conducted to measure net carbon fixation and subsequent respiratory loss and incorporation into dry matter relative to the net nitrogen assimilated by whole soybean plants dependent on N2 fixation or nitrate reduction. A unique two-compartment plant chamber is described which permitted 24-h measurement of both shoot and root gas exchanges on the same plants without disruption of the normal diurnal rhythm or whole plant integrity. Seeds for germination were individually selected for soundness and uniformity. They were washed for 5 min in a solution of 5% (v/v) Clorox and immediately rinsed with a minimum of four changes of deionized H20 over a second 5-min period. Seeds were then soaked for 5 min in 0.1 mm CaSO4 (-0.5 ml seed-'). For the nodulated treatment, Rhizobium japonicum (USDA strain 31 1B7 1, grown axenically on slants and used at an approximate rate of 1 X 106 bacteria/seed) was included in the CaSO4 soak. The seeds were planted at a depth of 1.5 cm in moist sand which had previously been autoclaved to kill residual microorganisms. The CaSO4 soak solution (+ inoculum) was poured into the sand at time of planting. Germination pans were covered with foil and placed in the growth chamber under the conditions described. MATERIALS AND METHODS Plant
The effects of herbicides on in vivo nitrate and nitrite reduction were determined by vacuum infiltrating sections of barley (Hordeum vulgareL.) or bean (Phaseolus vulgarisL.) leaves with solutions containing nitrate and herbicides. Herbicides causing a reduction of nitrite accumulation in the dark were considered to have inhibitory effects upon nitrate reduction and those causing an accumulation of nitrite in the light were considered to inhibit nitrite reduction. Only dinoseb (2-sec-butyl-4,6-dinitrophenol) and potassium azide significantly reduced nitrate reduction in both barley and bean. All of the herbicides which inhibit photosynthesis inhibited nitrite reduction but had no significant effect on nitrate reduction in barley and bean. Nitrite reduction in an atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] resistant pigweed (Amaranthus retroflexusL.) biotype was not affected by any triazine tested. However, these triazines significantly inhibited nitrite reduction in barley, bean, and the susceptible pigweed biotype. The results suggest that the in vivo nitrate reductase technique may be a useful technique for identifying chemicals which inhibit the flow of electrons to ferredoxin, thereby inhibiting nitrite reduction in light.
Efforts to assess the effect of acid rain on soybean [Glycine max (L.) Merr.] have shown variable results. Simulated acid rain has been reported to cause increases, decreases, and no significant effect on yield. Although few parameters were identical among the diverse studies reported in the literature, one common difference was the choice of cultivar. In this study, 20 soybean cultivars were screened to determine their relative sensitivity to simulated acid rain. Soybean was grown in 1984, 1985, and 1986 in field plots in east central Illinois. Plots were protected from ambient rain and treated twice weekly with simulated rain of pH 5.6 (control) or pH 3.0. Early each growing season visible leaf injury was noted for all 20 cultivars, and level of injury was significantly higher for plants receiving the more acidic treatment (pH 3.0). Average yield for the 20 cultivars was approximately the same when plants were treated with simulated rain of pH 3.0 as when plants were treated with stimulated rain of pH 5.6. Over all 3 yr, the group V cultivars Essex and Forrest yielded more grain and had more chaff dry matter, pods per hectare, and pods per plant with the more acidic pH treatment than with the control treatment. Only 'Williams 82' yielded significantly less with the more acidic pH treatment than with the control treatment. The results suggest soybean cultivars may respond differently to applications of highly acidified rainfall, but none of the cultivars tested were extremely sensitive to growth and yield reduction.
An important component of the effects of acid rain on our ecosystem is its impact on the yield of agricultural crops. Field experiments were conducted for 3 yr to determine the effects of simulated acid rain on seed yield of two soybean [Glycine max (L.) Merr.] cultivars, 'Amsoy 71' and 'Williams 82'. Soybeans were grown on a Flanagan silt loam (fine, montmorillonitic, mesic Aquic Argiudoll) and protected from ambient rainfall by movable rain exlusion shelters. Plants were treated biweekly with simulated rain of pH 5.6,4.6,4.2, 3.8,3.4, and 3.0. For Amsoy 71 there was a linear decrease in yield with increasing rainfall acidity for 1 of 3 yr but no significant effects for the other two. Thus acid rain appears to reduce the yield of some soybean cultivars slightly but this effect is not consistent from year to year. Amsoy 71 and Williams 82 soybean treated with the most acidic rain, pH 3.0, resulted in average yields for the 3 yr of the study of approximately 3 and 4% lower than the average yields for the other treatments, respectively. However, calculations from the response functions developed have shown that at current levels of rainfall acidities the effects on yield are very small. With an increase in rainfall acidity of 50°7o in Illinois the predicted yield decrease for Amsoy 71 and Williams 82 soybean would be less than 1%. Similarly the expected increase in yield of these cultivars would be 1% or less if acidity in the rainfall were reduced by 50%. While there may be beneficial effects of reduced S and N oxide emissions, these results suggest the resultant lower rainfall acidities are not likely to produce noticeable changes in yield of soybean. Additional index words: Glycinemax (L.) Merr., Rain exclusion shelters, Grain yields, pH, Sulfate, Nitrate. Porter, P.M., W.L. Banwart, J.J. Hassett, and R.L. Finke. 1987. Effects of simulated acid rain on yield response of two soybean cultivars. J. Environ. Qual. 16:433-437.
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