Drought stress can lower seed quality of soybean [Glycine max (L.) Merr.], possibly from stress related changes in seed‐nutrient concentration. Determinate soybean plants were grown under a mobile weather‐shelter near Ames, IA in 1985 and 1986 to determine if the timing of drought stress could influence seed‐Ca concentration and, subsequently, seed germination and quality. A randomized complete‐ block design was used to test the effect of withholding water during flowering (R2), full pod (R4), seed formation (R5), and full seed (R6) on the quality of the harvested seed. Drought stress was quantified by monitoring leaf temperatures. Each drought‐stress period received an equivalent amount of drought stress. Seed from a R5 drought stress had 85% germination compared with 96% germination for the nonstressed seed. This reduction in seed germination coincided with a 343 μg g−1 decrease in seed‐Ca concentration from a Ca level of 1648 μg g−1 in the nonstressed seed. Electrolytic conductivity per seed and seed‐Ca concentration were negatively correlated (r = −0.50). Germination percentage was significantly correlated (r = + 0.57) with seed‐Ca concentration and negatively correlated with P, Fe, and Zn (r = −0.49, −0.44, and −0.43, respectively). Application of 2.0 g L−1 of Ca(NO3)2 to the germination media of the R5‐stressed seed improved germination to the level obtained by nonstressed seed. The application of Ca(NO3)2 also improved the germination of R6‐stressed seed, which had a concentration of seed Ca equal to the control. This suggests that seed‐Ca concentration is not solely responsible for the decrease in germination percentage of drought‐stressed seed. Results indicate that drought stress during seed formation can reduce seed‐Ca concentration, but additional work is needed to clarify the role that Ca and other seed nutrients play in the germination of drought‐stressed seed.

The effects of drought stress on soybean (Glycine max (L.) Merr.] seed quality may be influenced by morphological plant characteristics and the timing of drought stress. Determinate soybean plants were grown near Ames, lA, to ascertain the influences of the timing and frequency of drought stress and pod position on seed quality (as mea· sured by germination, seedling dry weight, and seed leachate current). Plants were well-watered (high reproductive load) or drought-stressed at Dowering (R2) to decrease potential seed number (low reproductive load). Subsequent drought stresses were implemented at the full pod (R4), seed formation (RS), and full seed (R6) stages, in addition to a nonstressed treatment. The harvested pods were grouped into the top main stem, bottom main stem, and branch positions for seed quality testing. An R2 drought-stress (low reproductive load) did not prevent reductions in seed quality from occurring when additional stress was imposed at R4, RS, or R6 plant stages. However, the R2 + R4 stress treatment increased seedling weight by 7% and weight per seed by 12% compared with the R4 drought-stress treatment. Top main stem seeds were of better quality and greater weight than seed harvested from the bottom main stem or branch positions. Branch or bottom main stem seeds were most sensitive to quality reductions when drought occurred during the RS or R6 stages. The results indicate that variable seed quality resulting from drought stress can be attributed to the timing of stress and pod position on the plant.

Supplying both N forms (NH 4 + NO 3) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH 4 + NO 3 versus predominantly NO 3) and exogenous cytokinin treatment (six foliar applications of 22 ixM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH 4 + NO 3 nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO 3 . Cytokinin application to NO3-grown field plants increased grain yield to that of NH 4 + NO3-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH2 + NO 3 doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH 4 + NO 3 nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH4 + NO 3 plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO2-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH 4 + NO 3 may be partly associated with an increased endogenous cytokinin supply.

Phytoremediation of hydrocarbon‐contaminated soil shows promise as a low‐cost alternative to most remediation methods. This study evaluated seedling growth of six crop species in crude oil contaminated soils. The experiments were conducted in a greenhouse. Weathered crude oil was added to an Ipava silt loam soil at the rate of 0 (control), 10, 50 and 100 g of crude oil kg−1 of soil, which was then placed into pots. Irrigation was used to maintain soil moisture at approximately field capacity. Five seeds of Zea mays, Meticago sativa, Lolium perenne, Triticum aestivum, Glycine max or Vicia villosa were sown per pot. The experimental design was completely randomized with five replications per treatment. Germination and seedling height data were recorded on day 7, 14, 21 and 28. Plants were harvested on day 28, separated into shoots and roots and dried to measure biomass. Analysis of variance was used to determine treatment significance. Significant treatment mean values were separated using Tukey's Honestly Significant Difference Test. Based upon percent emergence and plant biomass production in contaminated soil, Z. mays and G. max seedlings show the greatest potential to enhance remediation.

Although the maize (Zea mays L.) plant can utilize either NH4−N or NO3−N, many hydroponic studies have shown that mixed‐N nutrition (NH4 + NO3) can optimize growth and yield. Results from field studies have been more erratic, however, and may be influenced by genotype. A 2‐yr field study was therefore conducted at Urbana, IL, to evaluate five maize genotypes (B73 × LH51, LH74 × LH51, LH74 × LH82, LHE136 × LH82, and LHE136 × LH123) for plant growth, nutrient content, grain yield, and canopy photosynthesis (Ps) when N was supplied either as calcium nitrate (NO3 plots) or urea plus a nitrification inhibitor (mixed‐N plots). Three of the five genotypes (B73 × LH51, LH74 × LH51, and LHE136 × LH82) increased grain yield (by 6∓8%) when supplied with mixed N, compared with plants supplied with predominantly NO3. However, NO3‐grown plants had equivalent or greater rates of canopy Ps (≈5–10%) than mixed N plants, and the duration of Ps was unaffected by N‐form treatment at photosynthetic photon flux densities of 900 or 1800 μmol photon m−2 s−1. Furthermore, N‐form treatment did not affect the duration of daily canopy Ps. Genotypes responsive to mixed N utilized distinct physiological strategies to achieve increased grain yields. For example, the genotype LHE136 × LH82 increased the partitioning of dry matter to the grain, whereas LH74 × LH51 and B73 × LH51 increased total dry matter production. Mixed‐N nutrition decreased the percentage of aborted kernels for the genotype B73 × LH51 and increased anthesis ovule number per earshoot for LH74 × LH82. Plants of the five genotypes supplied with mixed N also increased whole‐shoot N content at maturity (by 5–14%). Based on these data, we conclude that mixed‐N nutrition can moderately increase grain yield and productivity of certain maize genotypes by altering dry matter accumulation and partitioning, earshoot and ovule development, and N accumulation.