R. O. Akinwale

Genetic gains in grain yield under nitrogen stress following three decades of breeding for drought tolerance and Striga resistance in early maturing maize

Breeding for resistance to Striga hermonthica Del. (Benth) and tolerance to drought has been a major strategy to improve maize ( Zea mays L.) production and productivity in West and Central Africa during the last three decades. The three decades consisted of three breeding periods or eras based on the germplasm and methodologies used; that is, 1988–2000, 2001–06 and 2007–11. A total of 50 early maturing cultivars, combining Striga resistance with drought tolerance were developed, including 15, 16 and 19 cultivars for the three periods, respectively. Although the cultivars were not selected intentionally for low-nitrogen (N) tolerance, it was hypothesized that tolerance to low-N had been significantly improved while selecting for drought tolerance and Striga resistance. This hypothesis was tested by evaluating the 50 cultivars in 2010 and 2011 in Nigeria at Mokwa and Ile-Ife under both low-N (30 kg N/ha) and high-N (90 kg N/ha) levels. Under low-N conditions, grain yield improved from 2280 kg/ha during the first period to 2610 kg/ha during the third period, an increase of 165 kg/ha per period with r 2 of 0·99. Under high-N, yield increased from 3200 to 3650 kg/ha, an increase of 225 kg/ha and r 2 of 0·93. Relative gain per period was 30 kg/ha for the two N rates with r 2 values of 0·99 and 0·94 respectively. Grain yield performance of the 50 cultivars under low-N conditions adequately predicted their performance under high-N. Selection for Striga resistance and drought tolerance in early maturing maize populations enhanced low-N tolerance in the maize cultivars derived from the populations. The improvement was higher in later than earlier breeding periods.

Grain Iron and Zinc Concentrations and their Relationship with Selected Agronomic Traits in Early and Extra-Early Maize

ABSTRACT Improvement of iron (Fe) and zinc (Zn) concentrations in maize (Zea mays L.) is of prime importance in combating malnutrition and food insecurity in sub-Saharan Africa. The choice of suitable base population is crucial in making satisfactory progress from selection for a target trait. Twenty-four open-pollinated maize cultivars were evaluated for certain agronomic characters and for Fe and Zn concentrations in maize kernels to identify those with improved Fe and Zn concentrations. Relationship of Fe and Zn with agronomic traits was also studied. Data were subjected to analysis of variance and correlation analysis. Cultivar 2009TZEE-OR1STR had the highest zinc (5.29 mg/g) and iron content (11.42 mg/g) in grain. Grain yield had no relationship with Fe and Zn concentrations, suggesting that yield and micronutrient concentrations were not controlled by the same genetic system/mechanism. There was a significant positive correlation (r = 0.80**; r2 = 64%) between Fe and Zn concentrations, which indicated that the two micronutrients can be improved simultaneously in maize. Maturity significantly affected relationship of Fe and Zn concentrations with the agronomic traits. There was a significant correlation (r = 0.88**) between Fe and Zn only for extra-early germplasm and a significant correlation between Fe concentration and grain yield (r = 0.63**) only for early-maturity cultivars. In conclusion, 2009TZEE-OR1STR was the best cultivar in Fe and Zn concentrations as well as for grain yield and other agronomic traits and it could be promoted for further genetic improvement.

Genetic analysis of drought-adaptive traits at seedling stage in early-maturing maize inbred lines and field performance under stress conditions

Maize hybrids that are tolerant to drought at the seedling stage are needed to boost productivity in the rainforest agro-ecology of West Africa. Genetics of tolerance of maize seedling to drought stress is not well understood and is poorly documented. The objectives of this study were to screen early-maturing maize lines for seedling drought tolerance, determine the inheritance and the combining ability of selected inbred lines, and evaluate the performance of seedling drought-tolerant hybrids under field conditions. Forty-nine early maize lines were screened for drought tolerance at the seedling stage. Ten drought-tolerant and two susceptible inbred lines were selected and used in diallel crosses to generate 66 hybrids. The twelve inbred lines and their hybrids were evaluated under induced drought at seedling stage in the screen house and under marginal growing conditions on the field for two seasons. Data collected were subjected to analysis of variance using the DIALLEL-SAS program. Mean squares for both GCA and SCA were significant for most traits in all research environments, indicating that additive and non-additive gene actions are controlling seedling traits under stress conditions. However, for most traits, SCA was preponderant over GCA in all environments, indicating overdominating effect of non-additive gene action. Which in turn implied that the best improvement method for the traits is hybridization. Inbred TZEI 7 had the best GCA effect for seedling traits under screenhouse conditions and for grain yield and other agronomic traits under drought conditions in the field. Hybrids TZEI 357 × TZEI 411 and TZEI 380 × TZEI 410 showed superior SCA effects under screen house conditions. In conclusion, the study established wide genetic variability for drought tolerance at seedling stage among tropical early-maturing maize germplasm however, the non-additive gene action was more important for most seedling traits.