The majority of resource-poor families in sub-Saharan Africa (SSA) consume conventional maize that were deficient in major nutrients such as provitamin A (PVA) carotenoids and amino acids (lysine and tryptophan) (Oyekale et al. 2021). Such diets are fed to infants (from 2-3 months old) and babies of preschool age without nutritional supplements in several developing countries (Badu-Apraku et al. 2016). This has resulted in millions of deaths due to malnutrition and food insecurity in the region (Badu-Apraku et al. 2020). About 15% and 40% of preg-nant women and children, respectively, are vitamin A carotenoids deficient in SSA (WHO 2009). Nigeria has severe micronutrient deficiencies prevalence because about one in every five infants below five years and childbearing age women of 15 to 49 years are affected (NNHS 2018). Vitamin A controls vital metabolic develop-ments in humans (Oluwaseun et al. 2022). However, Vitamin A deficiency induces night sightlessness in nearly 15% and 40% of expectant mothers and infants, respec-tively, thereby enhancing immune system depression, growth delay, and childhood mortality due to measles, diarrhoea, and malaria (Muthayya et al. 2013).

Tryptophan and lysine amino acids are major protein building blocks needed by monogastric animals and humans (Oyekale et al. 2021). A deficiency of tryptophan resulted in increased pain sensitivity, impaired bone development, a decline in growth rate, lessened food intake, increased aggression, and nervousness (Moehn et al. 2012). Lysine deficiency causes a decrease in growth rate and fatigue among others. Meanwhile, QPM offers more than twice the lysine and tryptophan compared with conventional maize. It has the potential of providing humans with nearly the protein contents of 73% needed, as against 46% of conventional maize (Duo et al. 2021). It also has a bio-logical value of nearly 80% (the magnitude of human protein assimilation) compared to 40-58% conventional maize (Mbuya et al. 2010). Maize biofortification with enhanced levels of PVA and QPM is the most favourable approach for combating malnourishment and health chal-lenges such as kwashiorkor, ocular impairment, under-weight, and diseases vulnerability (Oyekale et al. 2021; Oluwaseun et al. 2022).

Maize streak virus transmitted by leafhopper (Cicadulina spp.) is a major biotic vector of maize in Africa, including Nigeria by reducing the quantity and quality of maize products (Sime et al. 2021). This disease occurred as mixed infections under favourable warmth and humid environ-ments (Akinbode et al. 2014). The incidence, severity, and losses of maize grain yield (about 0-100%) as a result of MSV disease are based on several factors, such as type of genotypes, heritable make-up, level of resistance, growth stage, and time of infections of the plant (Gichuru et al 2011). High MSV disease prevalence has been reported in Nigeria, Tanzania, Kenya, Zambia, Uganda, Mozambique, and South Africa (Gichuru et al. 2011; Bello 2017). Such disease occurrence posed a severe burden of hunger in Africa, especially wherever MSV-resistant cultivars were scarce. In Nigeria, the disease was pandemic in the 1970s in the prone agroecological areas. Meanwhile, the symptoms first occurred as chlorotic bands at the tertiary leaf veins and then protruded to rectangular tan-coloured lesions running parallel to the leaf veins. This thereby resulted in the blight of the entire leaf. The disease also caused a reduction of leaf photosynthesis, effective growth, and grain yield, leading to the premature death of the plant (Karavina 2014).

The development of resistance host-plant maize varieties is reported as an environmentally friendly, consistent, efficient, and viable strategy for governing foliar infections (Muiru et al. 2015). The majority of maize traits are quantitatively inherited; therefore, testcross and diallel cross are prerequisites for improving disease resistance in hybrid development (Bello 2017). These mating methods not only enable appropriate selection for hybrid-variety development, the addition of many entries, heterotic group-ing, and male and female interactions but also aid in choos-ing promising parents and crosses (Akhi et al. 2018). Developing testcross hybrids with at least one non-inbred parent is not only affordable and more comfortable but usually out yielded open-pollinated varieties. The Line by Tester strategy for testcrosses also enables the use of heterosis with comparatively minimal inbreeding depression whenever seeds were recycled.

Breeding of PVA-QPM hybrids with resistance to MSV by incorporating suitable genes from resistant donors requires adequate knowledge of MSV resistance genetics because the major genes conferring resistance varied among maize accessions (Lameck et al. 2017). Maize varieties with these genes need to be used during hybrid improvement to increase MSV resistance. However, this disease is controlled by major dominant and minor additive genetic effects which suggest that there are ample oppor-tunities for selecting general and specific combiners using genetic analysis (Bello 2017). Also, periodic testing of streak resistance levels in newly developed maize cultivars with the artificial infestation of MSV is sacrosanct in retaining recessive traits to enhance stable productivity. However, many maize varieties of varied maturing groups have been developed by Research thrusts in West and Central Africa (WCA) to alleviate the effects of vitamin A and protein malnourishment (Oyekale et al. 2021). Early MSV resistance PVA-QPM hybrids are yet to be developed, released, and commercially in the sub-region. This study aimed at developing early PVA-QPM topcross hybrids of conferred MSV disease resistance with increased grain yield, tryptophan, and carotenoid contents. This is of the view to identifying improved segregants among the maize genotypes either suited for further improvement or direct adoption by farmers in WCA. This could reduce protein and vitamin A malnutrition in the zone.

The trial sites and genetic materials

Five improved PVA-QPM inbred lines (TZEIORQ 13, TZEIORQ 24, TZEIORQ 29, TZEIORQ 59, and TZEQI 82) were obtained at the International Institute of Tropical Agriculture (IITA) Ibadan. Five promising open-pollinated MSV resistance cultivars (Pop 28 SR, Acr. 91 Suwan-1-Sr C1, IK.91 TZL Comp 3-Y C1, Ikenne 88 TZSR-Y-1 and TZB-SR SGY TZB-SR) were also collected from the Insti-tute and used as testers. The cultivars were identified as superior in grain yield, disease resistance, and adaptation to tropical Africa. Two yellow commercial hybrids of maize (Oba Super 2 and Oba Super 4) varietal checks were served as controls (Table 1). All the MSV resistance cultivars and PVA-QPM inbreds were crossed in a half-diallel fashion without reciprocals [n(n-1)/2] (parent numbers). Fifty-seven genotypes consisting of 45 F₁ testcross hybrids, 10 parents, and the two commercial hybrids were assessed in Nigeria, at the Lower Niger River Basin Development Authority, Oke-Oyi (80°30’N and 8°36’E) during 2019 and 2020 rain-fed regimes.

Table 1 . Description of five early maturing PVA-QPM inbreds, five MSV resistance cultivars, and two commercial hybrid checks.

Genotypes	Pedigree
PVA-QPM inbreds
TZEIORQ 13	2009-TZE OR2 DT STR-QPM S6 inb 7-1/3-1/2-2/2-3/3-1/1
TZEIORQ 24	2009-TZE OR2 DT STR-QPM S6 inb 26-1/1-1/2-4/6-2/3-1/1
TZEIORQ 29	2009-TZE OR2 DT STR QPM S6 inb 28-1/1-2/2-1/2-1/2-1/1
TZEIORQ 59	2009-TZE OR2 DT STR QPM S6 inb 50-2/2-1/3-2/3-2/2-1/1
TZEQI 82	TZE-COMP5-Y C6S6 Inb 25 × Pool 18 SR QPM BC1S6 2-3-1-1-6-6
MSV resistance cultivars
AK-9528-DMRSR Pop 28 SR	Pop 28 SR
Acr. 91 Suwan-1-Sr C1	Suwan-1
IK.91 TZL Comp 3-Y C1 TZL Comp 3	TZL Comp 3
Ikenne 88 TZSR-Y-1 TZSR-Y-1	TZSR-Y-1
TZB-SR SGY TZB-SR	TZB-SR
Commercial maize hybrid checks
Oba super 2	Commercial yellow maize hybrid
Oba super 4	Commercial yellow maize hybrid

Cross-pollination procedures

The maize cross-pollination was done by staggering seed sowing twice at seven days intervals. This is to capture the niche of different flowering dates. Before cross-polli-nation commenced, extruded ear shoot and shedding of pollen grains were observed on a day-to-day basis. Before silks protrusion of ears from the plants, cutting of ear shoots was done and enclosed with translucent shoot bags to control unwanted cross-pollination. The shoot bags were then covered carefully with the maize stalks of the testers (female). This was carried out to attain uniform protrusions of silks and prevent dislodging by wind and/or rainfall droplets. To achieve appropriate silks development, appro-priate spacing was allowed for clamping the shoot bags. Subsequently, the tassel bag was fastened with paper clips to the tassels to obtain pure pollen from a specific male inbred. As the silks emerged and were receptive to pollen fertilization, the plants were pollinated utilizing preferred genotype pollens. To pollinate, it is necessary to shake the stapled tassel bags lightly and dispensed the pollens on the silks of the desired ear. After that, they were unclamped and dislodged the grains against the silks. Finally, the shoots were fastened using tassel bags until the period of har-vesting to prevent unwanted pollens. Hybrids maize grains produced were stored distinctly in sacks for advanced evaluation.

Field techniques and management

The trial site was prepared by ploughing, harrowing, and ridging with a planting space of 0.75 m between ridges. Each entry contained a four-row 5 m plot, 50 cm within the row of about 53,333 plants ha^‒1. About 170 g/l Atrazine ha^‒1 and 3 kg/l Metolachlor active ingredients were sprayed as pre-emergence herbicides to control weed infestations after land preparation. The inbred parents, their 45 F₁ hybrids, with the 2 commercial checks were tested on 18th and 12th June 2019 and 2020, respectively. The leafhoppers’ resis-tance s were applied artificially and arranged to exploit the randomized complete block design of 4 replicates. About 3 seeds were planted in each hole and retained 2 at two weeks after sowing (WAS). Complimentary roguing was done at four WAS preceding fertilization. Fertilizers were supplied at a proportion of 40kg of P2O5 ha^‒1 (single superphos-phate), 80kg of N ha^‒1 (urea), and 40kg K2O ha^‒1 (muriate of potash).

Cicadulina mbila inoculation

The leafhoppers (Cicadulina mbila) were trapped when their densities were high after the cessation of the rainfall. The small cubical frame containing steel tightened with the black-green transparent net at a frame side was used to capture the leafhoppers from the infested areas. MSV- infected maize seedlings were given two days acquisition access period to the incubated leafhoppers. Before entering the plant’s leaf whorls, leafhoppers were carbon dioxide- anesthetized and immobilized to prevent them from flying away (Leuschner et al. 1980). Thereafter, they were exposed to a 2-day access period of inoculation between two and three-leaf stages for the virus infection severity to be expressed. Inoculation was done by administering 3 of them on the young leaves in a proximal exterior of each stand. About 2 days of access period of inoculation was applied to them before they were slew using Dimethoate 40 EC sprays. To restrain leafhoppers from virus transmission, Furadan (Carbosulfuran 5% m/v) was sprayed on the plant stands in specific plots. Before physiological maturity, Dimethoate 40EC was supplied at three weeks intervals.

MSV severity scoring

The infection severity was rated at an interval of 8 days from 2 weeks when the plants germinated till the flowering period. This is rated on an average of ten plants at the middle of the plot with an ordinal scale of one to five. The scoring includes 1 showing high resistance of below 10% with slight green streaks on the leaves, 2 = mild symptoms of 10 to 25% comprising shattered slight green with small yellow streaks on the leaves, 3 = reasonable streaking with 26 to 50% yellow-green streaks on the leaves, 4 = susceptibility with stern streaking with 51 to 75% of yellow streaks on the leaves with resultant plant premature death and little cobs, 5 = high susceptibility with 76 to 100% of severe stunting and streaking on the leaves with subsequent plant premature death and very small cobs (Bello 2017).

Grain yield determination

When the maize matured, harvesting and weighing of cobs were carried out per plot independently while the following formula was used to determine grain yield in kilograms per hectare:

Grain yield kg ha−1=100−MC×10,000m2×Fresh weight7.5cm2×100−12.5

The 12.5% reveals grain moisture content standard, 10,000 m2 denotes the plot ha^‒1, 7.5 m² connotes the plot area harvested (5 m × 0.75 × 9 2) and fresh weight indicates the cobs weight per plot during harvest, Based on the cob weight plot^‒1 measurement, grain yield was computed assuming a shelling percentage of an 80%. The grain yield was later converted to tonnes ha^‒1 (Bello et al. 2012). To evade border effects in all trials, edge plants at both row sides were removed during harvesting.

Proximate analysis of tryptophan content

Kjeldahl apparatus was used in grinding and defat of grain samples for each maize genotype. A single approach of papain hydrolysis was used for protein solubilization. Acetic acid was oxidized by iron ions to glyoxylic acid by applying sulphuric acid. Glyoxylic acid reacted with free tryptophan indole ring of bound dissolved proteins to generate the violet-purple reactants. The violet-purple colouration was assessed using the spectrophotometer at 560 nm. As described by Teklewold et al. (2015), the tryptophan optical density standard curve of concentration was read with the spectrophotometer and transformed into a percentage of tryptophan as follows:

% Tryptophan

= Factor × Corrected optical density at 560 nm

The optical density

= Optical density _{560-nm sample} − Optical density _{560-nm papain blanks mean}

Factor=Hydrolysate volumeWeight sample×Slope of the standard curve

Two readings were recorded for each genotype. Only tryptophan was analyzed in maize grain endosperm because lysine was greatly correlated with tryptophan (r > .9) (Nurit et al. 2009). Besides, the quantification of tryptophan is cheaper compared with lysine.

Carotenoid proximate analysis

Carotenoid was analyzed with high-performance liquid chromatography according to the protocol extraction method for dried maize kernels analysis (Howe and Tanumihardjo 2006; Obeng-Bio et al. 2020). From each of the self-pollinated cobs, 10 grams of grains were randomly sampled from 30 grains and dry-frozen at ‒80℃. The samples were then crushed into fine powders (0.5 mm) which were used for carotenoid analysis. The b-carotene (bC), b-cryptoxanthin (bCX), zeaxanthin (ZEA), lutein (LUT), (cis and trans isomers), and carotenoids a-carotene (aC) contents were quantified (Nurit et al. 2009). The overall sum of carotenoid contents was calculated as the total concentrations of LUT, ZEA bCX, bC, and aC. The PVA content was recorded for every sample as the bC sum plus half of aC and bCX. This measurement resulted in molecular compositions of aC and bCX which were assumed to contribute 50% of the PVA activity of bC (U.S. IMFNB 2001). All tryptophan and carotenoids of every trial were obtained from the 2 replicates to enhance quantification accuracy.

Statistical analyses

Method 2 (and hybrids and parents without reciprocal crosses) and Model I (Fixed model) diallel analysis were exploited as described by Griffing (1956). Specific combin-ing ability (SCA) and general combining ability (GCA) mean squares and individual error variances were evalu-ated with the SAS program (SAS 2018) from ANOVA. Following tests for homogeneity, independence, and nor-mality of variance, the combined analysis of the PVA- QPM inbreds, as well as testcrosses for the two years of evaluation with nine analyzed features was as follows:

yijkl=μ+genotypei+yj+blockrep×yjkl+repyjk+genotype×yij+εijkl

The m is the average total, genotype i is the ith effect of genotype, year j is the jth effect of year, genotype × year ij is the jth year, and ith genotype interactive effect. Also, rep(year)jk is the effect of kth replication in the jth year, block(rep × y) jkl is the effect of incomplete block lth within the jth year, and the kth replicates while ꜫijkl is the variance error. For untransformed data, standard residuals for all the effects of heritable traits were explored with mean squares error from the testcrosses. The least signi-ficant difference test was utilized to calculate the dif-ferences in the trait averages. The percentage coefficient of variation (PCV) was employed to measure the variation sum.

Maize grain yield

Grain yield decidedly differed among the parents in the two growing seasons of the field trials (Table 2). Among the five QPM-PVA inbreds used as parents, four (TZEIORQ 24, TZEQI 82, TZEIORQ 59, and TZEIORQ 13) had higher grain yield with a mean yield of 5.86 t ha^‒1 com-parable with commercial hybrid checks (5.44 t ha^‒1) with a 7.2% yield advantage. The AK-9528-DMRSR POP 28 SR (5.32 t ha^‒1) was very prolific among MSV-resistant cultivars but with a yield increase of 2% over the commercial checks. All the topcrosses varied strikingly with superior yield compared with corresponding parents and checks. The PVA-QPM × PVA-QPM hybrids were higher in yield potential while MSV-resistant × PVA-QPM was next, then MSV × MSV-resistant cultivars

Table 2 . Means of PVA-QPM lines, MSV resistance cultivars, selected testcrosses and commercial checks for grain yield, MSV disease rating, carotenoids, and tryptophan contents assessed in two years of cropping seasons.

Genotypes	Tryptopha n (%)	Grain yield (t ha‒1)	MSV (no.)	PVA µg g‒1	βC µg g‒1	ZEA µg g‒1	αC µg g‒1	LUT µg g‒1	βCX µg g‒1
TZEIORQ 13	3.97	5.83	3.52	4.01	2.02	7.63	0.47	11.24	2.98
TZEIORQ 24	3.98	5.91	2.21	3.87	2.45	7.77	0.49	12.25	2.81
TZEIORQ 29	3.91	5.24	3.01	3.76	2.23	7.81	0.51	12.33	2.85
TZEIORQ 59	3.93	5.84	2.34	3.84	2.41	7.67	0.56	12.67	2.91
TZEQI 82	3.95	5.86	3.11	3.76	2.52	7.98	0.51	12.66	2.96
AK-9528-DMRSR POP 28 SR	2.34	5.32	1.02	0.11	0.12	2.62	0.01	1.67	0.12
ACR. 91 SUWAN-1-SR C1	2.21	5.01	1.51	0.15	0.08	1.81	0.06	2.64	0.04
IK.91 TZL COMP 3-Y C1	2.33	5.22	1.43	0.02	0.04	2.53	0.08	2.77	0.03
IKENNE 88 TZSR-Y-1 TZSR-Y-1	2.48	5.15	1.72	0.06	0.03	2.68	0.04	2.11	0.01
TZB-SR SGY TZB-SR	2.12	5.11	1.65	0.12	0.02	2.63	0.02	2.47	0.02
Selected testcrosses
TZEIORQ 13 × TZEIORQ 24	4.06	6.12	3.56	4.25	2.81	8.6	0.69	14.22	2.81
TZEIORQ 13 × TZEIORQ 29	4.34	6.34	3.54	4.34	2.77	7.99	0.61	15.1	2.77
TZEIORQ 24 × TZEIORQ 29	4.09	6.32	2.96	4.25	2.69	8.55	0.72	14.34	2.69
TZEIORQ 29 × TZEIORQ 59	4.13	6.11	3.61	4.39	2.75	8.63	0.68	14.43	2.75
TZEIORQ 29 × TZEQI 82	3.97	6.18	3.88	4.28	2.98	8.69	0.59	15.29	2.98
TZEIORQ 59 × TZEQI 82	4.24	6.44	3.49	4.15	2.11	8.48	0.57	15.31	2.81
TZEIORQ 13 × AK-9528-DMRSR POP 28 SR	3.79	5.82	2.16	3.87	2.32	7.99	0.76	13.29	2.77
TZEIORQ 24 × AK-9528-DMRSR POP 28 SR	3.65	5.5	2.18	3.76	2.29	7.73	0.72	12.27	2.69
TZEIORQ 29 × AK-9528-DMRSR POP 28 SR	3.78	5.86	2.17	3.84	2.32	7.57	0.49	12.39	2.75
TZEIORQ 59 × AK-9528-DMRSR POP 28 SR	3.81	5.54	2.11	3.76	2.54	7.79	0.53	13.26	2.98
TZEQI 82 × AK-9528-DMRSR POP 28 SR	3.89	5.98	2	3.87	2.33	7.83	0.74	11.29	2.81
TZEIORQ 13 × ACR. 91 SUWAN-1-SR C1	3.61	5.83	2.66	3.76	2.51	7.77	0.53	13.44	2.77
TZEIORQ 29 × ACR. 91 SUWAN-1-SR C1	3.6	5.89	2.58	3.84	2.68	7.99	0.61	13.56	2.69
TZEQI 82 × ACR. 91 SUWAN-1-SR C1	3.87	5.97	2.64	3.79	2.11	7.78	0.59	12.52	2.75
TZEIORQ 24 × IK.91 TZL COMP 3-Y C1	3.68	5.87	2.56	3.87	2.45	7.57	0.63	12.76	2.98
TZEQI 82 × IK.91 TZL COMP 3-Y C1	3.88	5.96	2.68	3.76	2.32	7.7	0.71	13.89	2.81
TZEIORQ 24 × IKENNE 88 TZSR-Y-1 TZSR-Y-1	3.67	5.88	2.69	3.84	2.61	7.83	0.53	11.36	2.77
TZEIORQ 59 × IKENNE 88 TZSR-Y-1 TZSR-Y-1	3.69	5.75	2.56	3.76	2.45	7.75	0.64	13.51	2.69
TZEQI 82 × IKENNE 88 TZSR-Y-1 TZSR-Y-1	3.56	5.99	2.65	3.87	2.38	7.99	0.81	13.46	2.75
TZEIORQ 24 × TZB-SR SGY TZB-SR	3.64	5.76	2.66	3.79	2.29	7.72	0.67	12.73	2.94
TZEIORQ 59 × TZB-SR SGY TZB-SR	3.69	5.78	2.73	3.84	2.87	7.57	0.54	12.68	2.75
TZEQI 82 × TZB-SR SGY TZB-SR	2.59	5.96	2.71	3.77	2.44	7.71	0.49	13.72	2.91
AK-9528-DMRSR POP 28 SR × ACR. 91 SUWAN-1-SR C1	2.91	5.88	1	3.99	2.44	7.68	0.51	12.31	2.98
IK.91 TZL COMP 3-Y C1 × TZB-SR SGY TZB-SR	2.78	5.33	2.32	3.83	2.53	7.83	0.54	13.11	2.57
IKENNE 88 TZSR-Y-1 TZSR-Y-1 × TZB-SR SGY TZB-SR	2.59	5.56	2.43	3.91	2.5	7.72	0.5	12.28	2.93
Varietal checks
Oba Super 2	3.98	5.4	2.86	0.22	0.02	2.13	0.05	3.21	0.02
Oba Super 4	3.97	5.48	2.88	0.24	0.05	2.24	0.03	4.09	0.04
SE	0.074	0.029	0.055	0.011	0.047	0.023	0.031	0.051	0.011
CV (%)	5.34	5.91	10.11	7.93	9.63	11.34	8.93	10.44	7.32
LSD (0.05)	0.91	0.87	0.68	0.65	0.73	0.89	0.99	0.72	0.45

MSV: maize streak virus, PVA: total provitamin A, βCX: β-cryptoxanthin, ZEA: zeaxanthin, LUT: lutein, βC: β-carotene, αC: α-caro.

MSV disease expression

Grain yield decidedly differed among the parents in the two growing seasons of the field trials (Table 2). Among the five QPM-PVA inbreds used as parents, four (TZEIORQ 24, TZEQI 82, TZEIORQ 59, and TZEIORQ 13) had higher grain yield with a mean yield of 5.86 t ha^‒1 comparable with commercial hybrid checks (5.44 t ha^‒1) with a 7.2% yield advantage. The AK-9528-DMRSR POP 28 SR (5.32 t ha^‒1) was very prolific among MSV-resistant cultivars but with a yield increase of 2% over the com-mercial checks. All the topcrosses varied strikingly with superior yield compared with corresponding parents and checks. The PVA-QPM × PVA-QPM hybrids were higher in yield potential while MSV-resistant × PVA-QPM was next, then MSV × MSV-resistant cultivars.

Grain-quality protein and vitamin A

The AK-9528-DMRSR POP 28 SR combined well for high Tryptophan in some PVA-QPM testcrosses and MSV-resistant cultivars (Table 2).

Carotenoids assessment

Concerning PVA, two hybrids (TZEIORQ 13 × TZEIORQ 29 and TZEIORQ 59 × TZEQI 82) and four testcrosses (TZEQI 82 × IKENNE 88 TZSR-Y-1, TZEQI 82 × ACR. 91 SUWAN-1-SR C1, TZEQI 82 × AK-9528-DMRSR POP 28 SR, and TZEQI 82 × TZB-SR SGY TZB-SR) were of topmost performance among PVA-QPM hybrids. They also had high and comparable values among the selected testcrosses for other carotenoids (ZEA, LUT aC, bCX, and bC).

Combined analysis of variance

The MSV-resistant cultivars and PVA-QPM inbreds pooled analysis of variance for the virus resistance scoring, carotenoids, grain quality protein, and yield were assessed in the two cropping seasons (Table 3). Crosses mean squares vastly differed among the ten traits studied.

Table 3 . Pooled analysis of variance of PVA-QPM lines and their hybrids and MSV-resistant testcrosses for grain yield, MSV disease scoring, carotenoids, and tryptophan contents assessed in two years of cropping seasons.

Source of variation	Df	Grain yield (t ha‒1)	MSV (no.)	Tryptophan (%)	PVA	ZEA	LUT	aC	bC	bCX
Year	1	4.67	3.81	5.31	6.56	8.22	4.87	7.93	10.52	9.38
Rep (year)	6	7.24	5.78	12.78	10.63	14.72	11.43	13.65	11.57	12.11
Crosses	45	99.83**	87.69**	92.13**	78.89**	95.37**	92.19**	87.44**	91.72**	89.55**
Crosses × Year	45	10.67	12.83	11.87	13.11	12.89	10.09	13.65	11.23	9.11
GCA	9	87.32*	95.55**	88.78**	97.21**	95.34**	87.71**	99.67**	67.58**	78.79**
SCA	35	34.01*	43.21*	51.32*	48.67*	53.34*	41.89*	54.22*	49.46*	53.32*
Year × GCA	6	13.37	15.11	8.70	11.13	9.73	12.25	13.11	10.79	12.22
Year × SCA	35	21.12	10.99	11.22	12.67	9.28	10.54	13.27	10.44	11.56
Pool error	329	12.52	9.88	7.04	10.22	8.01	11.56	8.07	7.04	10.76
GCA/SCA (Baker ratio) %		1.45	0.78	0.56	0.82	0.74	0.31	0.75	0.38	0.50
CV%		9.8	11.2	7.55	10.82	11.78	9.54	10.59	12.66	11.83

** and * significant at 0.01 and probability levels, respectively.

MSV: maize streak virus, PVA: total provitamin A, bCX: b-cryptoxanthin, ZEA: zeaxanthin, LUT: lutein, bC: b-carotene, aC: a-caro.

General combining ability estimates

All traits studies, excluding MSV, are suitable for meaningful positive GCA effects since the aim is for improved disease resistance, carotenoids, grain quality protein, and yield (Table 4). The whole genotypes were positive and significant for GCA, suggesting better combinations for outstanding grain-yielding development, corresponding to Nyaligwa et al. (2014). This also indi-cates the additive genetic action in conditioning grain yield (Aly 2013). Hence, recurrent selection could be explored in developing maize varieties with improved MSV resistance. The PVA-QPM inbreds that signified no differential positive effects for MSV showed that similar MSV resis-tance levels could be created in their different combination hybrids. In leaf disease evaluation, negative effects of GCA exhibit resistance genetic expressions. For MSV resistance, the entire five cultivars of MSV resistance had GCA negative effects depicting their promising resistance suita-bility sources (Bello 2017). It is noteworthy that host-plant MSV resistance varieties are adjudged as the greatest economically viable and practical essential in conditioning MSV disease pandemics (Nyaligwa et al. 2014). It is also obvious that all PVA-QPM inbreds revealed effects of GCA for carotenoids and grain quality protein which signifies per se performance. This will enable desirable genotypes to be chosen in crop breeding programs. Positive with non-significant effects of MSV resistance cultivars symbolize that their similar performance could be achieved in their varied hybrid combinations (Nyaligwa et al. 2014). It is also evident that efficient breeding methods exploring improved genes in homozygous form and linkage block braking could substantially decrease losses of grain yield linked to MSV resistance.

Table 4 . Estimates of GCA for PVA-QPM lines and their hybrids and MSV-resistant testcrosses for grain yield, MSV disease scoring, carotenoids, and tryptophan contents assessed in two years of cropping seasons.

Genotypes	Tryptophan (%)	Grain yield (t ha‒1)	MSV (no.)	PVA	ZEA	LUT	aC	bC	bCX
TZEIORQ 13	80.35**	1.97**	0.26	89.96**	87.22**	111.10**	92.33**	53.91*	97.55**
TZEIORQ 24	91.97**	0.74**	0.34	91.74**	98.76**	91.88**	88.54**	92.93**	81.99**
TZEIORQ 29	100.44**	1.87**	0.23	84.87**	82.11**	87.74**	76.97**	47.25*	92.77**
TZEIORQ 59	98.98**	1.92**	0.14	98.31**	89.32**	76.81**	99.25**	89.88**	79.12**
TZEQI 82	86.87**	0.86**	0.35	87.88**	78.87**	92.93**	71.11**	45.74*	88.94**
AK-9528-DMRSR POP 28 SR	0.07	1.09**	‒1.99**	1.29	2.11	2.03	2.04	1.06	2.07
ACR. 91 SUWAN-1-SR C1	1.01	0.82**	‒0.71**	1.54	1.65	1.99	3.11	2.09	1.87
IK.91 TZL COMP 3-Y C1	0.08	1.88**	‒0.88**	1.19	1.09	1.86	0.04	1.05	2.07
IKENNE 88 TZSR-Y-1 TZSR-Y-1	1.05	0.91**	‒1.76**	1.09	1.42	2.54	1.73	1.11	3.05
TZB-SR SGY TZB-SR	0.09	1.93**	‒0.93**	0.11	0.77	1.39	0.67	0.68	2.01

** and * significant at 0.01 and 0.05 probability levels, respectively.

MSV: maize streak virus, PVA: total provitamin A, bCX: b-cryptoxanthin, ZEA: zeaxanthin, LUT: lutein, bC: b-carotene, aC: a-caro.

Specific combining ability estimates

Distinctly positive GCA effects are suitable for all traits studied, excluding MSV resistance since the aim is for improved carotenoids, grain quality protein, and yield (Table 4). The whole genotypes were positive and signi-ficant for GCA.

It appears that the remarkable yield expressed by PVA-QPM × PVA-QPM crosses could be credited to suita-ble allelic interactions among the PVA-QPM combined. The TZEQI 82 was a common parental inbred with superior grain yield among the selected topcrosses. The TZEIORQ 59 × TZEQI 82 (6.44 t ha^‒1) was the foremost yielding topcross with a yield gain of 2% above the checks. Besides, AK-9528-DMRSR POP 28 SR produced topcrosses of elevated yield with some PVA-QPM inbreds. Testcros-sed TZEQI 82 × AK-9528-DMRSR POP 28 SR, TZEQI 82 × ACR. 91 SUWAN-1-SR C1 and TZEQI 82 × IK.91 TZL COMP 3-Y C1 produced an average of 5.97 t ha^‒1, representing 8%, 10%, and 11% above the superior check, inbred, and MSV-resistant cultivar, respectively. These results corroborated with that of Bello (2017; Oluwaseun et al. 2022).

That MSV × MSV-resistant testcrosses were utmost resistant genotypes. MSV-resistant × PVA-QPM were next, while MSV × MSV-resistant topcrosses were most susceptible. These findings were earlier reported by Bello, (2017). AK-9528-DMRSR POP 28 SR was the best MSV- resistant cultivar with a 1.01 score and favourably com-bined with many QPM inbred lines in producing resistant topcrosses with increased yield. This indicates that AK-9528-DMRSR POP 28 SR was commonly resistant to the virus, containing great occurrences of desirable bene-factor genes than the rest parents. TZEQI 82 × AK- 9528-DMRSR POP 28 SR with the greatest yield among MSV-resistant × PVA-QPM testcrosses was also prominent with 30% exceedingly resistance over the checks. The two vulnerable checks scores signify the degree of the disease virulence and that the inoculation procedures used were effective.

The highest Tryptophan (3.98) expressed by TZEQI 82 × AK-9528-DMRSR POP 28 SR in the MSV-resistant × PVA-QPM testcrosses was comparable to the checks. It seems that AK-9528-DMRSR POP 28 SR not only possessed MSV-resistant alleles suitable for introgression with other maize genotypes but a favourable resource for quality protein and grain yield genes. The PVA-QPM inbreds derived from TZEIORQ 13, TZEIORQ 29, and TZEQI 82 that were crossed with MSV-resistant AK- 9528-DMRSR POP 28 SR appeared exceedingly promising for high virus resistance, grain quality protein, carotenoids, and yield.

Concerning PVA, two hybrids (TZEIORQ 13 × TZEIORQ 29 and TZEIORQ 59 × TZEQI 82) and four testcrosses (TZEQI 82 × IKENNE 88 TZSR-Y-1, TZEQI 82 × ACR. 91 SUWAN-1-SR C1, TZEQI 82 × AK-9528-DMRSR POP 28 SR, and TZEQI 82 × TZB-SR SGY TZB-SR) were of topmost performance among PVA-QPM hybrids. They also had high and comparable values among the selected testcrosses for other carotenoids (ZEA, LUT aC, bCX, and bC). This indicates that PVA-QPM inbreds of different pedigrees transformed to high grain yielding MSV-resistant attributes with justifiable quality protein are feasible to be adapted in savanna agroecology (Oluwaseun et al. 2022). Thus, these six genotypes (two PVA-QPM × PVA-QPM hybrids and four MSV-resistant × PVA-QPM topcrosses) identified in meeting these standards can be used as parents in developing novel hybrids. These could also be utilized in broadening the forthcoming improvement schemes. The AK-9528-DMRSR POP 28 SR is the common parent among the best topcrosses for MSV resistance, carotenoids, grain quality protein, and yield. These four topcrosses are recommended for direct cultivation for commercial pro-duction in this agroecology but after broad evaluation at varied years and locations to confirm their promising performance.

Crosses mean squares vastly differed among the ten traits studied, indicating broadly diverse parents were used and the genotypes contributed differently among the top-crosses. It also shows that genetic diversity is apparent in the improvement scheme. SCA mean squares of GCA were greatly significant for all the traits in the trials, denoting that the genetic variations governing these traits were primarily influenced by additive effects. Varied genetic frequency distribution in the genotypes was also obtained in this study. These findings not only accentuated the possibility of evolving divergent parents for hybrids improvement but assisted as unique sources of alleles in modifying the genic base of adapted genotypes for genetic gain sustainability in the hybridization schemes. Additive gene action governing maize grain yield was reported by many workers (Bello and Olawuyi, 2015). However, the mean squares results showed non-significant interaction between years and both SCA and GCA, highlighting that both gene actions (non-additive and additive) were comparable across the two years. Baker (1978) reported that the predominant additive genetic effects (GCA) and non-additive genetic effects (SCA) for grain yield and other traits showed the type of genetic effects in diallel crosses. In this study, the worth of the GCA/SCA fraction revealing above one for grain yield and below one for rest traits implied that additive inheritance components conditioning the grain yielding and non-additive inheritance components governing the carotenoids, MSV resistance, and quality protein grain.

All traits studies, excluding MSV, are suitable for mean-ingful positive GCA effects since the aim is for improved disease resistance, carotenoids, grain quality protein, and yield (Table 4). The whole genotypes were positive and significant for GCA, suggesting better combinations for outstanding grain-yielding development, corresponding to Nyaligwa et al. (2014). This also indicates the additive genetic action in conditioning grain yield (Aly 2013). Hence, recurrent selection could be explored in developing maize varieties with improved MSV resistance. The PVA-QPM inbreds that signified no differential positive effects for MSV showed that similar MSV resistance levels could be created in their different combination hybrids. In leaf disease evaluation, negative effects of GCA exhibit resistance genetic expressions. For MSV resistance, the entire five cultivars of MSV resistance had GCA negative effects depicting their promising resistance suitability sources (Bello 2017). It is noteworthy that host-plant MSV resistance varieties are adjudged as the greatest econo-mically viable and practical essential in conditioning MSV disease pandemics (Nyaligwa et al. 2014). It is also obvious that all PVA-QPM inbreds revealed effects of GCA for carotenoids and grain quality protein which signifies per se performance. This will enable desirable genotypes to be chosen in crop breeding programs. Positive with non-significant effects of MSV resistance cultivars symbolize that their similar performance could be achieved in their varied hybrid combinations (Nyaligwa et al. 2014). It is also evident that efficient breeding methods exploring improved genes in homozygous form and linkage block braking could substantially decrease losses of grain yield linked to MSV resistance.

Distinctly positive GCA effects are suitable for all traits studied, excluding MSV resistance since the aim is for improved carotenoids, grain quality protein, and yield. The whole genotypes were positive and significant for GCA, similar to Nyaligwa et al. (2014). Variances of SCA are generally assessed in the instance where substantial SCA effects are produced (Badu-Apraku et al. 2020). Typically, the highest estimates of SCA variance for yield were obtained for all traits of the selected twenty-two crosses, indicating dominant effects loci controlling the grain yield (Table 5). These results corroborated with the previous researchers (Rovaris et al. 2014). Regarding quality protein and carotenoid contents, positive and significant SCA were observed for PVA-QPM × MSV and PVA-QPM × PVA-QPM hybrids, while MSV × MSV demonstrated no worthy positive impacts. It is noticeable; however, that exploiting inbred in breeding synthetic and composite varieties is a reasonable approach to developing greater MSV resistance, quality protein, and carotenoid levels. Since MSV resistance allelic numbers of the studied parents are yet to be attained, an advanced genome-wide related analysis is necessary for guiding imminent allele pyramiding.

Table 5 . SCA effects of selected MSV-resistant testcrosses for grain yield, MSV disease scoring, carotenoids, and tryptophan contents assessed in two years of cropping seasons.

Crosses	Grain yield (t ha‒1)	MSV (no.)	Tryptophan (%)	PVA	ZEA	LUT	αC	βC	βCX
TZEIORQ 13 × TZEIORQ 24	0.99**	0.84	99.92**	78.61**	73.11**	79.45**	87.56**	78.51**	86.34**
TZEIORQ 13 × TZEIORQ 29	1.22**	0.56	111.74**	92.55**	98.22**	98.23**	88.12**	98.12**	89.56**
TZEIORQ 24 × TZEIORQ 29	3.45**	0.72	76.87**	77.52**	84.23**	87.34**	87.35**	88.47**	84.94**
TZEIORQ 29 × TZEIORQ 59	2.12**	0.89	87.44**	91.53**	91.56**	91.57**	99.12**	92.02**	91.23**
TZEIORQ 29 × TZEQI 82	1.98**	0.75	98.82**	104.76**	100.03**	99.83**	111.05**	112.60**	96.67**
TZEIORQ 59 × TZEQI 82	3.43**	0.96	81.91**	78.44**	92.04**	100.42**	93.48**	98.63**	104.25**
TZEIORQ 13 × AK-9528-DMRSR POP 28 SR	2.77**	‒1.76**	66.91**	51.67**	56.22**	61.34**	54.66**	51.43**	56.33**
TZEIORQ 24 × AK-9528-DMRSR POP 28 SR	1.01**	‒0.88**	70.42**	56.54**	67.53**	49.51**	65.11**	56.54**	60.98**
TZEIORQ 29 × AK-9528-DMRSR POP 28 SR	2.22**	‒0.91**	67.22**	61.33**	54.98**	21.98*	22.56*	6356**	61.54**
TZEIORQ 59 × AK-9528-DMRSR POP 28 SR	0.97**	‒1.83**	30.43*	56.11**	64.69**	67.54**	63.11**	67.56**	50.66**
TZEQI 82 × IK.91 TZL COMP 3-Y C1	0.88**	‒0.75**	67.78**	27.54*	24.11**	52.22**	67.78**	49.22**	69.33**
TZEIORQ 13 × ACR. 91 SUWAN-1-SR C1	0.75**	‒1.32**	56.11**	51.33**	61.34**	59.54**	59.23**	59.89**	59.78**
TZEIORQ 24 × ACR. 91 SUWAN-1-SR C1	1.11**	‒0.44**	49.78**	47.12**	56.67**	43.66**	25.99*	51.66**	51.32**
TZEIORQ 29 × TZB-SR SGY TZB-SR	1.56**	‒0.97**	51.11**	56.45**	49.76**	24.78*	53.65**	29.23*	48.76**
TZEIORQ 59 × IK.91 TZL COMP 3-Y C1	0.88**	‒1.43**	33.24*	61.41**	54.78**	63.11**	63.78**	48.11**	66.31**
TZEQI 82 × IKENNE 88 TZSR-Y-1 TZSR-Y-1	0.75**	‒0.89**	61.81**	28.56*	26.09**	54.99**	64.33**	56.89**	57.22**
TZEQI 82 × TZB-SR SGY TZB-SR	1.91**	‒0.94**	56.91**	23.56*	21.24**	53.22**	61.56**	61.24**	62.56**
AK-9528-DMRSR POP 28 SR × TZB-SR SGY TZB-SR	1.34**	‒1.41**	1.4	0.78	0.98	1.22	1.77	1.78	1.66
ACR. 91 SUWAN-1-SR C1 × IKENNE 88 TZSR-Y-1 TZSR-Y-1	2.54**	‒0.93**	1.83	1.11	1.22	1.75	1.56	1.01	1.4383
IK.91 TZL COMP 3-Y C1 × TZB-SR SGY TZB-SR	0.99**	‒1.86**	2.11	0.23	0.66	0.84	1.78	0.67	0.97
IKENNE 88 TZSR-Y-1 TZSR-Y-1 × TZB-SR SGY TZB-SR	1.76**	‒0.72**	0.81	0.87	0.98	0.93	0.77	0.91	0.11
TZB-SR SGY TZB-SR × TZB-SR SGY TZB-SR	1.55**	‒0.89**	1.94	0.11	1.32	0.77	0.89	1.55	1.67

** and * significant at 0.01 and 0.05 probability levels, respectively.

MSV: maize streak virus, PVA: total provitamin A, βCX: β-cryptoxanthin, ZEA: zeaxanthin, LUT: lutein, βC: β-carotene, αC: α-caro.

The authors appreciate the Anonymous Reviewers and Editor whose contributions have significantly enhanced the manuscript quality.