Field-grown Maize Research Articles

A N2-fixing endophytic Burkholderia sp. associated with maize plants cultivated in Mexico.

In the frame of a survey of potentially endophytic N2-fixing Burkholderia associated with maize in Mexico, its country of origin, the soil of an indigenous maize field near Oaxaca was studied. Under laboratory conditions, plant seedlings of two ancient maize varieties were used as a trap to select endophyte candidates from the soil sample. Among the N2 fixers isolated from inside plant tissues and able to grow on PCAT medium, the most abundant isolates belonged to genus Burkholderia (API 20NE, rrs sequences). Representative isolates obtained from roots and shoots of different plants appeared identical (rrs and nifH RFLP), showing that they were closely related. In addition, their 16S rDNA sequences differed from described Burkholderia species and, phylogenetically, they constituted a separate deep-branching new lineage in genus Burkholderia. This indicated that these isolates probably constituted a new species. An inoculation experiment confirmed that these N2-fixing Burkholderia isolates could densely colonize the plant tissues of maize. More isolates of this group were subsequently obtained from field-grown maize and teosinte plants. It was hypothesized that strains of this species had developed a sort of primitive symbiosis with one of their host plants, teosinte, which persisted during the domestication of teosinte into maize.

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.

Distributions of rare earths and heavy metals in field-grown maize after application of rare earth-containing fertilizer

Rare earths are widely applied in Chinese agriculture to improve crop nutrition through the use of fertilizers, yet little is known of their accumulation in field-grown crops. We have studied the distribution of 16 rare earths (Sc, Y and 14 lanthanide elements) in field-grown maize and the concentration of heavy metals in the grains after application of rare earth-containing fertilizer. When maize entered the vigorous vegetation growth stage (e.g. early stem-elongation stage), rare earth-containing fertilizer was applied to the soil with irrigation water. At 10 days after application of the rare earths, significantly dose-dependent accumulative effects of individual rare earth concentrations in the roots and the plant tops of maize were observed, with the exception of Sc and Lu. At the level of 2 kg rare earths ha −1, accumulative concentrations of most light rare earths (e.g. La, Ce, Pr and Nd) and Gd in the plant tops were much larger than those in the control. Concentrations of individual rare earths in a field-grown maize after application of rare earths decreased in the order of root≫leaf>stem>grain. During the maize growth period, selective accumulation of individual rare earths (e.g. La, Ce) in the roots seemed to be in dynamic equilibrium, and the distribution of these elements in the plant tops was variable. At a dosage of less than 10 kg rare earths ha −1, no apparent accumulative concentrations of individual rare earths appeared in the maize grains. Under the experimental conditions, application of rare earth-containing fertilizer did not induce an increase in the concentrations of heavy metals in the grains. We conclude that the present dosage of rare earths (<0.23 kg ha −1 year −1) currently applied in China can hardly affect the safety of maize grains in arable soil, even over a long period.

Phytosiderophore release from nodal, primary, and complete root systems in maize

Some maize (Zea mays L.) hybrids grown in high pH soil in Nebraska suffer from severely reduced yields caused by iron (Fe) deficiency chlorosis. Hybrids which recover from early season Fe-deficiency chlorosis and yield well are termed Fe-efficient or tolerant. Most Fe-efficient gramineous species respond to Fe-deficiency stress by releasing phytosiderophores (mugineic acid and its derivatives) into the rhizosphere, thereby increasing Fe availability and uptake of the Fe 3+ -phytosiderophore complex via a high affinity uptake system. Field-grown Fe-efficient maize recovers from Fe-deficiency chlorosis at a stage when nodal roots have become the dominant root system. Quantifying phytosiderophore release from hydroponically grown plants has been proposed as a viable alternative to time-consuming and variable field trials and has been used successfully to delineate among Fe-efficient and Fe-inefficient lines of oat (Avena sativa L.) and wheat (Triticum aestivum L.). Our objectives were (1) to determine if phytosiderophore release differed between nodal- and primary-root systems of maize. and (2) to compare phytosiderophore release from 12 hybrids. Root exudates secreted during daily 4-h collections were analyzed for their Fe-solubilizing ability, which was equated to phytosiderophore release. Nodal root systems released significantly more phytosiderophore than primary- or complete-root systems. In early experiments, an Fe-efficient hybrid (P3279) released more phytosiderophore from nodal roots than an Fe-inefficient hybrid (P3489). Tests of an additional 10 hybrids showed that phytosiderophore release varied significantly among the cultivars but did not clearly distinguish between hybrids classified as Fe-efficient or Fe-inefficient in individual company trials. We recommend using nodal roots when studying Fe-stress response mechanisms in maize.

Ovary abscisic acid concentration does not induce kernel abortion in field-grown maize subjected to drought

This study investigated the effects of drought of different duration and severity on ovary abscisic acid (ABA) concentration and yield components in field-grown maize ( Zea mays L. cv. Loft). The study was conducted in a field lysimeter of the Royal Veterinary and Agricultural University (KVL) in Højbakkegaard (55°40′N; 12°18′E; 28 masl), Denmark in 1997. Irrigation was withheld at four different dates to induce drought of different duration and severity at the reproductive stage of the plants. Plots were re-watered shortly after silking and kept at field capacity for the remainder of the season. Soil water status, plant height, and early morning leaf water potential were monitored during the treatment. Ovary ABA concentration was determined at four dates before and after fertilization. Final grain yield, total DM, harvest index (HI), mean kernel weight, kernel weight distribution, and kernel number per cob were determined at maturity. Plant height was significantly ( P<0.05) reduced by 40 and 25%, respectively, in the two most severe drought treatments. In the two shorter drought treatments no effect of drought stress on plant height or biomass was observed. Leaf water potential decreased slowly as a function of relative available soil water content and resulted in −0.4 MPa at the end of the longest and −0.12 MPa at the end of the shortest stress period. Under fully watered conditions, plot yields averaged 1400 g m −2 for total dry matter (DM) and 700 g m −2 for grain yield, with a HI of about 0.5. Initiation of a drying cycle close to flowering did not change yields. Long drying cycles resulted in significant ( P<0.05) yield reductions up to 70% of the fully watered controls. Kernel number per cob was reduced up to 60% under long drought conditions and not affected under short-term drought. Drought imposed about two weeks prior to fertilization resulted in 30% reduction in kernel number per cob, but this effect was balanced by an increase of 25% in mean kernel weight. Long and severe drought increased ovary ABA concentration prior to fertilization, whereas short-term drought did not. At fertilization no increase of ovary ABA as compared to fully watered controls was found in any treatment. It is concluded that drought induced grain yield losses in field grown maize cannot be attributed to kernel size reduction or kernel abortion due to ovary ABA concentrations as reported by some authors for studies on maize and wheat under controlled conditions, as ovary ABA concentrations peaked before zygote formation and endosperm development.

3D Visualization of Maize Stem by MRI Technology

Recent development in confocal and multi-photon microscopy allows 3D imaging of plant tissue in high resolution. However, other than physical sectioning, macroscopical study of plant organs in 3D remains a difficult task. Among various available technologies for macroscopical imaging (e.g., Xray macro-tomography, optical coherent tomography and MRI), MRI is an ideal choice for its contrasting modality in volumetric imaging of soft tissues. A 3T Biospect MRI system (Brucker, Germany)(FIG 1) equipped with a 6cm inner diameter micro-quadrature coil (FIG 2) for RF transmission and reception of MRI signals was used in this study. Spin echo based RARE sequence was used to obtain T2 weighted images with TR/TE = 3160.5/58.5ms and field-of-view of 1.67cm × 1.67cm (256 × 256 pixels) at a slice thickness of 0.8mm. This corresponds to a voxel size of 65 × 65 × 800μm. Data was obtained within 1/2 hour with number-of-excitations (nex) set at 16. Figure 4 (a-x) shows a series of MRI sections through a stem node (the node below the main ear insertion) from field-grown maize (Zea mays, van Odyssey sweet corn). The stem was fixed in 1:3 EtOH/acetic acid, washed thoroughly in water prior to imaging. Air bubbles trapped in the tissue were removed by vacuuming, to avoid imaging artifact due to low magnetic susceptibility of air. Figure 5 (a-g) shows reconstructed longitudinal sections. Three-dimensional reconstruction (FIG. 3) was performed by using Vaytek VoxBlast™ and AutoQuant’s AutoVisulize 3D™ software. in combination with image segmentation and tracing tools, the MRI technology will greatly enhance our capability in the understanding of vascular architecture and its development in plants.

Effect of primers hybridizing to different evolutionarily conserved regions of the small-subunit rRNA gene in PCR-based microbial community analyses and genetic profiling.

Genetic profiling techniques of microbial communities based on PCR-amplified signature genes, such as denaturing gradient gel electrophoresis or single-strand-conformation polymorphism (SSCP) analysis, are normally done with PCR products of less than 500-bp. The most common target for diversity analysis, the small-subunit rRNA genes, however, are larger, and thus, only partial sequences can be analyzed. Here, we compared the results obtained by PCR targeting different variable (V) regions (V2 and V3, V4 and V5, and V6 to V8) of the bacterial 16S rRNA gene with primers hybridizing to evolutionarily conserved flanking regions. SSCP analysis of single-stranded PCR products generated from 13 different bacterial species showed fewer bands with products containing V4-V5 (average, 1.7 bands per organism) than with V2-V3 (2.2 bands) and V6-V8 (2.3 bands). We found that the additional bands (>1 per organism) were caused by intraspecies operon heterogeneities or by more than one conformation of the same sequence. Community profiles, generated by PCR-SSCP from bacterial-cell consortia extracted from rhizospheres of field-grown maize (Zea mays), were analyzed by cloning and sequencing of the dominant bands. A total of 48 sequences could be attributed to 34 different strains from 10 taxonomical groups. Independent of the primer pairs, we found proteobacteria (alpha, beta, and gamma subgroups) and members of the genus Paenibacillus (low G+C gram-positive) to be the dominant organisms. Other groups, however, were only detected with single primer pairs. This study gives an example of how much the selection of different variable regions combined with different specificities of the flanking "universal" primers can affect a PCR-based microbial community analysis.

Burkholderia, a genus rich in plant-associated nitrogen fixers with wide environmental and geographic distribution.

The genus Burkholderia comprises 19 species, including Burkholderia vietnamiensis which is the only known N(2)-fixing species of this bacterial genus. The first isolates of B. vietnamiensis were recovered from the rhizosphere of rice plants grown in a phytotron, but its existence in natural environments and its geographic distribution were not reported. In the present study, most N(2)-fixing isolates recovered from the environment of field-grown maize and coffee plants cultivated in widely separated regions of Mexico were phenotypically identified as B. cepacia using the API 20NE system. Nevertheless, a number of these isolates recovered from inside of maize roots, as well as from the rhizosphere and rhizoplane of maize and coffee plants, showed similar or identical features to those of B. vietnamiensis TVV75(T). These features include nitrogenase activity with 10 different carbon sources, identical or very similar nifHDK hybridization patterns, very similar protein electrophoregrams, identical amplified 16S rDNA restriction (ARDRA) profiles, and levels of DNA-DNA reassociation higher than 70% with total DNA from strain TVV75(T). Although the ability to fix N(2) is not reported to be a common feature among the known species of the genus Burkholderia, the results obtained show that many diazotrophic Burkholderia isolates analyzed showed phenotypic and genotypic features different from those of the known N(2)-fixing species B. vietnamiensis as well as from those of B. kururiensis, a bacterium identified in the present study as a diazotrophic species. DNA-DNA reassociation assays confirmed the existence of N(2)-fixing Burkholderia species different from B. vietnamiensis. In addition, this study shows the wide geographic distribution and substantial capability of N(2)-fixing Burkholderia spp. for colonizing diverse host plants in distantly separated environments.

Prospects for reducing fumonisin contamination of maize through genetic modification.

Fumonisins (FB) are mycotoxins found in (italic)Fusarium verticillioides-infected maize grain worldwide. Attention has focused on FBs because of their widespread occurrence, acute toxicity to certain livestock, and their potential carcinogenicity. FBs are present at low levels in most field-grown maize but may spike to high levels depending on both the environment and genetics of the host plant. Among the strategies for reducing risk of FB contamination in maize supplied to the market, development and deployment of Fusarium ear mold-resistant maize germplasm is a high priority. Breeding for increased ear mold tolerance and reduced mycotoxin levels is being practiced today in both commercial and public programs, but the amount of resistance achievable may be limited due to complicated genetics and/or linkage to undesirable agronomic traits. Molecular markers can be employed to speed up the incorporation of chromosomal regions that have a quantitative effect on resistance (quantitative trait loci). Transgenic approaches to ear mold/mycotoxin resistance are now feasible as well. These potentially include genetically enhanced resistance to insect feeding, increased fungal resistance, and detoxification/prevention of mycotoxins in the grain. An example of the first of these approaches is already on the market, namely transgenic maize expressing Bacillus thuringiensis (Bt) toxin, targeted to the European corn borer. Some Bt maize hybrids have the potential to reduce FB levels in field-harvested grain, presumably through reduced feeding of Bt-susceptible insects in ear tissues. However, improved ear mold resistance per se is still an important goal, as the plant will still be vulnerable to noninsect routes of entry to (italic)Fusarium. A second approach, transgene-mediated control of the ability of Fusarium to infect and colonize the ear, could potentially be achieved through overexpression of specific antifungal proteins and metabolites, or enhancement of the plant's own defense systems in kernel tissues. This has not yet been accomplished in maize, although promising results have been obtained recently in other monocots versus other fungal and bacterial pathogens. Achieving reproducible and stable enhanced ear mold resistance under field conditions will be immensely challenging for biotechnologists. A third approach, transgene strategies aimed at preventing mycotoxin biosynthesis, or detoxifying mycotoxins in planta, could provide further protection for the grower in environments where FBs present a risk to the crop even when the maize is relatively resistant to Fusarium mold. In one example of such a strategy, enzymes that degrade FBs have been identified in a filamentous saprophytic fungus isolated from maize, and corresponding genes have been cloned and are currently being tested in transgenic maize.

Bleeding Rate of Field-Grown Maize with Reference to Root System Development.

Bleeding rate could be an effective index for the physiological activity of a whole root system in field-grown crops, because it is based on active water absorption caused by root pressure. We examined the bleeding rate of field-grown maize with reference to root system development. The bleeding rate decreased rapidly just after detopping, which suggested negative effects of shoot removal on bleeding in maize. The bleeding rate of different plants at every hour showed that it has a diurnal change pattern with the maximum value occurring at around 8 a.m. Then we examined the bleeding rate early in the morning with reference to shoot growth. It increased with shoot growth and root development to reach the maximum value at around tasselling and decreased thereafter. The number and diameter of nodal roots emerging acropetally along the stem were increasing until tasselling. Therefore significant positive correlations were evident between the bleeding rate and the root morphological traits, such as total number, diameter, and cross area of nodal roots of a whole root system until tasseling, although these relationships were not clear afterwards. Physiological activity per unit root mass was estimated on the assumption that bleeding rate is a product of root morphology and its physiological activity. The physiological activity per unit root mass increased in the former half of the growing period, then decreased, possibly because of the relative relationship between the formation of new roots and the senescense of existing roots.

QTL analysis of drought-related traits and grain yield in relation to genetic variation for leaf abscisic acid concentration in field-grown maize

QTL analysis of drought-related traits and grain yield in relation to genetic variation for leaf abscisic acid concentration in field-grown maize

QTL analysis of drought-related traits and grain yield in relation to genetic variation for leaf abscisic acid concentration in field-grown maize

deficit (Zeevaart and Creelman, 1988; Bray, 1997). ABA modulates the expression of a large number of genes Abscisic acid (ABA) concentration is a quantitatively whose products may protect the cell from the harmful inherited trait which plays a pivotal role in the eVects of an excessive water loss (Bray, 1993; Close, 1996, response of plants to drought stress. A recent study 1997; Ingram and Bartels, 1996). It has been hypothesized identified 17 quantitative trait loci (QTLs) controlling that ABA may be involved in controlling leaf expansion bulk-leaf ABA concentration (L-ABA) in a maize (Zea during water deficit via modulation of cell wall-associated mays L.) population of 80 F 4 random families tested peroxidase activity (Bacon et al., 1997). Under conditions for two years under droughted field conditions. Sixteen of water deficit, an increased ABA concentration has also of the QTL regions influencing L-ABA also harboured been shown to increase the root to shoot ratio (Sharp, QTLs for one or more of the following traits: stomatal 1996), another important trait influencing drought tolerconductance, a drought sensitivity index, leaf temper- ance. The adaptive role of an increased ABA concentraature, leaf relative water content, anthesis-silking tion is well represented by the consequent reduction in interval, and grain yield. The analysis of the effects of stomatal conductance and in water lost by transpiration each QTL region on the investigated traits indicated ( Trejo et al., 1995). Even though it has been shown that that L-ABA mainly represented an indicator of the level ABA in the xylem sap more tightly controls stomatal of drought stress experienced by the plant at the time conductance than ABA in the bulk-leaf tissue (Davies of sampling because an increase in L-ABA was most and Zhang, 1991; Tardieu et al., 1992, 1993; Jackson, commonly associated with a decrease in both stomatal 1993), the diYculty in collecting a large number of conductance and grain yield as well as an increase in samples of xylem sap has led many authors to focus on leaf temperature. Opposite results were observed at bulk-leaf ABA concentration (L-ABA). one QTL region on chromosome 7 near the RFLP locus Although the above-mentioned findings clearly indicate asg8. A model is presented to interpret these con- a positive role of ABA in increasing plant survival under trasting results in terms of pleiotropic effects. drought, it remains diYcult to predict the global eVect which genetic variation in L-ABA may have on crop

Absorption and translocation of sludge-borne zinc in field-grown maize (Zea mays L.)

The absorption and translocation of sludge-borne zinc (Zn) in field-grown maize was studied in a long-term sewage sludge field trial on an acid sandy soil at Bordeaux, France. Since 1974, Zn-contaminated sludge had been applied at two levels of 10t dry matter (DM)ha−1year−1 (SS 10) and 100t DMha−1 per 2 years (SS 100) on annually cropped maize plots. Treatment with farmyard manure (FYM) at a rate of 10t DMha−1year−1 served as control. Plants were examined at six different growth stages from seedling to grain maturity. At each stage, the whole plant was separated into its different organs and Zn concentration and total Zn amount was determined in at least 12 different plant parts. The Zn transfer coefficient (plant Zn concentration/total soil Zn concentration) was highest in young FYM plants, followed by SS 10-amended ones, and was relatively low in SS 100 plants. Zn repartition between the different plant parts was more uniform in FYM-treated plants compared with SS 10 and SS 100 plants. High application of sewage sludge caused significantly raised Zn concentrations in all plant parts over the whole growing season. We determined an initial Zn accumulation with concentrations above 400μg Zng−1 DM in the roots and above 500μg Zng−1 DM in the lower stalks of the young maize plant. At silage stage, we measured Zn values between 300 and 500μg Zng−1 DM in the upper leaf and stalk parts of SS 100-treated plants. The increased Zn concentrations in sludge-treated plants coincided with a dry matter reduction of total shoots in SS 100-treated plants. The upper leaves and stalk parts occurred as the most important concentration and storage sites. In contrast, in the grains we determined low Zn concentrations of about 50μg Zng−1 DM until harvest without significant treatment effects or yield reductions. These observations provide evidence for a raised Zn exposure to animals and humans by vegetative aerial maize plant parts used as fodder.

Response of field-grown maize to applied magnesium in acidic soils in north-eastern Australia

Split-plot field experiments, with main plots consisting of various rates of calcitic lime and single rates of dolomite, gypsum, and calcium silicate, were conducted at each of 4 sites to determine the effect of band-applied magnesium (Mg) on maize yield. The sites were acidic with pH values of 4.5, 4.9, 5.0, and 6.1 and exchangeable Mg levels of 0.16, 0.10, 6.0, and 2.0 cmol(+)/kg, respectively. Magnesium significantly (P &lt; 0.05) increased grain yield at the 2 low-Mg sites, both of which were strongly acidic and responsive to lime application, but the nature of the Mg × lime interaction was different at each of the 2 responsive sites. The absence of a response to Mg at lime rates ≥1 t/ha at one responsive site was attributed to the presence of small amounts of Mg in the calcitic lime and/or an improved root environment enabling better exploitation of the soil Mg. Supplying a readily soluble source of Mg in the fertiliser band also resulted in increased grain yield in the gypsum, dolomite, and calcium silicate treatments at the 2 Mg-responsive sites. When the initial soil pH was strongly acidic, exchangeable Mg levels increased with increasing lime rate, suggesting that the small quantities of Mg that occur in the majority of liming materials may be of importance with respect to Mg nutrition. In contrast, gypsum application exacerbated the Mg deficiency at one site. The relationship between grain yield response and soil Mg level across all sites indicated that above an exchangeable Mg level of 0.27 cmol(+)/kg there would be little likelihood of a response to applied Mg.

The Responses of Field-grown Sunflower and Maize to Mechanical Support

The effects of mechanical support on two contrasting species of herbaceous annual, the dicot sunflower (Helianthus annuusL.) and the monocot maize (Zea maysL.), were investigated by comparing the growth and mechanical properties of supported plants and those which were left to sway freely in the wind. Providing support had its greatest effect on the more highly-stressed basal areas of the plants, such as the lower stem and the base of the lateral roots. The diameter of the stem bases of both species was approx. 10% lower in supported plants, but there was no difference between treatments in the diameter of the stem above 50 cm. Roots of both species also showed a reduction in rigidity and bending strength of 40–50% in the supported plants compared with freely swaying plants. There was a significant reduction in the partitioning of biomass to the root systems of supported plants of both species. There were differences in the way in which sunflower and maize responded to the provision of support; in sunflower, the reduction in lateral diameter was about twice that in maize, whereas in maize the decrease in the number of first-order laterals was twice that of sunflower. This study suggests that thigmomorphogenesis may be a localized response, but that different species can respond in different ways to mechanical stimulation.

Raffinose accumulation related to desiccation tolerance during maize ( Zea mays L.) seed development and maturation

Summary A comprehensive analysis of soluble carbohydrates in developing embryos of field-grown maize seed was used to establish the relationship of sucrose and raffinose levels and their mass ratio to the occurrence of desiccation tolerance in immature whole kernels and in isolated embryos. Kernel, endosperm, and embryo dry mass were maximum at 56 d after pollination (DAP). Freshly harvested seeds germinated between 64 and 86 DAP, but after fast drying at 12 % relative humidity, seeds germinated between 30 and 36 DAP and between 14 and 22 DAP after slow drying on the ear. Freshly harvested isolated embryos germinated between 14 and 16 DAP, between 26 and 45 DAP after fast drying, and between 18 and 22 DAP after slow drying. Sucrose accumulated independendy of desiccation tolerance. Raffinose accumulation was not required for germination but was associated with survival after drying. Desiccation tolerance was not observed in the absence of raffinose accumulation, but the presence of small quantities of raffinose was not always associated with the onset of desiccation tolerance during early embryo development. Desiccation tolerance in maize embryos was associated with sucrose-to-raffinose mass ratios less than 20∶1 and stage 2–3 in embryo development.

Uptake and partitioning of sludge-borne copper in field-grown maize ( Zea mays L.)

Accumulation of sludge-borne copper (Cu) by field-grown maize and its distribution between the different plant organs was studied in detail in a long-term sewage sludge field trial. Since 1974, field plots on a coarse sandy soil have been amended each year with farmyard manure (FYM) at a rate of 10 t dry matter (DM) ha −1 year −1 and with sewage sludge at the two levels of 10 t DM ha −1 year −1 (SS 10) and 100 t DM ha −1 per 2 years (SS 100). All field plots have been cropped annually with maize. In 1993, five replicate plants per treatment were examined at six different growth stages from seedling to grain maturity. Each plant was separated into at least 12 different parts and the Cu content of each was determined. Regarding growth parameters, no visible deleterious effects on plant development due to the different soil treatments could be observed, although the dry matter yield of roots and stalks of SS 100-treated plants was significantly reduced. Significantly increased Cu concentrations of up to 60 mg Cu kg −1 DM in the roots of young SS 100-grown maize plants and of up to 20 mg Cu kg −1 DM in the upper leaves at silage stage were found. No critical Cu amounts were reached in the grains until harvest.

Responses of Stomatal Conductance in Field-grown Maize Leaves to Certain Environmental Factors over a Long Term.

Using measurements made under varying environmental conditions for a long period (1992-1994) in a maize field, we examined the characteristics of stomatal response to environmental variables. We found that, the characteristics could not be specified by a single relationship between the stomatal conductance and environmental variables. If the nature of the response was examined by dividing into a long-term variation and a diurnal variation, and by separating into two processes of both stomata opening and closing, the characteristics in each process could be clarified.The response functions and the relative importance of each environmental factor to the stomatal conductance were different for long-term and diurnal periods. For any particular day, the stomatal conductance was influenced by both the potential stomatal conductance (PSC) and the relative degree of stomatal opening (RDO), in which PSC was determined by the mean of environmental variables and RDO depended on the relative variation of environmental factors in daytime.In the processes of stomatal opening and closing, the curves of stomatal response to photosynthetic photon flux density and to leaf water potential showed an identical tendency, but stomatal response to saturation deficit and to leaf temperature in the two processes differed remarkably and showed a hysteresis phenomenon.

Genotypic differences for temperature response of leaf appearance rate and leaf elongation rate in field-grown maize

La mise en place du systeme foliaire est le principal facteur limitant le rendement dans les regions caracterisees par un printemps long et frais telles que l'Europe du Nord. L'objectif de ce travail etait d'etudier l'influence de la temperature sur le developpement et la croissance foliaires. A la difference de la plupart des etudes anterieures, il a ete conduit en conditions de plein champ et l'accent a ete mis sur la variabilite genetique pour differents caracteres de tolerance au froid. Douze genotypes de mais ont ete semes dans le nord de la France le 4 avril 1991 (semis precoce) et le 10 mai 1991 (semis normal a tardif). Le rythme d'apparition des jeunes feuilles et la vitesse d'allongement foliaire (feuilles de rang 6, 9 et 12) ont ete estimes sur des periodes successives de 3 ou 4 j. Pour les genotypes etudies, le rythme moyen d'apparition des feuilles variait de 0,17 a 0,22 feuille par jour. La vitesse moyenne d'allongement foliaire variait de 10 a 21 mm par jour pour la feuille 6 et de 29 a 58 mm par jour pour la feuille 9. Le rythme d'apparition des feuilles etait lie de facon lineaire a la temperature moyenne du sol (R 2 = 0,47). Les vitesses moyennes d'allongement etaient presque exclusivement determinees par la temperature moyenne du sol (R 2 > 0,85). Parmi les genotypes etudies, la temperature seuil variait de 7,1 a 12,6°C pour le rythme d'apparition des feuilles; de 8,3 a 12,8°C pour la vitesse d'allongement de la 6 e feuille; de 7,1 a 13,5°C pour la vitesse d'allongement de la 9 e feuille. Il a ete possible de definir pour chaque genotype un indice de tolerance aux variations de temperature. De tels resultats devraient permettre au selectionneur d'obtenir des genotypes adaptes aux conditions de printemps frais et pourraient etre integres dans des modeles de simulation specifiques du genotype

Determining Leaf Toughness and Its Role in Maize Resistance to the European Com Borer (Lepidoptera: Pyralidae)

An instron technique was peveloped for determining leaf toughness of maize ( Zea mays L.) and its relation to maize resistance to the European corn borer, Ostrinia nubilalis (Hubner). Leaf tissue of field-grown maize was mounted onto a stainless steel plate, and the force required to puncture the leaf was determined using an instron. A rounded cylindrical probe attached to the instron gave consistent leaf-toughness readings. Significant negative correlations were found between leaf toughness and leaf-damage ratings taken at midwhorl and plant tasseling for five different groups of maize germplasm that included multiple borer-resistant cultivars and inbreds, Mexican landraces, BS9 selection cycles, and North American inbreds. Leaf toughness appears to be an important defense mechanism in maize across diverse groups of germplasm. Instron measurements of leaf toughness can be used to monitor leaf toughness in resistance breeding programs and as a tool to better understand phytochemical contributions to leaf toughness and behavioral ecology of lepidopteran folivores feeding on maize.