Stimulation par la lumière rouge lointain de la synthèse de la δ-aminolévulinate déshydratase des cotylédons de radis

Stimulation of de novo synthesis of δ-aminolevulinate dehydralasc of radishes grown under far-red light .
Density labelling studies of δ-aminolevulinate dehydratase (ALAD) in cotyledons of radish ( Raphanus sativus L. cv. Longue Rave Saumonée) seedlings demonstrate that far-red light stimulates de novo synthesis of ALAD and that the turn-over of this enzyme is very poor. Cycloheximide reduces considerably both the increase of ALAD activity and the incorporation of deuterium in ALAD, which indicates that ALAD synthesis depends upon cytoplasmic ribosomes.     

Interaction of coupled nonlinear oscillators having different intrinsic resting levels

The interaction among coupled oscillators is governed by oscillator properties (intrinsic frequency and amplitude) and coupling mechanisms. This study considers another oscillator property, the intrinsic resting level, and evaluates its role in governing oscillator interactions. The results of computer experiments on a chain of either three or five bidirectionally coupled nonlinear oscillators, suggest that an intrinsic resting level gradient, if present, is one of the factors governing the interaction between coupled oscillators. If there is no intrinsic frequency gradient, then an intrinsic resting level gradient is sufficient to produce many features of interaction among coupled oscillators. If both intrinsic frequency and intrinsic resting level gradients are present, then both of them determine the manner in which the coupled oscillators interact with each other.     

Phloem unloading in the potato tuber. Pathways and sites of ATPase

Summary Potential pathways for sucrose unloading in the potato tuber were examined by light and electron microscopy. Abundant plasmodesmata connected sieve elements with surrounding parenchyma elements and also sieve elements with companion cells. Plasmodesmata were rarer, however, between companion cells and parenchyma elements. These observations suggest that sucrose may leave the sieve elements and enter the storage parenchyma cells directly via the symplast and that transport through the companion cell may not be a prerequisite for unloading. Plasmodesmata, grouped together in primary pit fields, were also abundant between storage cells, and isolated storage cells, separated enzymically, showed considerable variation in plasmodesmatal distribution between cells and also on different faces of a single cell. Deposition of starch was found to occur in the tuber cortex while an endodermis with Casparian strip was present external to the phloem, suggesting that assimilates initially enter the cortical storage cells by an entirely symplastic pathway. The possible involvement of ATPase in the unloading process was examined cytochemically, using a lead-salt precipitation method. By contrast with previous findings for phloem no evidence was found for ATPase activity that was unique to the sieve element-companion cell complex. The present observations favour the view that phloem unloading in the potato tuber is a symplastic and passive process.     

Duplicated chromosome segments in maize (Zea mays L.): further evidence from hexokinase isozymes

Summary The genetic control of hexokinase isozymes (ATP: d-hexose-6-phosphotransferase, E.C. 2.7.7.1, HEX) in maize (Zea mays L.) was studied by starch gel electrophoresis. Genetic analysis of a large number of inbred lines and crosses indicates that the major isozymes observed are encoded by two nuclear loci, designated Hex1 and Hex2. Five active allozymes and one null variant are associated with Hex1, while Hex2 has nine active alleles in addition to a null variant. Alleles at both loci govern the presence of single bands, with no intragenic or intergenic heteromers visible, suggesting that maize HEX's are active as monomers. Organelle preparations demonstrate that the products of both loci are cytosolic. All alleles, including the nulls, segregate normally in crosses. Vigorous and fertile plants were synthesized that were homozygous for null alleles at both loci, suggesting that other hexosephosphorylating enzymes exist in maize that are undetected with our assay conditions. Linkage analyses and crosses with B-A translocation stocks place Hex1 on the short arm of chromosome 3, 27 centimorgans from Pgd2 (phosphogluconate dehydrogenase) and Hex2 on the long arm of chromosome 6, approximately 45 centimorgans from Pgd1. It is suggested that the parallel linkages among these two pairs of duplicated genes reflects an evolutionary history involving chromosome segment duplication or polyploidy.Paper No. 10170 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC     

Dry matter and nitrogen accumulation and partitioning in selected soybean genotypes of different derivation

Summary Experiments were conducted to determine if changes in the accumulation and partitioning of dry matter (DM) and nitrogen (N) in soybean [Glycine max (L.) Merr.] were associated with agronomic improvements and to assess the degree of genetic variation present for these traits. Fifteen maturity group II soybean genotypes including three ancestral cultivars, three modern cultivars, and nine agronomically superior plant introductions (PI's) were grown in replicated tests at four locations in the eastern U.S. The DM and N of stems, pod walls, and seeds were determined at maturity, and the apparent harvest indices (HI) and the apparent nitrogen harvest indices (NHI) were calculated. Pod DM partitioning was calculated as the ratio of seed DM to total pod DM and pod N partitioning was the ratio of seed N to total pod N. The mean DM accumulation of the modern cultivars was significantly greater than that of the ancestral cultivars and PI's. The apparent HI and the pod DM partitioning of both the modern and ancestral cultivars were significantly higher than that of the PI's. The three modern cultivars demonstrated the highest N accumulation. As a group, the modern cultivars consistently showed maximal accumulation and partitioning of DM and N suggesting that these physiological traits are associated with agronomic improvement. No individual PI was found to possess DM or N accumulation or partitioning which significantly exceeded the best modern cultivar or ancestral cultivar, indicating that genotypes with accumulation or partitioning characteristics which exceed available germplasm may be difficult to identify. Seed yield was correlated (P<0.05) with both DM (r=0.61) and N (r=0.57) accumulation.     

Associations between quantitative traits and enzyme loci in the F2 population of a maize hybrid

Summary Univariate and multivariate analyses were used to identify associations between eight enzyme marker loci and 11 quantitative traits of maize (Zea mays L.). The material analyzed included inbred lines Wf9 and Pa405, single-cross hybrid Wf9 X Pa405, and the F2 generation of the selfed single-cross hybrid. Each enzyme locus assayed was associated with at least one quantitative trait, and all quantitative traits were associated with genotypes at particular enzyme loci. Significant associations also were found between the level of heterozygosity per individual and nine of 11 quantitative traits. The total contribution to heterosis, for seed yield per plant, of genes linked with the eight enzyme loci, was 27% of the F2 mean and 18% of the difference in mean between the F1 hybrid and the inbred parents. Genes linked with Glu1 accounted for nearly one third of the total dominance effect detected by the eight enzyme loci. The chromosome segments marked by loci with significant effects on seed yield were markedly overdominant. The large heterotic effects of chromosome segments marked by particular loci suggest that enzyme loci could be used to help transfer genes responsible for heterosis to inbred lines. We conclude that analyses of additional inbred lines, F1 hybrids, and F2 populations in more environments will halp identify specific associations between enzyme loci, or chromosome segments which they mark, and important agronomic traits.Cooperative investigations of the USDA, ARS and Dept. of Plant Sciences, South Dakota State Univ. (SDSU), Brookings, Journal Series No. 2039; and the Institute of Animal Resource Ecology, Univ. of British Columbia, Vancouver, B.C. V6T 1W5, Canada     

Epistasis in maize (Zea mays L.)

Summary Three flint and three dent maize (Zea mays L.) inbred lines, their possible F₁ crosses, F₂ and backcross progenies, and all possible three-way crosses were evaluated in a three-year experiment for yield, ear moisture, and plant height. The purpose was to estimate genetic parameters in European breeding materials from (i) generation means analysis, (ii) diallel analysis of generation means, and (iii) analysis of F₁ and three-way cross hybrids. Method (i) was based on the F-metric model and methods (ii) and (iii) on the Eberhart-Gardner (1966) genetic model; both models extended for heterotic maternal effects.Differences among generation means for yield and plant height were mainly attributable to dominance effects. Epistatic effects were significantly different from zero in a few crosses and considerably reduced heterosis in both traits. Additive x additive and domiance x dominance effects for yield were consistently positive and negative, respectively. Significant maternal effects were established to the advantage of generations with a heterozygous seed parent. In the diallel analysis, mean squares for dominance effects were greater than for additive effects for yield and plant height but smaller for ear moisture. Though significant for yield and plant height, epistatic variation was small compared to additive and dominance variation. Estimates of additive x additive epistasis for yield were significantly negative in 11 of 15 crosses, suggesting that advantageous gene combinations in the lines had been disrupted by recombination in the segregating generations. The analysis of hybrids supported the above findings regarding the analysis of variance. However, the estimates of additive x additive epistasis for yield were considerably smaller and only minimally correlated with those from the diallel analysis. Use of noninbred materials as opposed to materials with different levels of inbreeding is considered the main reason for the discrepancies in the results.     

Carbon monoxide sensitivity of cytochrome oxidase in wheat leaves in relation to dwarfing genes (Rht) in wheat cultivars

Summary Leaves of young seedlings of a number of tall cultivars of wheat, lacking the dwarfing Rht genes, readily responded to a brief 2 min exposure to CO, as assessed by in vivo aerobic assay of nitrate reductase. This test depends on the inhibition of cytochrome c oxidase by CO, which in turn renders cytosolic NADH available for the reduction of nitrate to nitrite in vivo. Semi-dwarf cultivars of wheat (Rht present) did not respond to CO in this way. Since CO forms a complex only with reduced cytochrome a₃, the results indicate differences in the redox state of cytochrome a₃, during in situ respiration of leaves from tall and semi-dwarf plants which are likely to be under genetic control.     

Accumulation and subcellular distribution of cations in relation to the growth of potassium-deficient barley

Abstract Growth of barley (Hordeum vulgare L., cv. Georgie) was insensitive to soil K content above about 150 mg kg^?1, but at lower levels it declined. The reduction in yield was greater in soils containing approximately 10 mg Na kg^?1 than in soils with about 90 mg kg^?1 of Na. Growth was unaffected by changes in shoot K concentration above 75 mol m^?3, but declined at lower concentrations, and the decrease was less in plants grown in soils with high Na. Growth responses were not simply related to tissue K concentrations because plants grown in soils with extra Na had higher yields but lower K concentrations. When soil Na was low, plants accumulated Ca as tissue K declined, but when Na was provided this ion was accumulated. Plant Mg concentrations were generally low but increased as K decreased. The Ca and Mg were osmotically active. There were highly significant inverse linear relationships between yield and either the Ca or Mg concentrations in the shoots. X-ray microanalysis was used to examine the compartmentation of cations in leaves from barley plants (cv. Clipper) grown in nutrient solutions with high and low K concentrations. In plants grown with 2.5 mol m^?3 K, this was the major cation in both the cytoplasm and vacuole of mesophyll cells. However, in plants grown with 0.02 mol m^?3 K it declined to undetectable levels in the vacuole, although it was still detectable in the cytoplasm. In all plants, Ca was mainly located in epidermal cells. The implication of the results for explaining responses to K. in terms of compartmentation of solutes is discussed.     

Restitution of polarity in statocytes from centrifuged roots*

Abstract The structural polarity of statocytes from cress roots is changed by centrifugation. Upon low- dose centrifugation (3000 g min), the extent of stratification depends on statocyte position, i.e., central statocytes are affected more than lateral ones. Upon higher doses of centrifugation (60,000 and 360,000 g min), a uniform density gradient is established in all statocytes. If, after centrifugation, the roots are exposed to gravity again, the endoplasmic reticulum (ER) cisternae are relocated parallel to the periclinal cell walls within a few minutes; this relocation is independent of the direction of gravity in relation to the root axis, and independent of the previously applied centrifugation dose. This supports the notion that polarity is determined genetically. Cytochalasin B treatment, before and during centrifugation, totally inhibits the relocation of ER. After removing the drug by rinsing the roots, the statocytes restore cell polarity and relocate ER. These results indicate that relocation of ER cisternae may be mediated by microfilaments. When centrifuged roots are exposed to 1 g in the horizontal position, the latent period of gravitropism increases by 8–10 min relative to controls, regardless of the previously applied centrifugation doses. The kinetics of curvature are virtually identical. Since the increase in the latent period coincides with the time needed for most statocytes to restore the distal cell pole, it is evident that perception of gravity is correlated to the integrity of the distal cell pole.