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Selection against pink stem borer (Sesamia nonagrioidesLef) (PSB) attack was performed in the maize (Zea maysL.) synthetic EPS12. The direct response of the EPS12 population to three cycles of selection to reduce tunnel length damage by PSB while maintaining yield was evaluated. S0(cycles of selection) and S1(selfed cycles of selection) generations and testcrosses to three testers were evaluated under artificial infestation with two corn borers, PSB and European corn borer (Ostrinia nubilalisHubner) (ECB). Genotypes and the genotype × environment interaction were significant for tunnel length and yield. Differences among cycles of selection were significant for tunnel length, and the linear decrease for this trait (−1.80 cm cycle−1) achieved during selection accounted for most of these differences. Yield was not significantly reduced with selection. Inbreeding for yield significantly increased due to selection. EPS12 crossed to EP42 showed a significant increase in yield with selection, while crosses to other testers showed a nonsignificant reduction in yield. Although crosses to EP42 were not significantly different for tunnel length, the high heterosis between EPS12 and EP42 and the increased yield of EPS12 × EP42 during selection suggest that inbred lines developed from advanced cycles of EPS12 could be crossed to EP42 to generate promising hybrids. In general, resistance to PSB and ECB was improved, while yield was maintained, inbreeding was increased, and yield of the cross EP42 × EPS12 was improved.

Early planting can contribute to increased grain yield of maize (Zea maysL.), but it requires cold tolerance. A limited number of cold‐tolerant maize genotypes have been reported. The objectives of this study were to test a new strategy to improve cold tolerance in maize searching for broad × narrow genetic combinations that may be useful as base populations for breeding programs, to compare genotype performance under cold‐controlled and field conditions, and to establish the major genetic effects involved in crosses between cold‐tolerant inbred lines and populations. Nine cold‐tolerant populations were crossed to five inbred lines and evaluated in a cold chamber and in the field. Most inbred line × population crosses performed better than populations per se or hybrids used as checks, both in the cold chamber and in the field, suggesting that broad × narrow genetic combination could be a suitable start point for further breeding programs for cold tolerance. The crosses between the inbred line EP80 and northwestern Spanish populations are the most promising base germplasm. In particular, EP80 × Puenteareas showed the greatest yield and good performance at the first stages of development under cold conditions. In addition, EP80 and Puenteareas showed favorable general combining ability for most traits. Early vigor rating would be the most suitable trait to select maize genotypes with superior cold tolerance during emergence and postemergence stages, because it was the only trait for which differences among genotypes were observed in both the cold chamber and the field. Although evaluation under controlled conditions is essential to test cold tolerance, field evaluations are complementary because no association was found between traits evaluated in both conditions.

AbstractThe pink stem borer (PSB) (Sesamia nonagrioidesLef?bvre, 1827; Lepidoptera: Noctuidae) is the main insect pest of maize (Zea maysL., Poaceae) in the Mediterranean area. Maize varieties partially resistant to PSB have been identified, but few studies have documented the mechanisms of resistance involved. The objectives of this research were to study the levels of leaf and sheath antibiosis of a maize population, EPS12, and determine whether antibiosis improved over the selection process for less stalk tunneling. A hybrid check and two Bacillus thuringiensis(Bt) hybrids were also examined. Several bioassays were carried out in a control chamber to evaluate antibiosis of maize leaves and sheaths against PSB larvae. Leaf antibiosis was significantly higher for EPS12 than for the hybrid check and sheath antibiosis was low for all non-Bt hybrids studied. We conclude that leaf and sheath antibiosis was not significantly improved during the selection process. Future efforts should look for other possible mechanisms of resistance, such as stem rind antibiosis.

Various pests, such as those in the order Lepidoptera, frequently feed on young maize (Zea mays) plants and pose a significant threat to plant development and survival. To manage this problem, maize generates a wide variety of responses to attack by pests, from activation of wound-response pathways to the release of volatile compounds. Mp708, an inbred line resistant to feeding by the larvae of the fall armyworm (Spodoptera frugiperda J.E. Smith Lepidoptera: Noctuidae), has been developed through traditional breeding methods, but its underlying mechanisms of resistance are still not completely understood. Mp708 has been shown to have a moderately high constitutive expression of jasmonic acid (JA) before infestation by fall armyworm. However, Tx601, a genotype susceptible to feeding by fall armyworm, activates JA pathway only in response to feeding, suggesting that Mp708 is “primed” to respond swiftly to an attack. Current research indicates that fall armyworm show a lack of preference to feeding on Mp708, leading to the hypothesis that volatiles constitutively released by the plant may also play an important role in its resistance. Analysis of volatiles released by Mp708 and Tx601 in the presence and absence of fall armyworm larvae identified (E)-β-caryophyllene, a terpenoid associated with resistance, released constitutively in Mp708. Fall armyworm fed samples of both Mp708 and Tx601 showed high transcript number of tps23, the gene responsible for the synthesis of (E)-β-caryophyllene. In addition, fall armyworm larvae show a preference for Tx601 whorl tissue over Mp708 tissue, and the dosage of Tx601 whorl with (E)-β-caryophyllene repels the fall armyworm.