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Production of hybrid corn (Zea mays L) depends on cross-pollination between male and female inbred parents. As such, reproductive isolation of seed fields is required to ensure genetic purity of the hybrid progeny. Customer demand for improved genetic purity prompted the seed industry to examine the level of genetic purity resulting from current isolation practices. A 3-yr study was conducted to monitor purity of hybrid seed produced in 315 fields from 24 seed companies in North America. Each field was near a commercial corn field shedding pollen synchronously with the seed parent: a worse case scenario. Seed samples were collected at five locations along a 200-m transect established perpendicular to the nearest potential adventitious pollen source, and 100 seed from each location were subjected to isozyme analysis to determine percent out-crossing. Isolation distance, seed field size, field block size, number of border rows, adventitious field size, male:female row ratio, male population, and male pollen class were analyzed as continuous predictors and as variable class predictors of out-crossing at the field margins and field midpoints. Year had a significant (p ≤ 0.05) impact on observed out-crossing. Isolation distance, border rows, and male pollen class were significant predictors of out-crossing at the field margin but not at the field midpoint. A significant interaction between isolation distance and border rows was observed at both the field margin and midpoint. There also was an interaction between male:female ratio and male pollen class on out-crossing at the field margin. These results indicate that current practices used to isolate hybrid seed fields often achieve the goal of producing ≥99% genetically pure seed, but much higher levels of out-crossing can and do occur. Because out-crossing generally was greater and more variable at the field margins than at the field midpoint, adjustments to field management that focus on minimizing out-crossing at the field margins should lead to consistently high levels of genetic purity from hybrid seed fields.

Hybrid maize (Zea mays L.) seed artificially dried to 120 g H 2 O kg -1 moisture in air temperatures of either 35 (LT) or 45°C (HT) was used to study the effect of desiccation temperature on mitochondrial function during early imbibition. In embryo culture experiments, embryo growth was delayed for embryos excised from HT treated seed compared with LT samples. After 1 h of imbibition, the rate of O 2 uptake by LT axis tissue was greater than for HT dried samples. Uptake rates continued to increase for LT samples through the first 6 h of imbibition while rates for HT axes remained constant. Mitochondria isolated from LT axis tissue that had undergone imbibition for 4 h exhibited respiratory control with oxidative phosphorylation efficiency values of 0.79 and 0.77 for NADH and succinate assays, respectively. Mitochondria from imbibing HT axis tissue exhibited no respiratory control. Electron micrographs of cells from the radicle meristem region of both treatments showed that mitochondrial development during the initial 24 h of imbibition was impaired in HT samples. These results suggest that mitochondrial function and development, during the initial stages of embryonic tissue hydration, is impaired when maize seed are artificially dried with high air temperatures. We conclude that the development of mitochondrial function during this period is a key element in determining the rate of germination and subsequent seedling growth.

Marzak and Oum-rabia wheat seeds were produced under three temperature regimes (20:15, 28:21, 36:29 °C) starting 10 d after anthesis through harvest. Nucleotide levels and respiratory activity of mitochondria isolated from imbibing embryos were determined. Mitochondrial structure from the radicle meristem region of imbibed embryos was examined under electron microscopy. Embryos from low-temperature treatments showed rapid accumulation of adenosine triphosphate (ATP) and higher energy levels and rates of oxygen uptake than embryos from high-temperature treatments. Embryos from medium-temperature treatments exhibited intermediate values. Parallel to these metabolic changes during early seed germination, results from electron microscopy revealed visible differences in mitochondrial structure. Mitochondria from the low-temperature regime were well developed with visible membranes and cristae; those from the high-temperature regime were degenerating. These results provide clear evidence of the influence of parent temperature conditions on the seed metabolism during early stages of germination. Key words: Embryo, mitochondria, nucleotide, oxygen uptake, vigor, wheat

Membrane phospholipids play an important role in acclimation of plants to environmental stresses. Phospholipid composition and thermal properties in maturing maize (Zea mays L.) seed were studied to relate high-temperature desiccation tolerance to membrane stabilization. A preconditioning process (treatment at 35 °C prior to high-temperature drying) was used to induce the high-temperature desiccation tolerance. Phosphatidylcholine (PC) accumulated, resulting in an increase in the PC/phosphatidylethanolamine (PE) ratio from 3.6 to 8 within 48 h as the high-temperature desiccation tolerance was induced during preconditioning. The increase in PC/ PE ratio coincided with a decrease in both phase transition temperature and enthalpy of transition, indicating more stable membranes. The improved stability could be related to the high-temperature desiccation tolerance and membrane function after preconditioning. A shift in the fatty acid composition of the membrane lipids from linoleic acid (18:2) to oleic acid (18:1) during preconditioning indicates a more saturated fatty acid composition. This shift in fatty acids may result in membranes that more easily cope with high-temperature desiccation, as contrasted to a low-temperature effect. The results suggest that alterations in phospholipid molecular species and changes in fatty acid composition to a more saturated composition in maize seed during preconditioning and maturation could be common mechanisms in high-temperature desiccation tolerance.

This study proposes to further clarify the effect of drying temperature and rate on vigour, membrane integrity, mitochondrial competence and synthetic processes in susceptible genotypes. Hybrid maize of the seed parent B73 by H99xH95 as pollen parent was harvested periodically, husked and dried under four temperature regimes to 12% mc providing drying times from 4 to 300 h. Standard germination results were generally high and cold test values were similar regardless of temperature or rate. Dry weights from seedlings grown at 25°C were responsive to drying temperature and rate while cold test dry weights showed little difference except for the rapid drying treatment in which root development was consistently depressed more than shoot. Membrane integrity was not compromised in seed dried in normal thinlayer drying but the rapidly dried material exhibited a linear increase in leakage with time. Oxygen consumption by imbibing embryos was reduced by high drying temperature but not rate. Incomplete alignment of lipid bodies along the cell wall following rapid drying treatments as compared to complete alignment in all others suggests roles in both drying and imbibition. Percentage of a 66 kDa protein (associated with maturation) in proportion to total non heat-denatured protein increased in response to drying but not at the rapid rate.

Description d'une egreneuse pour le soja recolte avant maturite complete pour l'experimentation agronomique

Seed performance in early spring plantings varies with temperature and soil atmospheric conditions, but nothing has been reported on the combined stress of low temperature and low O2 levels on soybean (Glycine max (L.) Merr.) seed germination. Artificially aged and unaged soybean seed of two cold-tolerant and two cold-intolerant genotypes were exposed to either 10 or 25 °C and 0.2, 2.0 or 20.0% O2 during imbibition and germination in a controlled environment. Germination and seedling dry weights were determined immediately after completing the temperature-O2 treatments and after completing a 3-d grow-out period in growth chambers set at 25 °C. Decreasing the O2 available to germinating seeds was more detrimental to the germination process at 25 than at 10 °C. Reductions in the amount and speed of germination and seedling vigor resulting from low temperature and low O2 levels did not continue after the 3-d grow-out period, but the effects of artificially aged seed did remain. The cold-tolerant genotypes germinated better and had greater seedling vigor than the cold-intolerant genotypes at 10 and 25 °C and at 2.0 and 20.0% O2. Seeds did not germinate at 0.2% O2 either at 10 or 25 °C, but were subsequently able to germinate normally during the 3-d grow-out period. These results indicate that genotypes respond differently to low temperature and low O2 and that soybeans may undergo a temporary cessation of germination at 0.2% O2. Reduced oxygen availability decreased seed and seedling performance of high-quality seeds more than of low-quality seeds and more at higher temperature than at lower temperature.Key words: Soybean, seed quality, vigor, germination, cold tolerance, artificially aged seed

General agreement does not exist regarding the influence of seed corn (Zea mays L.) vigor on subsequent crop performance. Studies were conducted to determine the characters through which seed corn vigor is expressed, how long the influence persists and the relationship among the characters identified. Seeds of two corn hybrids A632 X (H99 X H95) and B73 X (H99 X H95), dried at 35 °C and 50 °C were grown in the field in each of 2 years and in the growth chamber in Ames, Iowa. A complete range of growth parameters was measured in the laboratory and growth chamber and field. Of these parameters, emergence, emergence rate, plant height, stem diameter, shoot dry matter at 4 and 6 weeks and penultimate leaf length and width were characters found to be influenced by seed vigor. As the plants matured, the influence of seed vigor decreased. Positive and negative relationships were found to exist between some of the characters influenced by seed vigor. However, seed vigor had no influence on yield.

The relationship between seed corn vigor and some crop characters in uniform and reduced plant populations is poorly defined. This study, was designed to define the crop characters through which seed vigor is expressed in reduced and uniform populations of corn and to clarify which experimental technique should be used in seed vigor studies. Seeds of two corn hybrids, A632 x (H99 x H95) and B73 x (H99 x H95), were dried at two temperatures, 35 °C and 50 oC, and planted near Ames, Iowa, in 1983. The two drying temperatures created differential vigor levels between the seed lots. There were 21 crop characters measured in two populations, reduced populations due to lack of seed vigor and uniform population (thinning to stand). Early vegetative growth was influenced by seed vigor, but as the plant aged, the influence of seed vigor decreased. Increased early plant height, up to 6 weeks old, small stem diameter, low shoot dry-matter content, delay in days from sowing to 50 % tasseling and to 50 % silking, narrow penultimate and ear leaf, and reduction in the number of ears and tiller production were observed in the uniform population. The differences in population density due to seed vigor did not have a significant effect on final grain yield inasmuch as plants in the reduced populations were able to compensate completely for missing stands.

Of 323 pregnancies with meconium-stained amniotic fluid at 36-42 weeks' gestation, 68 (21%) had a pH less than 7.20 in umbilical arterial blood, 21 (7%) had a pH less than 7.15, and only three newborns (0.9%) had true metabolic acidemia. At birth, of the 74 newborns with normal electronic fetal heart rate (FHR) tracings, eight (11%) had an umbilical arterial pH less than 7.20. There was a significantly higher frequency of acidemia (defined as pH less than 7.20) in newborns with both baseline and periodic FHR abnormalities. Although there was a significant difference (P less than .05) in the frequency of meconium found below the cords in these neonates with an umbilical artery pH less than 7.20 compared with those with values exceeding 7.20, there was no significant difference in the frequency of clinical meconium aspiration syndrome. We conclude that meconium-stained amniotic fluid correlates poorly with infant condition at birth as reflected by umbilical cord acid-base measurements.