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Soybean growth and yield models require good predictions of vegetative and reproductive development stages, as a function of specific environmental variables. Parameters for predicting R5 (beginning of seed growth) and R7 (physiological maturity) dates of four soybean cultivars were estimated for a development rate model. R5 is predicted by accumulating a daily rate of development, which depends on night length and temperature, starting at Rl (flowering) until a threshold is reached. Daily development rate is computed by a multiplicative relationship containing two linear‐plateau functions: one for describing the variation in development rate with night length under optimal temperature and the other describing variation with temperature under optimal night length. The downhill simplex method was used to estimate phenological parameters for each cultivar, minimizing the error sum of squares between observed and simulated dates of R5 occurrence. The same type of model and methodology was used to estimate parameters for R7 prediction, but beginning at R5 until a threshold is reached. The results indicate that as plants develop from V1 (first true leaf) to R7 they become more responsive to photoperiod and less sensitive to temperature. Differences among cultivars with respect to optimal night length tend to diminish as the plants approach physiological maturity; with respect to temperature, the reverse happens. The optimal temperature for reproductive development varied between 25 and 29 °C, without great differences among cultivars of differing maturity, but with a slight increasing trend from V1 toward R7.

The absorption characteristics and mechanisms of pertechnetate (TcO47) uptake by hydroponically grown soybean seedlings (Glycinemax cv Williams) were determined. Absorption from 10 micromolar solutioDs was linear for at least 6 hours, with 30% of the absorbed TcO47 being transferred to the shoot. Evaluation of concentration-dependent absorption rates from solutions containing 0.02 to 10 micromolar TcO4-shows the presence of multiphasic absorption isotherms with calculated K, values of 0.09, 8.9, and 54 micromolar for intact seedlings. The uptake of TcO4-was inhibited by a 4-fold concentration excess of sulfate, phosphate, selenate, molybdate, and permangnate; no reduction was noted with borate, nitrate, tugstate, perrhenate, iodate, or vanadate. Analyses of the kinetics of interaction between TcO47 and inhibiting anions show permangnate to be a noncompetitive inhibitor, while sulfate, phosphate, and selenate, and molybdate exhibit characteristics of competitive inhibitors of TcO4-transport suggesting involvement of a common transport process. Technetium-99 is currently projected to be one of the most important isotopes associated with the nuclear fuel cycle based on potential long-term dose to man. This is based on a number of criteria including fission yield (6%), long half-life (2.13 x l0 year), mobility and persistence in soils as the pertechnetate ion (TcO4-), and its comparatively high availability for plant absorption (11, 19). Wildung et al. (18) have shown that TcO4-added to soil was readily accumulated by soybean and wheat seedlings over a 30-d period with reported CR2 values (CR = activity/g tissue dry weight. activity/g soil dry weight) of 380 to 170 for soil concentrations of0.001 to 0.1 Mg Tc/g soil. At concentrations above 0.1 Mg/g, severe toxicity and seedling death resulted. At soil concentrations of 1 pg Tc/g soil, CR values have been reported to range from 50 to 400 for cheatgrass and tumbleweed grown on arid soils (16). Landa et al. (12) showed a similar range in CR values for six plant species grown hydroponically and for wheat grown on 11 different soils. While CR values for TcO4-are high compared with values for fission products and non-nutrient pollutant elements (<1), they are of a magnitude observed for trace' nutrient ions such as Mo, Mn, Cu, Fe, and Zn (4). The solubility ofTcO4-in soils, although an important aspect of its soil-plant behavior, does not totally explain the high affinity of plants for TcO4-. The observed phytotoxicity of9Tc was investigated by Berlyn et al. (2). Their results failed to demonstrate cytological evidence ' Work performed for the United States Department of Energy under Contract DE-AC06-76RL0 1830. 2 Abbreviation: CR, concentration ratio. for radiation-induced damage and suggested an interaction or substitution of Tc for an essential nutrient. Pertechnetate has been shown to be toxic to a number of algae and bacteria, and interfere with both respiration and photosynthesis (9) at concentrations in excess of 1 mm. The potential interaction of Tc with nutrient...

Defoliation during reproductive growth of soybean [Glycine max (L.) Merr.] may significantly reduce yields. The objective of this study was to elucidate the relationships among organism‐induced defoliation, CO2 exchange rates, and reproductive growth in fieldgrown soybean. Soybean (‘Bragg’) plots were sprayed a factorial design with diflubenzuron (0.035 kg a.i./ha) and a benomyl‐maneb mixture (1.12 a.i./ha) on two dates, first at early flowering (5 August) and again at early pod set (19 August). Diflubenzuron application prevented defoliation by velvetbean caterpillar (Anticarsia gemmatalis Hubner). By 12 September, LAI was reduced from 5.5 to 2.8, leaf dry weight from 185 to 110 g/m2, and midday light interception from 94 to 72% in defoliated as compared to non‐defoliated plots. While canopy specific leaf weight (SLW) increased in all treatments during the experimental period, SLW was 1.0 mg/cm2 higher in defoliated than non‐defoliated plots. On 4 September defoliated plots were sprayed with diflubenzuron (0.035 kg a.i./ha) and carbaryl (0.56 kg a.i./ha) prevent further leaf loss.Canopy CO2 exchange was measured at various photosynthetic photon flux densities (PPFD) to develop photosynthesis light response curves. Net canopy photosynthesis at 1,500 μE·m−2·sec−1 PPFD (PN1500) declined steadily in all treatments during seed growth. PN1500 and plant dark respiration, respectively, were reduced by 6.8 and 3.3 mg CO2·dm−2·hour−1 in defoliated compared to non‐defoliated plots. Soil CO2 efflux rates did not differ significantly (P < 0.05) between treatments. Differences in pod growth rates (7.6 and 9.21 g·m−2·day−1 and yields (434 and 502 g·m−2) between defoliated and non‐defoliated plots, respectively, were attributed to differences in canopy photosynthetic capacity. There was no apparent effect of defoliation on seed growth duration or seed abortion. Rather, yield reduction in defoliated plots was related primarily to slowing of the individual seed growth rate. Balance of carbon flux to seeds showed relationships between instantaneous canopy CO2 exchange measurements and sequential harvest data.

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