3.1. Analysis of Variance
Under CDHS conditions, the mean sum of squares for GCAm was significantly different at
p ≤ 0.05 for all traits except ears per plant, anthesis-silking interval, grain yield and cob length (
Table 3). Conversely, the mean squares for GCAf were significantly different at
p ≤ 0.05 for most traits except plant height, ears per plant, chlorophyll content and cob length only. Highly significant effects (
p ≤ 0.01) were observed for female × male interaction in all traits except for plant height, ears per plant, number of rows per ear, canopy temperature and cob length. The results obtained showed that the mean sum of squares of GCAm and GCAf combined were higher than SCA mean squares of the hybrids.
Under optimum conditions, the mean sum of squares for GCAm was significantly different for all traits except ears per plant, the number of plants per plot and chlorophyll content (
Table 4). Significant GCAf effects were observed for all traits excluding ears per plant and canopy temperature. Significant SCA effects (
p ≤ 0.05) were observed for most traits except for ears per plant and chlorophyll content.
3.2. General Combining Ability Analysis
Estimates of GCA effects for different traits were either negative or positive. Generally, inbred lines having higher and positive GCA values demonstrated good general combining ability characteristics for that trait [
23]. However, negative GCA effects were desirable for some traits [
24]. Positive GCA effects for yield were desirable. Under CDHS conditions, L30, L6, L5, L17 and L2 exhibited positive GCA effects (
Table 5), indicating that these inbred lines were good parental sources of genes for yield. Three of these inbred lines (L2, L5 and L6) were drought-tolerant at the seedling stage, while two of them (L30 and L17) were susceptible to drought at the seedling stage (
Table 1). Inbred lines with negative GCA effects (L14, L16, L13, L18 and L27) were described as poor parental sources of genes for yield. All these poor combiners for yield, except L16, were found to be susceptible to drought at the seedling stage.
Positive GCA effects for cob length were also desirable. Inbred lines L5, L14, L13, L16 and L27 were the only inbred lines that combined well for this trait. The rest of the inbred lines were poor combiners. The worst combiner for cob length was L18. Estimates of GCA for the number of rows per ear were both positive and negative. Positive GCA effects implied the presence of many rows per ear, which could be translated into more yield for the respective genotype. Inbred lines L18, L2, L14, L16 and L30 had positive GCA effects and were described as good parental sources of genes for the number of rows per ear. The rest of the inbred lines were poor sources of genes for increased number of rows per ear. Positive GCA effects for the number of ears per plant were desirable. Inbred lines L5, L14, L18 and L27 were generally good combiners whereas L2, L16, L6, L13, L17 and L30 were poor combiners for the number of ears per plant. Under CDHS conditions, positive GCA effect values of chlorophyll content were desirable. Positive GCA depicted the plant’s ability to maintain chlorophyll content, which would enable such plants to photosynthesize when others were senescing. Inbred lines L2, L13, L16, L17, L30, L14 and L27 were good combiners for chlorophyll content. However, L6, L5 and L18 did not combine well for this trait.
Negative GCA effects were desirable for days to 50% anthesis and silking since it demonstrated the ability of the plant to flower early under stressed conditions. According to Shavrukov et al. [
25], plants that flower early show a mannerism of drought and/or heat stress evasion. Inbred lines L2, L5, L13, L16 and L17 had negative GCA effects for both days to 50% anthesis and days to 50% silking. Inbred line L14 had negative GCA effects for days to 50% anthesis only. The other inbred lines showed positive GCA values implicating that they were late maturing genotypes. Late maturing genotypes are not desirable since yield is greatly reduced under stressed conditions. Negative GCA values for the anthesis-silking interval were desirable since it implied good synchronization of male (tasseling) and female (silking) flowering. Inbred lines L2, L17, L18 and L30 recorded negative GCA effect values thus making them desirable sources of genes for shortening the anthesis-silking interval in breeding programs. Inbred lines L5, L6, L13, L14, L16 and L27 recorded positive GCA effects, indicating that these inbred lines were poor parental sources of genes controlling flowering. Negative GCA effects for canopy temperature and plant height were desirable under CDHS conditions. Negative GCA effects for canopy temperature implied that canopy temperature was not elevated under CDHS conditions. Inbred lines L2, L13, L14, L30 and L27 were good combiners for canopy temperature while L6, L16, L17, L5 and L18 were poor combiners. Plants showing negative GCA effects for plant height implied that the plants were short. The development of short plants is desirable since such plants are not prone to logging unlike tall plants [
24]. Short plants, as shown by negative GCA effect values, were therefore desirable. Inbred lines L2, L5, L14, L13 and L16 were generally good parental sources of genes for plant height whereas L18, L27, L30, L17 and L6 were poor parental sources.
Under optimum conditions, inbred lines L2, L5, L14, L16, L17 and L30 combined well for grain yield while L6, L13, L18 and L27 were poor combiners (
Table 6). All inbred lines except L2, L18 and L27 were good combiners for cob length. Inbred lines L2, L14, L17 and L18 were the only good combiners for number of rows per ear. Approximately 50% of the inbred lines showed positive GCA effects for ears per plant under optimum conditions. Inbred lines L5, L14, L16, L17 and L18 were generally good combiners for ears per plant while L2, L6, L13, L27 and L30 were poor combiners. Combining ability effects for the rest of the traits under optimum conditions are shown in
Table 6.
Based on the results obtained, different inbred lines combined well for different traits under CDHS and optimum conditions. None of the inbred lines tested generally combined well for all the traits measured across environments. However, L2 was a good parental source of genes for most traits, except for cob length and ears per plant under CDHS, and under optimum conditions. This inbred line was previously observed to have drought tolerance at the seedling stage (
Table 1).
3.3. Specific Combining Ability Analysis
Significant SCA effects (p ≤ 0.05) were observed for most of the traits under CDHS, and under optimum conditions. Under CDHS, no significant effects were observed for canopy temperature, cob length, plant height and the number of ears per plant. Under optimum conditions, no significant effects were observed for grain yield, cob length, canopy temperature and the number of ears per plant. Negative and positive SCA effect values were recorded under CDHS, and under optimum conditions. Depending on the trait, cross combinations with high positive SCA values were denoted as best specific combiners for that trait while those with negative SCA effects were described as poor combiners and vice-versa. Positive SCA effect values for grain yield, cob length, the number of rows per ear, ears per plant and chlorophyll content were desirable. As such, SCHs exhibiting high positive SCA values for these traits were considered as good specific combiners while those exhibiting negative SCA values were regarded as poor combiners.
Under CDHS, 50% of the hybrids combined well for grain yield. Cross combinations L5*L18 (0.36), L14*L13 (0.35), L6*L13 (0.34) and L2*L30 (0.33) combined well for grain yield (
Table 7), recording total grain yields of 3.90, 3.53, 4.00 and 4.25 t/ha, respectively (
Table 8). These hybrids performed better than or approximately similar to the drought-tolerant check ZM 1523 (3.57t/ha). Three of these SCHs (L5*L18; L6*L13 and L2*L30) were generated from crosses between genotypes that were drought-tolerant and drought-susceptible at the seedling stage (
Table 1). The remaining SCH (L14*L13) was generated from a cross between two genotypes that were susceptible to drought stress at the seedling stage (
Table 1). Single cross hybrid L6*L16 (−0.54) with grain yield of 1.46 t/ha was the worst specific combiner for grain yield under stressed conditions. It was notable that this SCH was generated from a cross between genotypes classified as being drought-tolerant at the seedling stage (
Table 1). Percentage losses in grain yield of hybrids subjected to CDHS relative to optimum conditions ranged from 10.7% (L5*L18) to 70.7% (L14*L17). The average yield loss for all the 24 SCHs was 50,4% (data not shown).
Approximately 54% of the SCHs had positive SCA effects for the number of rows per ear. Hybrids L14*L13 (3.17), L5*L18 (2.25) and L2*L30 (2.00) were the best cross combinations for this trait while L14*L17 (−2.67), with the highest negative SCA value, was the worst cross combination. With regards to chlorophyll content, SCHs L6*L18 (5.91), L5*L27 (4.83), L2*L18 (4.63) and L14*L18 (4.30) recorded the highest positive SCA effect values and these cross combinations were good combiners for chlorophyll content. In contrast, L5*L18 (−14.83) was the worst cross combination for this trait. Negative SCA effects for anthesis-silking interval and days to 50% silking and anthesis were desired. Negative SCA effects for anthesis-silking interval demonstrated that flowering synchronization was good while negative days to 50% silking and anthesis demonstrated early flowering of the SCHs. Single cross hybrids L2*L18 (−2.49), L2*L30 (−2.32), L5*L27 and L6*L17 (−1.76) were the top five early flowering cross combinations with regards to days to 50% silking. Single cross hybrid L2*L16 (2.60) was the worst cross combination for days to 50% silking. Single cross hybrids L2*L30 (−3.49), L14*L18 (−2.38) and L6*L17 (−2.29) were the top three early flowering cross combinations while SCH L5*L17 (1.93) was the worst combiner with regards to days to 50% anthesis. Only 42% of the SCHs were good combiners for anthesis-silking interval under CDHS conditions. The top three best cross combinations for anthesis-silking interval were L2*L18 (−4.39), L14*L30 (−1.86) and L5*L16 (−1.69), while SCHs L14*L18 (3.72), L2*L17 (1.19) and L5*L30 (1.14) were considered as the three worst combiners.
Under optimum conditions, the best cross combinations for the number of rows per ear were SCHs L2*L27 (1.08), L5*L16 (0.61) and L6*L13 (0.53). Single cross hybrids L2*L18 (6.57), L5*L16 (6.20) and L14*L18 (4.32) combined well for chlorophyll content (
Table 9). On the other hand, SCHs L5*L13 (−0.81), L2*L17 (−0.67), L2*L18 (−0.67) and L5*L30 (−0.56) were the worst three combinations for number of rows per ear whereas L5*L18 (−13.03), L2*L16 (−4.96) and L6*L16 (−2.17) were the worst three combiners for chlorophyll content.
With regards to days to 50% silking, SCHs L6*L16 (−1.53), L2*L18 (−0.89) and L14*L18 (−0.89) were the best top three combiners while L14*L27 (1.61), L6*1L8 (1.56) and L2*L17 (1.03) were the worst combiners. Approximately 50% of the SCHs combined well for days to 50% anthesis. The top three best combiners for days to 50% anthesis were SCHs L5*L27 (−1.78), L14*L16 (−1.22) and L14*L18 (−0.97) whereas L14*L27 (2.44), L5*L16 (1.56) and L5*L18 (1.47) were the bottom three worst combiners. Single cross hybrids L5*L18 (−1.25), L6*L16(−0.86) and L14*L27 (−0.83) were the top three best combiners for anthesis-silking interval while L6*L18 (1.64), L14*L16 (1.58) and L5*L27 (1.17) were the bottom three worst combiners. Based on the results obtained, no cross combination performed well for all traits recorded across environments. However, hybrid L6*13 was a good combiner for grain yield across the environments whereas, L2*18 and L14*18 combined well for chlorophyll content across environments.