Abstract:
Recurrent drought and parasitism by Striga hermonthica Del. Benth constitute the two most
important stresses limiting maize (Zea mays L.) production and productivity in sub-Saharan
Africa (SSA). Yield losses can reach up to 85% when the two stresses occur simultaneously in
the field. The use of resistant varieties is more sustainable, economical, and efficient for African
farmers. Several early (90-95 days to maturity) Striga resistant maize hybrids have been
commercialized in the sub-region. However, the levels of resistance are not as high as desired as
they still support Striga emergence thus increasing the Striga seed bank in the soil each season.
The International Institute of Tropical Agriculture (IITA) has developed new early maturing
maize inbreds containing novel Striga resistance genes from Zea diploperennis. Knowledge and
understanding of the mode of gene action conferring Striga resistance and drought tolerance in
these new early maturing maize inbreds would be invaluable in developing hybrids adapted to
both stresses in the sub-region. The objectives of this study were to: (i) determine the genetic
diversity and reaction of these early maturing maize inbred lines under Striga infestation and
drought environments, (ii) determine the mode of inheritance of Striga resistance in an early
maturing inbred line containing resistance genes from Zea diploperennis, (iii) determine the
combining abilities for grain yield and other agronomic traits and heterotic groups of 30 drought
tolerant, early maturing inbreds with the Striga resistance genes, (iv) identify high yielding and
stable hybrids under Striga-infested, drought and optimal growing conditions (v) examine the
inter-trait relationship of early maturing maize hybrids under the Striga infestation and drought.
Genetic diversity among 36 early maturing inbred lines was assessed using 8145 SNP markers.
The cluster analysis and population structure analysis separated the inbred lines into four distinct
groups based on their genetic distance indicating high level of genetic variability among the lines. Using the base indices for selection, 22% of the inbred lines combined resistance to Striga
and tolerance to drought. Generation mean analysis was used to study the inheritance of
resistance to Striga in the early maturing maize inbred line, TZdEI 352, containing genes from
Zea diploperennis to facilitate its effective use in resistance breeding programmes in SSA. Only
models that incorporated epistasis in addition to additive and dominance gene effects were
adequate in explaining variation in the six generations studied. Epistasis played an important role
in Striga resistance genes from Zea diploperennis in tropical maize. One hundred and fifty
hybrids derived from crosses involving the 30 inbreds utilizing North Carolina Design II plus six
hybrid checks were evaluated under artificial Striga infestation at Mokwa and Abuja, drought at
Ikenne, Bagauda, Minjibir and optimal environments at Ikenne, Mokwa and Abuja, in 2013 and
2015. Significant GCA and SCA effects for grain yield and most measured traits were detected
under the three research conditions. The higher values of GCA over SCA obtained for grain
yield, flowering traits, plant and ear heights, husk cover, Striga damage and number of emerged
Striga plants at 8 and 10 weeks after planting under Striga infested and optimal environments,
indicated that they were controlled by additive gene action. The non-additive gene action was
more important than the additive gene action for days to silking, anthesis-silking interval, ear
height, stalk and root lodging, ears per plants, ear and plant aspects while additive gene action
was more important for grain yield, plant height, husk cover, and stay green characteristics under
drought environments. There were no maternal effects in the expression of the traits either under
Striga infestation, drought or optimal growing environments. Inbreds TZdEI 268, TZdEI 352 and
TZdEI 173 had superior positive GCA-male and GCA-female effects for grain yield and
negative GCA-male and GCA-female effects for Striga damage and number of emerged Striga
plants under Striga infestation indicating that they contributed to higher grain yield in their
University of Ghana http://ugspace.ug.edu.gh
iv
hybrids and could be used to improve tropical germplasm for Striga resistance. The lines TZdEI
492 and TZdEI 378 with outstanding positive GCA effects for grain yield under drought
environments could be used to improve tropical germplasm for drought tolerance. The inbred
lines were classified into four heterotic groups across the research environments using GCA
effects of multiple traits. The inbred lines classified into each heterotic group may be recombined
to form populations that could be improved through recurrent selection. Grain yield ranged from
1134 kg ha-1 for TZEI 26 x TZEI 5 to 5362 kg ha-1 for TZdEI 173 x TZdEI 280 under Striga
infestation, 579 kg ha-1 for TZdEI 314 x TZdEI 378 to 3601 kg ha-1 for TZdEI 479 x TZdEI 260
under drought and 2376 kg ha-1 for TZdEI 82 x TZdEI 71 to 7769 kg ha-1 for TZdEI 260 x
TZdEI 396 under optimal conditions. The additive main effects and multiplicative interaction
analysis identified TZdEI 173 x TZdEI 280, TZdEI 173 x TZdEI 492, TZdEI 441 x TZdEI 260,
TZdEI 82 x TZdEI 260, TZdEI 71 x TZdEI 396, TZdEI 396 x TZdEI 131, TZdEI 396 x TZdEI
264, TZdEI 98 x TZdEI 352, TZdEI 157 x TZdEI 352, TZEI 18 x TZdEI 357, TZdEI 268 x
TZdEI 378, TZdEI 157 x TZdEI 280, TZdEI 492 x TZdEI 441 and TZEI 60 x TZEI 5 as the
highest yielding and stable hybrids with combined Striga resistance and drought tolerance genes.
Also, they had reduced Striga emergence and host plant damage. These hybrids should be tested
in multi-location and on-farm trials to confirm the consistency in performance and promoted for
release and commercialization in the Striga endemic areas with short duration of rainfall in West
and Central Africa to contribute to increased maize productivity, poverty alleviation and reduced
Striga seed bank in the soil. Striga resistant and drought tolerant hybrids with outstanding
performance across stress environments could be obtained through accumulation of favorable
alleles for stress tolerance in parental lines. Ear aspect was identified as the most reliable
secondary trait for indirect selection for grain yield under both Striga-infested and drought.