Genetic analysis of maize inbred lines for tolerance to drought and low nitrogen

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1 JONARES, Vol. 1, pp 29-36, June Genetic analysis of maize inbred lines for tolerance to drought and low nitrogen Silvestro K. Meseka 1*, Abebe Menkir 2, Abu Elhassan S. Ibrahim 3 and Sam. O. Ajala 2 1 Agricultural Research and Technology Corporation, P. O. Box 126, Wad Medani, Sudan 2 International Institute of Tropical Agriculture, Oyo Road, PMB 5320, Ibadan, Nigeria 3 Faculty of Agricultural Sciences, University of Gezira, P. O. Box 20, Wad Medani, Sudan Abstract The frequent occurrence of drought combined with low levels of soil nitrogen (Low N) limit maize (Zea mays L.) production in sub-saharan Africa. The knowledge of inheritance of tolerance to drought and low N would be useful for developing hybrids that combine tolerance to drought and low N stresses. This study evaluates under optimal, drought and low N conditions (i) the effects of zero, one and two doses of tolerance to drought in parents on yield performance of their hybrid, (ii) the modes of inheritance of grain yield in inbred lines selected for contrasting responses to drought, (iii) the relationship between per se performance of parents and their hybrids. Ninety-six hybrids from twenty-four inbred lines were produced using the Design II mating scheme. Lines and their hybrids were evaluated separately in trials under drought, low N and optimal conditions in a total of five environments in Nigeria. Drought tolerant inbred lines had consistently high grain yield across environments. Most hybrids with at least one tolerant parent produced tolerant hybrids, whereas most crosses between susceptible lines generated susceptible hybrids. Additive gene action conditioned grain yield under drought, while non-additive genetic effects influenced grain yield under low N. Midparent values were good predictors of hybrid performance for grain yield and other traits under stress. Significant SCA mean squares observed for grain yield under drought stress as well as high non-additive gene action value for grain yield under low N suggest that hybrid development could be employed to exploit non-additive gene action in drought and low N production environments. I. INTRODUCTION The frequent occurrence of drought combined with low levels of soil nitrogen (Low N) and high cost of agricultural inputs are important factors limiting maize (Zea mays L.) production in sub-saharan Africa (SSA). In maize, drought alone causes an average yield loss of about 17 60% (Edmeades et al., 1999), whereas low N causes an average yield losses ranging between 10 50% (Logrono and Lothrop, 1997) per year. Drought can occur at any growth stage of maize, but the crop is more sensitive at flowering and grain-filling periods (Edmeades et al., 1997; Menkir and Akintunde, 2001). On the other hand, low N is frequently found in farmer s fields where fertilizer application is far below the recommended or not commonly used. Furthermore, some farmers are hesitant to use inorganic N fertilizer in their maize fields due to the high risk of drought. One effective strategy to reduce yield losses and fertilizer cost is to develop maize genotypes that combine tolerance to drought with high nitrogen use efficiency and high yield potential. Information on gene action conditioning grain yield of tropical maize under drought and low N has been limited and contradictory. For example, Guei & Wassom (1992) reported that non-additive genetic effects control grain yield under drought, whereas Betran et al.(2003) found that additive genetic effects condition grain yield under drought stress. Under low N, Betran et al.(2003) reported that non-additive genetic effects control grain yield, contrary to earlier reported by Kling et al.(1997) that additive genetic effects control grain yield of tropical maize under low N. These results indicate that the genetic factors controlling grain yield of tropical maize under both drought and low N has not been fully understood. Several studies (Lafitte and Banziger, 1997; Zambezi and Mwambula, 1997; Banziger et al., 1999) have highlighted good performance of tropical maize genotypes selected for drought tolerance under low N conditions. Lafitte and Banziger (1997) reported that four maize populations (Tuxpino Sequia C8, Laposta Sequia C3, POOL 26 Sequia and POOL 18 C3) improved through recurrent selection under drought combined tolerance to both drought and low N. Zambezi and Mwambula (1997) also found that improvement of drought tolerance in a maize population, Tuxpino Sequia, resulted in improved performance under low N. However, these studies have not examined the relationships between inbred lines selected for drought and the performance of their hybrids under both drought and low N conditions. Understanding such relationships and their genetic basis would aid the development of effective breeding strategies to improve maize genotypes for drought and low N environments. This study was designed to evaluate the following under drought, low N, well-watered and high N conditions (i) the effects of zero, one and two doses of tolerance to drought in parents on yield performance of their hybrid, (ii) the modes of inheritance of grain yield in inbred lines selected for contrasting responses to drought, and (iii) the relationship between per se performance of parents and their hybrids.

2 JONARES, Vol. 1, pp 29-36, June II. MATERIALS AND METHODS Twenty-four maize inbred lines developed at the International Institute of Tropical Agriculture (IITA) were crossed in a Design II mating scheme (Comstock & Robinson, 1948). The inbred lines were classified using standardized base index involving anthesis-silking interval (ASI), number of ears per plant, grain yield and leaf death scores traits that are sensitive to drought stress. Twelve of the inbred lines were tolerant to drought (T), while the other twelve were susceptible (S) to drought stress (Table 1). parent in a second set. A total of 96 hybrids were produced (6 sets x 16 hybrids) in the 2001 rainy season at IITA in Ibadan, Nigeria (7 o 30 / N, 3 o 54 / E, altitude 224 m). The 24 inbred lines and their 96 F1 hybrids were evaluated separately in two trials planted side by side in 5 environments. The hybrids along with four checks were arranged in a 10 x 10 triple lattice design, while the parental lines were arranged in a randomized complete block design with three replications. Each hybrid or parental line was planted in a 3 m row plot spaced 0.75 m apart with 0.25 m between plants within each row. Within a row, two seeds were planted in a hill and thinned to one plant after emergence to attain a population Table 1: Means for grain yield (Mg ha-1) of parental lines used in Design II crosses tested under severe water stress (SS), mild water stress (MS), low nitrogen (LN), well-watered (WW) and high nitrogen (HN) conditions in Nigeria between 2002 and Grain yield (Mg ha -1 ) Inbred Category SS MS LN WW HN Across 9006 T T POP10 T T T T KU1409 T (POOL 26 Sequia)C3F2 T T T (KU1403x1368)STR T (TZMI501xKU1414x501) T S S Fun.47-3 S S GH 24 S (KU1403x1368) S S S S S Mok Pion-Y-S4 S (KU1403x1368)BC2 S Mean S.E± Probability of F for lines *** *** *** *** *** *** Probability of lines x year ns ** ** *** *** *** ** Significantly different from zero at P < *** Significantly different from zero at P < ns Not significantly different from zero at P = T = tolerant lines and S = susceptible lines. To reduce the number of crosses required, the 24 inbred lines were divided into six sets each of four inbred lines. The four inbred lines in one set were used as females and crossed with four inbred lines in another set used as males. Each inbred line was used as female parent in one set and as a male density of 53,333 plants ha -1. In the two trials, gramazone was applied as herbicide at 5 L ha -1 of paraquat (1, 1 -dimethyl- 4,4 -bipyridinium). Subsequent manual weeding was done to keep the trials weed-free. Apart from the targeted stress, the management of trials at each location was the same.

3 JONARES, Vol. 1, pp 29-36, June Three environments comprised of severe water stress (SS), mild water stress (MS) and well-watered (WW). The trials were conducted at Ikenne, Nigeria (6 o 53 / N, 3 o 42 / E, altitude 60 m) during the dry season. Maize crop planted during this period is completely dependent on irrigation. The experiment was conducted in two blocks with two irrigation treatments. Block 1 was referred to as well-watered (WW) treatment and block 2 as water stress (MS) treatment. The sprinkler irrigation system was used to supply sufficient water every week to all treatments in both blocks during the first 5 weeks following germination. Block 1, thereafter referred to as WW, continued to receive irrigation each week until physiological maturity. In block 2, water stress was imposed by withdrawing irrigation at 5 weeks after planting (WAP) to ensure drought stress at flowering and grain filling periods. No irrigation was applied during the remainder of the growing period. However, there were rains in 2003 and 2004 during the months of January (42.0 and 29.4 mm) and February (27.4 and 42.4 mm) that coincided with flowering and grain filling periods. Throughout the text, block 2 in 2002 and 2005 will be referred to as severe water stress (SS) and block 2 in 2003 and 2004 as mild-water stress (MS). A compound fertilizer was applied at the rate of 60 kg N, 60 kg P and 60 kg K ha -1 at the time of sowing. An additional 60 kg N ha -1 was applied as top dressing 4 weeks later. Two experiments were conducted under low and high N conditions at Mokwa, Nigeria (9 o 18 / N, 5 o 04 / E, altitude 457 m) during the rainy season in 2002 and At Mokwa, the experimental field was divided into low and high N blocks that received different N treatments. The low N block has been depleted of soil nitrogen by growing high density of maize without additional N fertilizer. All the experiments received 60 kg ha -1 each of triple super-phosphate and potassium at planting. In addition, 20 kg N ha -1 was split into two and was applied to the low N block as urea at the rate of 10 kg N ha -1 at two and six WAP. In a high N block, 90 kg N ha -1 was applied as a single dose at 2 WAP. Days to anthesis and silking were recorded as the number of days from planting to when 50% of the plants shed pollen and showed emerged silks, respectively. Plant and ear heights were measured as the distance from the base of the plant to the height of the first tassel branch and the node bearing the upper ear, respectively. Plant aspect was rated on a scale of 1 to 5, where 1 = excellent overall phenotypic appeal and 5 = poor overall phenotypic appeal. Ear aspect was scored on a scale of 1 to 5, where 1 = clean, uniform, large, and well-filled ears and 5 = rotten, variable, small, and partially filled ears. Visual leaf death was scored only under drought and low N at 10 and 12 WAP on a scale of 1 to 9, where 1 = less than 10% senesced leaf and 9 = more than 80% senesced leaf area below the ear. Anthesis-silking interval (ASI) and the number of ears plant -1 were computed as the interval in days between silking and anthesis and the proportion of total number of ears divided by the number of plants harvested, respectively. All ears harvested from each plot were shelled to determine percentage moisture and grain yield adjusted to 15% moisture was computed from the shelled grain weight. Individual analysis of variance were conducted for each trial using a mixed model in SAS (SAS Institute, 2001) with genotypes (hybrids and inbred lines) being considered as fixed effects, replications and test environments random effects. Each irrigation or N treatment-year combination was considered a test environment. The ANOVA for both inbred and hybrid trials were performed with PROC GLM in SAS using a RANDOM statement with the TEST option. The grain yield data for low N block in 2002 was highly variable due to available soil N caused by the previous cassava crop residue; therefore, the data was subjected to square root transformation (Gomez and Gomez, 1984). For the hybrid trials, ANOVAs were computed for each environment to generate entry means adjusted for block effects according to the lattice design (Cochran and Cox, 1960). The hybrids (sets) component of the variation was divided into variation due to male (sets), female (sets), and female x male (sets) interaction. The F test for male (sets), female (sets), and female x male (sets) mean squares were computed using the mean squares for their respective interaction with environment. The mean square attributable to environment x female x male (sets) was tested using the pooled error mean squares. The main effects of male (sets) and female (sets) represent the GCA effect while the female x male (sets) interaction represents SCA effect (Hallauer and Miranda, 1988). Line x Tester analysis was calculated for grain yield using adjusted means after the check entries were omitted following the procedure of Singh and Chaudhary (1985). For each trait, the midparent values for a cross was computed as the mean of the two parental line means for each environment. To determine the relationship between parental lines and their hybrids, Spearman s rank correlation analysis between pairs of trait means of midparent values and hybrid means were calculated for each environment. Midparent heterosis was computed for traits measured in each environment. III. RESULTS Per se performance of inbred lines The inbred lines selected for this study exhibited significant differences in grain yield across the environments (Table 1). Mean grain yield for inbred lines ranged from 0.57 Mg ha -1 under severe water stress to 1.53 mg ha -1 under high nitrogen. The average grain yield across environment was 1.02 mg ha -1. At Ikenne, grain yields of inbred tested under severe water stress and mild water stress were 42 and 72% of grain yield under well-watered conditions, respectively, while at Mokwa, grain yield of inbred lines under low N was only 44% of yield obtained under high N. Significant (P < 0.01) line x environment interaction was detected for grain yields in each environment except SS. Most of the drought tolerant inbred lines had mean grain yields higher than trial mean, while most susceptible lines had mean grain yields lower than the trial mean under SS, MS and LN (Table 1). The highest yielding inbred lines across environments include 4058, KU1409, (TZMI501xKU1414xTZMI501), 161, 1824, (KU1403x1368), 161, 9613 and Effect of different doses of drought tolerance in inbred lines on performance of their hybrids

4 JONARES, Vol. 1, pp 29-36, June The mean grain yields of hybrids generated from crosses of inbred lines with different doses of tolerance to drought is presented in Table 2. Table 2: Means of grain yield (Mg ha -1 ) and their standard errors for different combinations of inbred lines evaluated at five test environments in Nigeria between 2002 and Inbred line combinations Environ ment Severe water stress Mild water stress Low nitrogen Wellwatered High nitrogen T x T T x S S x T S x S Chec ±S.E k T = Tolerant and S = susceptible. Mean grain yield of hybrids ranged from 1.58 Mg ha -1 under LN to 5.14 Mg ha -1 under WW conditions. On the average, drought stress reduced grain yield of hybrids by 40 and 63% under MS and SS, respectively, while low N stress reduced grain yield of hybrids by 52%. Mean grain yield of hybrids of tolerant x tolerant (T x T) crosses were higher than hybrids of susceptible x susceptible (S x S) crosses across environments. Under low N, mean grain yield of hybrids of T x S and the checks were highest and those of S x S crosses were lowest. Mean grain yield of T x S crosses were slightly higher than S x T crosses under SS, LN, WW and HN, while under MS, hybrids of S x T crosses had higher grain yield than T x S (Table 2). accounted for 36% of the total variation in grain yield under LN. Combining ability estimates In the combined analysis, year and sets were significant sources of variation for grain yield across environments. Mean squares for both males (sets) and females (sets) were significantly different among hybrids for grain in each environment (Table 3). The interaction of males (sets) and females (sets) with year were significant for grain yield only in WW, for males (sets) with year in LN, for females (sets) with year in SS and MS environments. The females x males (sets) interaction was highly significant (P < 0.01) across environments, whereas, its interaction with environments was significant only under MS and WW conditions (Table 3). Partitioning of the hybrid sums of squares showed that GCA accounted for >55% of the variation among hybrids for grain yield under SS, MS, WW and HN, while under low N, GCA accounted for only 47% of the total variation among hybrids for grain yield (Table 4). Three drought tolerant inbred lines including 1824, KU1409 and (TZMI501xKU1403x501) had positive GCA effects when used in crosses as both male and female parents across environments, 9006 expressed positive GCA effects when used in crosses as both males and females only under SS, MS, WW, HN, while others such as 4058, 4001 and 161 had positive GCA effects when used in crosses either as males or females under SS, MS and LN (Table 5). Among the susceptible lines, 9432 had positive GCA effects when used in crosses as both males and females under SS, MS, LN and HN, whereas Fun.47-3 and Mok Pion-Y-S4 expressed positive GCA effects when used in crosses as both male and female parents only under SS and LN and as females under MS. The estimates of SCA effects of hybrids groups were positive for 57% of T x S, 55% of T x T crosses and negative for >50% of the hybrids of S x T and S x S crosses under SS, whereas under LN, the estimates of SCA effects of the hybrids groups were positive Table 3: Mean squares from the combined ANOVAs for grain yield of crosses of 24 maize inbred lines tested for two years under severe water stress (SS), mild water stress (MS), low nitrogen (LN), high nitrogen (HN) and for four years under well-watered (WW) conditions in Nigeria between 2002 and Source D.F SS MS LN HN D.F WW Year *** 11.91*** 20.02*** 4.16* *** Set *** 4.96*** 0.82** 3.69*** *** Male (Sets) *** 5.90*** 0.52** 4.39*** *** Female (Sets) *** 4.40*** 0.34* 3.97*** *** Female x Male (Sets) ** 1.96*** 0.32** 1.39** *** Year x Male (Sets) * *** Year x Female (Sets) * 1.59** ** Year x Female x Male (Sets) ** * Pooled error * Significantly different from zero at P < ** Significantly different from zero at P < *** Significantly different from zero at P < Results from regression analyses showed that the variation in grain yield under WW conditions accounted for 18 and 34% of the total variation in grain yields under SS and MS, respectively, while the variation in grain yield under HN for 56% of T x T, 52% of T x S and for 50% of S x T crosses, while >50% of S x S crosses had negative (data not shown). Relationship between per se performance of the parental lines and their hybrids

5 JONARES, Vol. 1, pp 29-36, June Spearman s rank correlation coefficients between hybrid means and mid-parent values differed significantly and were positive for all traits across environments except for number of ears per plant under LN. Table 4: Percentages of the sums of squares for crosses attributable to general (GCA) and specific combining ability (SCA) for grain yield (Mg ha -1 ) under severe water stress (SS), mild water stress (MS), low nitrogen (LN), wellwatered (WW) and high nitrogen (HN). Females Environment Males GCA GCA SCA Severe water stress Mild water stress Low nitrogen Well-watered High nitrogen The correlations of mid-parent values and the mean performance of their hybrids were high for plant height across environments, for ear height under SS, LN, WW, HN, for days to silking under MS, LN, WW, HN, for the second leaf death score under both SS and MS and for the first leaf death score only under SS conditions, and was moderate for grain yield and numbers of ears per plant across environments (Table 6). Grain yield of inbred lines accounted for >20% under SS, while under MS, LN, WW and HN it accounted for <15% of the total variation in grain yields among hybrids. Average heterosis for each trait was consistent in all environments except for ASI under SM, HN and for plant and ear aspects under both low and high N (Table 6). Negative heterotic values obtained under SS and LN for days to silking, ASI, leaf death scores indicate that hybrids matured earlier, had shorter ASI, longer stay-green than their corresponding parental lines, while its positive values for plant height, number of ears per plant and grain yield indicate that hybrids grew taller, had higher number of ears per plant and produced higher grain yields than their parental lines. IV. DISCUSSION The intensity of stress observed for drought and low soil nitrogen resulting in grain yield reduction of 58 and 56% for inbred lines in this study fall within the range of yield reduction occasioned by stress levels applied during selection of inbred lines for tolerance to drought or low N by other workers (Bolanos & Edmeades, 1993; Lafitte and Edmeades, 1994; Betran et al., 2003). Although the interaction of lines with year was significant for grain yield under both MS, LN, HN and WW, the interaction was not significant under SS suggesting that the parental lines had consistent grain yield under SS conditions. The majority of drought tolerant inbred lines had high grain yields under both drought and low N stresses, indicating that inbred lines selected for drought tolerance could be good source materials for developing tolerant maize hybrids for both drought and low N environments. Our results indicate that the relatively high grain yield of inbred lines under both drought and low N is indicative of increased tolerance to both stresses, a result that is consistent with other studies (Duvick, 1992; Tollenaar and Lee, 2002). As indicated by Goodman (1985), the choice of breeding materials determines the future success or failure of a breeding program. In our study, hybrids of T x T crosses produced high grain yield across environments, indicating that the genetic gain made through combination of drought tolerance of parental lines into their hybrids translated into spill-over effects in low N and optimal production environments. Also drought tolerant inbred lines combined well with some drought susceptible lines, providing further evidence that good performance across stress levels can be achieved by using at least one drought tolerant parent in hybrid combinations. These results are in agreement with those obtained by earlier workers (Williams et al., 1969; Duvick, 1997; Betran et al., 2003) who found that improvement of maize genotypes for tolerance to abiotic stress is associated with the ability to maximize grain yield under nonstress growing conditions. Our results revealed that sufficient variability that may be exploited for enhancing the development of maize hybrids for both drought and low N environments exists in drought tolerant parental lines. The genetic effects controlling the grain yield under drought in this set of hybrids were different from that under low N. Additive effects condition grain yield under drought stress, while non-additive effects controls grain yield under low N stress. The significant SCA mean squares for grain yield under drought stress as well as high non-additive gene action and heterotic value observed for grain yield under low N suggest that hybrid development could be employed to exploit nonadditive gene action in drought and low N production environments. These results corroborate those obtained by Betran et al. (2003) highlighting the need to use drought tolerant maize inbred lines as parents of single-cross hybrids to improve grain yield under both drought and low N stresses. Our results further revealed that crosses exhibiting high SCA effects could result from any combination of T x T, T x S, S x T and to a lesser extent from S x S parents. The majority of hybrids with at least one drought tolerant parent had significant SCA effects, indicating the importance of using inbred-hybrid approach in breeding for stress environments (Kirkham et al., 1984). Ward (1994) reported that the correlation of the same pair of traits in parents and hybrids increase in stress environments. In our study, the correlations of mid-parent values and mean performance of their hybrids for grain yield was higher under SS than under MS, LN, WW and HN conditions, indicating that inbred lines selected for tolerance to drought generally produced tolerant hybrids. Cox & Frey (1984) postulate that if gene action for all traits is predominantly additive then parental lines could be chosen based on performance per se as attested by the significant parent-hybrid correlations. In our study, inbred lines could be chosen on the basis of the per se performance under SS conditions. However, comparative yield trails of inbred lines and their hybrids are still needed because their specific hybrid combinations can best determine the performance of inbred lines.

6 JONARES, Vol. 1, pp 29-36, June Table 5: Estimates of general combining ability effects for grain yield of 24 maize inbred lines evaluated in sets of factorial crosses under severe water stress, mild water stress, low nitrogen, well-watered and high nitrogen conditions in Nigeria between 2002 and Severe water stress Mild water stress Low nitrogen Well-watered High nitrogen Inbred Category Male Female Male Female Male Female Male Female Male Female 9006 T 0.41** 0.27** 0.79** 1.20** ** 0.86** 0.20* 0.26** 4058 T ** ** ** 0.54** POP10 T ** -0.20** -0.14** 0.73** 0.23** -0.36** -0.35** 1824 T 0.38** 0.17** 0.23** 0.42** 0.23** 0.10* 0.46** 0.27** ** 9613 T ** * -0.31** 4001 T * * ** KU1409 T ** 0.33** 0.41** 0.26** 0.22** * 0.37** 0.47** (POOL 26 Sequia)C3F2 T ** -0.14** 0.37** 0.34** -0.46** -0.59** 9450 T 0.10* * T ** * ** 0.65** (KU1403x1368)STR T ** -0.12** ** (TZMI501xKU1414x501) T ** 0.34** 0.75** 0.12* ** ** 5012 S S ** -0.12** ** -0.22** Fun.47-3 S 0.24** ** 0.11* ** 9432 S ** ** 0.12** ** 0.45** GH 24 S * * 0.34** -0.70** (KU1403x1368) S ** -0.16** ** -1.04** 1808 S ** 9071 S ** ** ** 9485 S ** ** 0.25** 4008 S ** ** ** -0.34** 0.35** Mok Pion-Y-S4 S 0.26** * ** -0.28** (KU1403x1368)BC2 S ** ** -0.40** SED± *, ** Significantly different from zero at 0.05 and 0.01 levels of probability, respectively. T = tolerant lines and S = susceptible lines. = the standard error of difference between two GCAs Table 6: Correlation coefficients of the same pair of traits of 24 maize parental lines and their 96 hybrids and estimates of average heterosis under severe water stress (SS), mild water stress (MS), low nitrogen (LN), well-watered (WW) and high nitrogen (HN). Coefficient of correlation ( r ) Average heterosis (%) Trait SS MS LN WW HN SS MS LN WW HN Days to anthesis (days) 0.55*** 0.58*** 0.45*** 0.70*** 0.61*** ** * Days to silking (days) 0.54*** 0.64*** 0.61*** 0.65*** 0.67*** ** * Anthesis-silking (days) 0.36** 0.53*** 0.51** 0.41*** 0.50*** * Plant height (cm) 0.74*** 0.64*** 0.70*** 0.66*** 0.76*** 47.63** 35.97** 34** 39.81** 35** Ear height (cm) 0.62*** 0.51*** 0.60*** 0.61*** 0.65*** 56.61** 49.60** 41** 50.67** 39** Plant aspect (1-5) a 0.42*** 0.26** 0.29*** 0.59*** 0.26** Ear plant-1 (no.) 0.34** 0.35** *** 0.25* Ear aspect (1-5) b 0.44*** 0.27** 0.44*** 0.49*** 0.53*** Grain yield (Mg ha -1 ) 0.46*** 0.30** 0.22* 0.34** 0.36** * * 229** ** 114* Leaf death1 (1-9) c 0.68*** 0.45*** 0.39*** Leaf death2 (1-9) c 0.66*** 0.73*** 0.57*** * Significantly different from zero at P < ** Significantly different from zero at P < *** Significantly different from zero at P < a Plant aspect scores, a scale of 1 to 5, where 1 = excellent plant type with good agronomic traits and 5 = poor plant type with poor agronomic traits. b Ear aspect scores, a scale of 1 to 5, where 1 = clean, uniform, large and well-filled ears and 5 = ears with undesirable features. c Leaf death scores, a scale of 1 to 9, where 1 < 10% dead leaf area and 9 > 80% dead leaf area.

7 JONARES, Vol. 1, pp 29-36, June In our study, mid-parent values were good indicators of grain yield and other traits in hybrids under drought stress, indicating that the genetic gain made under drought stress translates into gains in low N and optimal growing environments. Significant SCA mean squares for grain yield under drought stress as well as high non-additive gene action and heterotic value observed for grain yield under low N suggest that hybrid development could be employed to exploit non-additive gene action in drought and low N production environments. V. ACKNOWLEDGMENT This work was funded by Maize Improvement unit of the International Institute of Tropical Agriculture, Ibadan, Nigeria. The contributions of all support staffs of the maize unit during conduct of trials are highly appreciated. VI. 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8 JONARES, Vol. 1, pp 29-36, June Zambezi, B.T., and Mwambula The impact of drought and low soil nitrogen on maize production in the SADC region. pp In: G.O.Edmeades et al. (eds.), Developing Drought- and Low N-Tolerant Maize. Proceedings of a Symposium, March 25 29, 1996, CIMMYT, El Batan, Mexico.biography. Personal hobbies will be deleted from the biography.