Among the phenotypic, biochemical, and molecular methods employed in assessment of genetic diversity, the phenotypic method has proven efficient for the assessment, description and classification of germplasm collections to enhance their use in maize breeding. The objectives of the present study were: (i) to assess the extent of genetic diversity in a collection of Egyptian commercial maize hybrids and populations, through field evaluation under water and N stressed and non-stressed conditions, using morphological data based on Principle Component Analysis (PCA), (ii) to measure the genetic distance among these genotypes using UPGMA cluster analysis and (iii) to assess the relationship between grain yield and yield-related traits of maize genotypes using GT-biplot analysis. A two-year field experiment was conducted in a split-split plot design with 3 replications, where 2 irrigation regimes, three N rates and 19 maize genotypes occupied the main plots, sub plots and sub-sub plots, respectively. The germplasm was assessed for 21 agronomic traits. Highly significant differences (P ≤ 0.01) were observed among the maize hybrids and populations for all measured traits.  Results of the GT biplot in the present study indicated that high values of 100-Kernel weight, ears/plant, kernels/plant, kernels/row, plant height, nitrogen use efficiency, nitrogen utilization efficiency, and grain nitrogen content and short ASI could be considered reliable secondary traits for improving grain yield under stressed and non-stressed conditions. The highest genetic distance was found between G9 (SC-2055) and each of G15 (American Early Dent), G18 (Midland) or G19 (Ried Type). The Agglomerative Hierarchical Clustering based on phenotypic data assigned the maize genotypes into five groups. The different groups obtained can be useful for deriving the inbred lines with diverse features and diversifying the heterotic pools.

One of the best biometrical methods for estimating genetic diversity among germplasm collections is multivariate analysis; it is used to study their variability and genetic relatedness in order to increase their value in plant breeding programs. The objectives of the present study were to: (i) evaluate the magnitude of genetic diversity, based on phenotypic data, among 19 maize genotypes, under drought and/or low N stressed conditions in the field, using principle component analysis (PCA) and (ii) assess the interrelationships between maize grain yield and its related traits under such stressed conditions using genotype × trait (GT) biplot analysis. An experiment was conducted in two seasons using a split-split plot design with 3 replications, where 2 irrigation regimes (well-watered and water stressed at flowering) occupied the main plots, three N rates (high N, medium N and low N) occupied the sub plots and 19 maize genotypes occupied the sub-sub plots. The genotypes were evaluated for 19 agronomic traits. Analysis of variance was performed under each of the six environments. Significant differences (p≤0.01) were recorded among the maize genotypes for all studied traits under each environment. The best genotypes for each trait were identified. Results of the GT biplot indicated that high means of 100-kernel weight (100-KW), ears/plant (EPP), ear height, days to silking, days to anthesis, plant height, and chlorophyll concentration index (CCI) under water stress (WS), kernels/row (KPR), EPP, 100-KW and CCI under low N and KPR, EPP and 100-KW under WS combined with low N environment and low values of anthesis-silking interval (ASI) under the three stressed environments could be considered selection criteria for high grain yield under respective stressed environments and for drought and/or low N tolerance. It is recommended to select for high values of KPR, EPP and 100-KW and low value of ASI in order to increase grain yield under such stressed conditions.

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Deficit irrigation and low nitrogen (N) fertilization rate cause negative effects on maize grain productivity; such effects differ from genotype to genotype. The main objectives of the present investigation were to: (i) identify the drought and low N tolerant genotypes, (ii) estimate the superiority of tolerant to sensitive genotypes and (iii) assess the differential response of maize genotypes to elevated levels of nitrogen. Maize genotypes were evaluated in 2016 and 2017 seasons under three N rates: high; 285.6, medium; 166.6 and low; 47.6 kg N/ha and two irrigation regimes: well-watered (WW) and water stressed (WS) at flowering. A split-split-plot design with three replications was used. Main plots were allotted to irrigation regimes, sub-plots to N rates and sub-sub-plots to genotypes. The tolerant (T) and sensitive (S) genotypes were identified under each stress. Grain yield/plant (GYPP) of T genotypes was significantly (P≤ 0.01) superior to as compared to S genotypes by 109.5%, 39.6% and 141.9% under Low-N, drought, and drought combined with low-N, respectively. Superiority of T over S genotypes in GYPP was associated with significant (P≤ 0.01) superiority in ears/plant (11.5, 13.15 and 11.99%), 100-kernel weight (38.65, 30.46 and 30.99%), kernels/row (22.81, 11.28 and 20.07%), Nitrogen use efficiency (109.49, 39.62 and 141.89%) and shortening in anthesis-silking interval (-44.56, -29.58 and -29.08%), under the three environments, respectively. A significant linear response on elevated levels of nitrogen was shown by 13 genotypes, but a quadratic response was shown by six genotypes. The present study suggested that further investigation should be conducted to identify the optimum N fertilization rate for each newly developed variety of maize.

The secondary trait for a given abiotic stress tolerance, should be of strong correlation (r) with grain yield, high heritability (h2b) and high genetic advance (GA) under stressed conditions. The main objective of the present investigation was to identify secondary trait(s) for drought and/or low-N tolerance in maize genotypes. A two-year experiment was conducted, using a split-split-plot design. Main plots were allotted to two irrigation regimes, i.e. well watering (WW) and water stress at flowering (WS), sub-plots to three N fertilizer rates, i.e. low (LN), medium (MN) and high (HN) and sub-sub-plots to nineteen maize genotypes. Analysis of variance of randomized complete blocks design (RCBD) was also performed under each of the six environments (WW-HM, WW-MN, WW-LN, WS-HN, WS-MN and WS-LN). Tolerance to drought and/or low-N was strongly correlated with grain yield/plant (GYPP) under stressed environments. GYPP had high (h2b) and (GA); thus it is considered the best indicator of drought, low N or both stresses tolerance.  The best secondary traits are high 100-kernel weight (100-KW), ears/plant (EPP), kernels/row (KPR), and short anthesis-silking interval (ASI) for low-N tolerance, high EPP, 100-KW, plant height (PH) and short ASI for drought tolerance, high 100-KW, EPP, KPR, PH and short ASI, for tolerance to drought combined with low N, and high 100-KW, rows/ear (RPE) and KPR under optimum conditions (WW-HN), since they show high (r), high (h2b) and high (GA) estimates under the respective environments. Under low-N and/or drought, future research should focus on the incorporation of secondary traits such as EPP, KPR, 100-KW, PH, ASI in the selection programs along with the grain yield trait.