Combining ability and genotype by environment interaction of selected maize inbred lines for performance under low nitrogen and drought stress.
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Harsh drought and infertile soils have increasingly become important abiotic stresses affecting maize production in Sub-Saharan Africa. Varieties with improved water use efficiency and/or N would be beneficial for subsistence farmers. Unfortunately, few breeding Programs focus attention on breeding for low N and drought stress conditions. CIMMYT Breeding Programs in Sub-Saharan Africa in the recent past has devoted their effort to breed for low N and drought tolerance and several germplasm including inbred lines and populations have been developed and availed to the National Programs and some of the elite inbred lines have now been acquired by the Maize Program at the National Agricultural Crop Resources Research Institute (NaCRRI) in Uganda. The main objective of this study was to assess performance of selected group of CIMMYT maize inbred lines to achieve superior hybrids with tolerance to drought and low soil nitrogen stress. The specific objectives were to determine: (1) the combining ability (GCA and SCA) of selected maize inbred lines for performance under drought stress and low nitrogen, including the interaction of GCA and SCA with environments, (2) the effects of low-N and drought stress on yield of single crosses of selected inbred lines, (3) the relationship of grain yield with other traits that might be used for indirect selection under stress, and finally (4) the patterns of G x E interactions of hybrids. Twelve parents were crossed in a half-diallel mating design. The resultant F1 hybrids were evaluated from 2008B-2009B in four managed environments (optimum N and low-N, well-watered and managed drought-stress) in two locations, namely, Kenya Agriculture Research Institute (KARI) substation at Kiboko -Kenya and National Crop Resources Research Institute (NaCRRI) Namulonge-Uganda. Data was collected on days to anthesis, days to silking, anthesis–silking interval, ears per plant, leaf senescence and yellowing, and grain yield. Results obtained indicated significant variation among the single-cross hybrids for grain yield and other agronomic traits. Additive genetic effects for grain yield (GY), number of ears per plant (EPP) and anthesis-silking interval (ASI) were generally more important than were non-additive genetic effects. The narrow-sense coefficient of genetic determination (NS-CGD, analogous to heritability, but for a fixed set of parents) was low for grain yield in stress environments, but was high for ASI and leaf senescence. Genotype by environment variance contribution to the total phenotypic variance was much higher than was the genotypic contribution across stress environments (46% vs 3%). The average grain-yield reduction due to stress was 76 % under drought and 50 % under low-N environments. Strong correlations were observed between yield and ASI (-0.67) and EPP (0.80) under drought stress. Some of the traits (ASI, leaf senescence) also exhibited high NS-CGD and, therefore, confirming that they can be used for indirect selection of performance under drought and low-N stress. AMMI and GGE showed that hybrid G8 (CZL 00003 x CML 341) was most high yielding and stable variety. Inbred lines CZL 02004 and CZL 03002 consistently exhibited positive GCA effects for grain yield in all environments. The most beneficial GCA effects for ASI were shown by inbred lines CZL 02004 under managed drought stress and by CML 379 under low-N. Inbred lines with good potential for hybrid production include CZL 02004, CZL 03002, CML 78, and CML 379. These inbred lines had the highest positive GCA effects for grain yield and most beneficial GCA for ASI across stress conditions tested and appear to be suitable for the low soil fertility and drought prone environments. Selection for performance in drought and low-N stresses should focus on high yielding hybrids from parents with desirable GCA values for yield, ASI, and EPP under drought stress.