Peanut seed and pods are susceptible to contamination by aflatoxin (AF), a carcinogenic mycotoxin produced by Aspergillus flavus Links Fr. and A. parasiticus Speare. Efforts to evaluate peanut lines for resistance to AF contamination have been impeded by limitations to the methodologies available for AF detection. AF cannot be seen by visible light and its detection involves grinding seed tissue in organic solvents, separation of phases, and detection by ELISA, high performance liquid chromatography (HPLC) or thin layer chromatography. These methodologies are time-consuming, expensive, labor-intensive, and are uninformative in defining the tissues of the peanut seed and pod that are most frequently contaminated with AF. Aspergillus AF mutants which accumulate norsolorinic acid (NOR), an orange-pigmented AF pathway intermediate, provide an easy and convenient mean to detect AF contamination. A visual rating scheme for NOR contamination of peanut seed was developed that correlated favorably to HPLC detection of both NOR and AF (r = 0.96 and 0.95, respectively). When screening the 38 plant progenies that comprise Tamspan 90 (a spanish cultivar), NOR was first seen in the intercotyledonary cavity and the interfacial surface of cotyledons and testae in seeds examined from infected pods. Immature pods were often heavily contaminated with NOR. Six of the 38 lines accumulated low levels of NOR in two laboratory tests. Additional studies are needed to determine if these results are predictive of aflatoxin contamination under field conditions.
Recently, the peanut ( Arachis hypogaea L.) industry has expressed a greater need for higher percentages of fancy pods and extra large kernels (ELK), especially for use in large-seeded in-shell products. Genetic control of these traits has been reported to range in complexity from simple inheritance to the inclusion of multiple modifier genes. This study was conducted to determine the general combining ability (GCA) effects of 50 peanut genotypes on pod and seed size. Each genotype was used five times as a female parent and five times as a male parent in a partial diallel crossing program. F 1 hybrids were grown and their pods were harvested for measurements of pod and seed size. The F 2 generation was planted the following year and similar measurements were recorded using the single pod descent procedure. Individual F 2 plants were harvested and pod and seed characteristics measured for segregation information within four crosses. General combining ability effects were not well correlated between generations (r=0.53-0.56) or with the same traits measured on pure-line parents (r=0.32-0.42). PI 298845, PI 314897, PI 325079, Jenkins Jumbo, and Fla 393-8-1-1-1-1-1-2 had consistently large positive GCA effects on pod and seed weight. F 2 segregation patterns indicated that some crosses exhibit predominantly additive gene action while one cross (PI 270818 / PI 269111) showed dominance toward smaller pods. Transgressive segregation occurred for pod and seed size traits in four crosses. Substantial genetic variability for pod and seed size remains in the peanut germplasm collection.
Symbiotic nitrogen fixation in peanut ( Arachis hypogaea L.) may be improved by genetically manipulating the host plant. This requires an understanding of the inheritance of the traits involved in nitrogen fixation. The objectives of this study were to determine the inheritance of several N 2 fixation-related traits for two peanut crosses based on Mather and Jink's fixation-related traits for two peanut crosses based on Mather and Jink's additive-dominance model, and to determine if epistasis was important in the inheritance of these traits. A generation means analysis usingparents, reciprocal F 1 s and F 2 s, and two back-cross generations was conducted for both crosses. Plants of different generations were grown in modified Leonard jars in the greenhouse for about 60 days at which time nodule number and dry weight, shoot dry weight, nitrogenase activity, and specific activity were measured. Means of the traits for the generations from both crosses (Robut 33-1 x NC 4 and Robut 33-1 x Argentine) showed significant differences. Reciprocal differences were found for most traits measured in the cross of Robut 33-1 x Argentine, a cross of Virginia x Spanish botanical types. Lack of fit of the additive-dominance model indicated significant epistasis for inheritance of nodule number, nodule weight, top dry weight, and nitrogenase activity in both crosses. Three types of digenic interactions (additive x additive, additive x dominance and dominance x dominance) were found. The presence of nonadditive genetic effects suggests that early generation selection would be ineffective.
The peanut ( Arachis hypogaea L.) generally is considered promiscuous since it forms symbioses with a diverse group of Bradyrhizobium. However, specific cultivar-strain combinations like Robut 33–1 and strain NC92 have resulted in significant yield increases, suggesting that host-strain combinations may be selected for superior nitrogen fixation and yield. The objectives of this study were to measure nitrogen fixation-related traits during the growing season and evaluate the interactions between host peanut genotypes and Bradyrhizobium strains under field conditions in North Carolina. A factorial experiment with four cultivars and five inoculants was conducted in two years at two locations (Clayton and Lewiston) in North Carolina. Traits measured during the growing season were nodule number and weight, root weight (1983 only), shoot weight, pod weight, nitrogenase activity and specific nitrogenase activity. In 1984, fruit yield was measured at harvest. Results indicated that cultivars and strains were different for most traits in 1983 at Clayton but significant host-strain interactions occurred only for nodule weight at 60 days after planting (DAP) and root weight at 132 DAP. In 1984, Clayton results indicated cultivar-strain interactions for all traits at 73 DAP and for several traits at 109 DAP. At Lewiston only cultivar differences were important. The Clayton fields had low populations of native Bradyrhizobium while Lewiston had a high level of the bacteria. Inoculation produced substantial yield increases at Clayton but not at Lewiston. Robut 33–1 inoculated with strain NC92 did not yield more than with other strains. Further study is needed to explain why repeated increases in yield were obtained with Robut 33–1/NC92 in tropical studies but not in North Carolina. The possibility still exists that superior cultivar-strain combinations can be identified.