The infection of soybean roots by the soil bacterium Bradyrhizobium japonicum results in the formation of specialized organs on the roots called nodules. Within the nodules, a differentiated form of the bacterium (the bacteroid) reduces atmospheric nitrogen to ammonia. Nodule development involves a complex interaction requiring the coordinated expression of nodule-specific bacterial (bacteroidin) and nodule-specific plant (nodulin) genes. A number of nodulin gene products from soybean have been characterized, but the functional roles of only a few have been determined. This thesis describes the isolation and characterization of four nodule-specific cDNA clones (and a leghemoglobin clone) from a cDNA library that was derived from soybean nodule poly (A)$\sp+$ enriched RNA. mRNA corresponding to these cDNA clones, could be detected in RNA isolated from the soybean nodule; but could not be detected in RNA isolated from the root. The pattern of expression of the genes corresponding to the cDNA clones was investigated at various times following infection of the soybean Glycine max (L.) Merrill by Bradyrhizobium japonicum. It appears that these nodulin mRNAs are expressed just prior to the onset of nitrogen fixation and accumulate to their maximum levels shortly thereafter. Sequence characterization of the four clones indicated that they represented members of a previously described nodulin multi-gene family: the "Nodulin A" family (Verma and Delauney, 1988). The "Nodulin-A" family represents a group of abundantly transcribed nodulins, whose function is unknown. Three of these clones, 9-11-B, 36-1-A, and 6-9-F, support the previously published sequences of nodulin-23 (Mauro et al., 1985; Sengupta-Gopalan et al., 1986), nodulin-22 (Sandal et al., 1987), and nodulin-44 (Sengupta-Gopalan et al., 1986) respectively. A fourth clone, 15-9-A, represents a new member of this multi-gene family. Northern blot hybridization with oligo-nucleotide probes specific to this cDNA clone, indicates that 15-9-A represents an abundantly transcribed 1 kb mRNA. Our analyses suggest that 15-9-A is a member of a subfamily of three closely related genes within the larger "Nodulin-A" family. 15-9-A is related to the nodulin-20 sequence of Sandal et al. (1987), which our analyses show is also a member of this subfamily. The 15-9-A cDNA is 100% identical to nodulin-20 in the region that is 5$\sp\prime$ to the position of the nodulin-20 intron, and 89% similar in the region that is 3$\sp\prime$ to the intron. Although the region of greatest divergence between the two sequences is in the 3$\sp\prime$ coding region, each sequence codes for a proline-rich carboxy terminus. In addition, we have determined that a 0.8 kb mRNA that cross-hybridizes to 15-9-A represents a third gene within this subfamily. We have also detected the presence of "Nodulin-A"-like sequences in the DNA of alfalfa, several actinorhizal plants, spinach, and corn. This raises the possibility that "Nodulin-A" like sequences have a functional role in plants other than soybean. We have isolated four alder genomic clones which cross hybridize to the cDNA clone 9-11-B, as an approach to characterizing a "Nodulin-A" sequence from another plant. Further characterization of these clones should determine their relatedness to the soybean sequences and may provide clues as to their possible functional significance.

Estimates of genetic diversity present in gene pools have important implications for breeders and germplasm curators. They constitute the raw material for plant improvement and can provide protection against genetic vulnerability to biotic and abiotic stresses. The purpose of this thesis was to derive Pedigree and Amplified Restricted Fragment Polymorphism (AFLP)-based Genetic Diversity Estimates (GDEPED and GDEAFLP, respectively) among all currently registered 13 Canadian durum wheat cultivars in order to test the hypothesis that the actual level of genetic variation at the DNA level is lower than what is measured from pedigree data. Two objectives were set in this thesis. The first objective was to estimate the genetic diversity level among cultivars using two independent methods namely AFLP and pedigree. The second objective of this thesis was to use cultivar-specific AFLP markers for identification of cultivars and transform these markers into sequence tagged site (STS) markers for routine identification of durum wheat cultivars. (Abstract shortened by UMI.)

The Senegal River Basin (SRB), located in the Sahel region of West Africa is undergoing fundamental environmental, hydrologic and socio-economic transitions simultaneously. The tri-nation (Senegal, Mauritania and Mali) river basin development authority, the Organisation pour la Mise en Valeur du Fleuve Senegal (OMVS) is attempting to execute a shift to irrigated rice production in the river basin for domestic consumption, to ease the severe foreign exchange shortfalls these riparian nations face. Compounding the severe effects of the drought on the river basin ecology is the negative impact of the state imposed agricultural policy of rice production. Rice production in the arid river valley has been a financial and social failure. This study postulates an alternative utilization of the scarce water resources in the basin. The water demand pattern for an alternative natural resources management focused agricultural development policy is based on the irrigation water requirements of well-researched village-scale irrigation projects in the SRB, and intensive irrigated agro-forestry projects. The agro-forestry production system analyzed has the joint objectives of using irrigation to re-establish a protective, diverse and productive bio-mass cover in the desertifying river valley, and to reverse the tide of drought induced migration from rural to urban areas. A comparative river system simulation study was conducted to analyze the effects of both the rice production development policy (policy RP) and the natural resources management policy (policy NRM), on the full agricultural development potential of the SRB. Alternative hydrologic scenarios were generated for the simulation study according to the Senegal River time series analysis, for the historical level, 1970s level drought and 1980s level drought. For all hydrologic scenarios the lower over-all demand pattern exerted by policy NRM allowed a higher full development potential than for policy RP. (Abstract shortened by UMI.)

By means of ANOVA, Duncan's test Student's t test and linear regressions, it was demonstrated that there was no significant correlation between the quantity of gramine and the morphological characters considered (geographic origin, spike row number, spike form, cultivation season, glume length, yield, water content of fresh leaves, seed mean mass, and seed colour). By "Duncan's multiple range test" it was shown that there was a significant (F = 26,81; P 0.0001) variation in gramine content between the different genotypes (minimum = 0 & maximum = 2674). However, canonical discriminant analyses and "cluster"ing by the MODECLUS procedure failed to provide justification for grouping of varieties susceptible to aphid attack, leading to the conclusion that gramine content cannot be sufficiently closely related to morphological characters to serve as a basis for taxonomy. Finally, additional experimentation revealed that the presence of gramine alone is often not sufficient to confer protection against aphid attack, and other factors must be considered, such as anatomy or the existence of other protective constituents. Such research could lead to the development of new resistant biotypes. (Abstract shortened by UMI.)

Of 40 identified phytoseiid mites which occur in Burundi, only Eseius fustis was regularly observed on cassava, whereas E. talinga and E. bwende were rarely present. Iphiseius degenerans was also observed but not as frequently as E. fustis. Predatory insects (Holobus fageli, H. pallidicornis, Stethorus aethiops, S. fujunus and some families like Chrysopidae, hemerobiidae and Anhocoridae) were recorded on cassava during CGM outbreaks. Of two predatory phytoseiids introduced (Typhlodromalus limonicus and Neoseiulus idaeus) and released twice during the study period on two varieties of cassava (Nakarasi and Mpambayabashengera), only one was recovered in the rainy season when the CGM population is normally low. This may indicate its potential use as a biological control agent. Mean mite densities, leaf damage, pubescence and leaf cyanide content of twenty cassava varieties were evaluated for resistance. Pubescence was negatively correlated with leaf damage (r = $-$0,46; p 0.05) whereas no relation was found between cyanide and leaf damage. (Abstract shortened by UMI.)

The structure of the DNA-protein complex in the insecticidal crystals produced by Bacillus thuringiensis var. kurstaki HD-73 was investigated. The role of DNA in the activation mechanism of the crystals was determined by monitoring the fate of the DNA and protein during the activation process with gut juice enzymes from spruce budworm, Choristeneura fumiferana. The results demonstrate that the DNA is co-processed with the protein, giving rise to novel DNA-protein complexes as intermediates. The role of the DNA appears to be mainly structural, providing a framework for the sequestering of the protein into the crystal. Based on the results obtained, a structural model for the arrangement of the DNA-protein complex is proposed. An activation model is also proposed, which accounts for the processing of the DNA and protein components of the crystals. The model offers explanations for the unusual mechanism of proteolytic activation of the Bt crystal protein. The sunlight inactivation (photo-inactivation) of Bt crystals was studied by UV-irradiation studies. The results of these studies suggest that the photo-inactivation is due to the formation of covalent linkages between the DNA and crystal protein. UV irradiation at 302 nm for 8 hours was sufficient to cause the total loss of biological activity of the crystals. Incorporation of water-soluble antioxidants into crystal preparations inhibits the photo-inactivation reaction. Ascorbic acid was found to be an effective antioxidant and inhibited the photo-inactivation of Bt crystals. In the presence of ascorbic acid, the toxicity of the irradiated crystals did not decrease significantly under the same conditions and the retention of toxicity is estimated to be approximately one order of magnitude longer than unprotected crystals. The inactivation of crystals was inducible in the presence of a free-radical generator, suggesting that a free-radical mechanism is responsible for the photo-inactivation of the crystals. A highly sensitive method has been developed for the determination of the C-terminal sequence of proteins. The C-terminal sequence of chymotrypsin was determined using this methodology. C-terminal peptides from ribonuclease and from the crystal protein from Bacillus thuringiensis var. kurstaki HD-73 were also isolated using this procedure.

This study examines the genetics and genetic variation of maize resistance to Diabrotica virgifera virgifera LeConte (rootworm). Rootworm resistance is a complex, multi-trait characteristic with inheritance mechanisms not fully understood or explained. In order to better understand the mechanisms of rootworm resistance and the genetic parameters that condition this trait, three studies were undertaken. The first study examined the genetic variation of components of resistance in twenty-three DEKALB Genetics Corporation hybrids. They were assessed for hydroxamic acid content under uniform greenhouse conditions and compared with field assessments over two growing seasons for resistance to the Western Corn Rootworm in terms of root pull resistance, root damage ratings and root lodging. Hydroxamic acid levels of all hybrids were found to be moderately low resulting in low antibiosis. It appears that rootworm resistance in these hybrids is achieved mainly through tolerance, not antibiosis. The study suggests an opportunity for improvement of resistance in commercial inbreds through selection for antibiosis and hydroxamic acid content. In the second study, the combining ability, additive and dominance effects and the heritability of several traits important to rootworm resistance in maize were determined. GCA and SCA effects were found to be significant in the inheritance of hydroxamate content, root weight and plant height. GCA effects were found to be significant in relation to low root damage ratings. Heritability in a narrow sense as determined by the midparent/offspring regression was found to be 0.34 for hydroxamate content, 0.26 for root damage ratings and 0.35 for plant height. Additive genetic effects were found to be significant in the inheritance of root damage ratings, both additive and dominance effects contribute to the inheritance of plant height and dominance effects were found to be most important in the expression of root weight. In the third study, two inbred lines were used to develop an F2 :F3 population to further investigate the genetic parameters governing some of the traits important to rootworm resistance. The inbred lines used were 93n445 (susceptible) and 118.31 (resistant). Dominance effects were found to be significant in the inheritance of plant height, additive effects were found to be significant in the inheritance of root size and additive and dominance effects were found to be significant in the inheritance of root damage ratings. Heritability in a narrow sense was found to be 54% for root size, 55% for root damage ratings, 84.7% for stalk circumference and 89% for plant height for the inbreds used in this study. (Abstract shortened by UMI.)

Many studies have shown that mycorrhizal symbiosis can induce synthesis of phenolic compounds associated with plant protection against environmental stress factors. This protection could be beneficial for the plant, especially at early stages of mycorrhizal colonization. Therefore, it is hypothesized that mycorrhizal symbiosis induces changes in phenolic synthesis in the early stages of corn growth. Phenolic synthesis was studied by analyzing the activity of phenylalanine ammonia-lyase and peroxidase and determining the concentration of phenolic compounds by spectrophotometry and HPLC. Two corn cultivars, Pioneer 3737 and Pride K127, were grown in the greenhouse, colonized or not with Glomus intraradices Schenck and Smith (Premier Tech). Plants were harvested from the second to the tenth week of growth. Mycorrhizal colonization was around 20% for both maize cultivars Mycorrhizae tend to enhance PAL activity in corn roots at the fifth week. Peroxidase activity was significantly higher in mycorrhizal plants for all weeks studied. The concentrations of proteins and soluble phenolics were influenced by cultivars and mycorrhizae at certain weeks. Hydroxamic acid concentration was not different in mycorrhizal and non-mycorrhizal corn roots of Pioneer 3737. The low root colonization might have influenced the lack of differences between mycorrhizal and control plants. Mycorrhizal symbiosis seemed to induce an enzymatic defense response in maize at early stages of growth and colonization.

This thesis examines fusarium ear rot epidemiology in maize (Zea mays L.) and host plant resistance to Fusarium species causing ear rot, in eastern Canada, through three major groups of experiments. A three-year disease survey that determines the causal agents of maize ear rot in Ontario; a three-year study on the stability of different genotypes to two different inoculation techniques across different environments; and a two-year study on fungal infection from an inoculum source using methods to detect infection spread. A survey of maize ear rot was conducted in eastern and western Ontario from 1991-1993 in order to develop a predictive model of ear rot incidence. Field data were collected on disease incidence, insect and/or bird damage, ear tip infection (silk), rainfall, and temperature. Isolations were made in the laboratory from samples showing evidence of infection, and the five prevalent Fusarium species (F. graminearum, F. moniliforme, F. proliferatum, F. sporotrichioides, and F. subglutinans) were enumerated. The profile of Fusarium species isolated varied considerably from year to year, and in a certain proportion between sites. Fusarium subglutinans was the most frequently isolated species and was associated with low rainfall in July. Fusarium moniliforme and F. proliferatum occurred mostly following the hot, dry summer of 1991. Fusarium graminearum presence increased significantly with higher precipitation in July. Presence of F. sporotrichioides was not associated directly with the incidence of ear rot. Insect damage and infection through the ear tip were the most important mode of fungal entry into maize ears. In the second major field study undertaken in east-central Ontario and south-western Quebec, the interaction between genotypes and environments was described using a Joint Regression Model and an Additive Main effects and Multiplicative Interaction effects (AMMI) model. The AMMI model was able to capture 68% of the interaction across both inoculation methods, compared to 84% for the Joint regression model. The number of cool and wet days (relative humidity over 80%) observed in August was more critical for silk channel inoculation infections, and the number of wet days was more critical to achieve severe ear rot with the kernel-stab inoculation technique. A disease spread study showed that the Power Law model is a suitable model to predict fusarium ear rot spread over a distance below five metres from a small inoculum source of F. graminearum (strain sc DAOM 180378). (Abstract shortened by UMI.)

This study focuses on switchgrass, Panicum virgatum L., a North American tallgrass prairie species that has been used in agriculture for only 50 years. We hypothesized that wild variants of this species are more mycorrhizal dependent than the cultivars due to selection against mycorrhizae in agricultural conditions. Mycorrhizae, naturally occurring associations between roots and symbiotic fungi, are known to benefit plants in most environments. These associations have been proposed as a means for crop improvement in agriculture. However, current agricultural practices tend to diminish the potential for crops to benefit from mycorrhizae. Two greenhouse experiments with a factorial design were performed using switchgrass inoculated or not with Glomus intraradices Schenck and Smith. In the first experiment, four wild varieties (PH, ONP, Ojibway and Pterophylla) and six cultivars (Forestburg, Summer, Shelter, Caddo, NU and Trailblazer) were grown for 12 weeks. From those, 3 wild types (PH, ONP and Ojibway) and 3 cultivars (Forestburg, Caddo and NU) were chosen for the second experiment. Switchgrass plants were analyzed for various physiological, root and mineral parameters in order to assess their overall mycorrhizal dependency (MD). (Abstract shortened by UMI.)