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Abstract Enhancement of maize productivity from land unit area in Egypt could be achieved via growing tolerant hybrids to high plant density (HD) stress in combination with using the optimum plant density irrigation. A set of inbred lines with obvious diversity in their adaptive traits to HD and water stress (WS) were used in the present study as parents of diallel crosses (without reciprocals) to study the inheritance of these traits, identify tolerant genotypes to HD and water stress and determine the optimum plant density and irrigation for maximizing grain yield/fed (GYPF). The F1 crosses were made in 2012 and evaluation of inbreds and hybrids were carried out in 2013 and 2014 seasons, at the agricultural experiment and research station, Faculty of Agriculture, Cairo university, Giza, Egypt. A split-split plot design in randomized complete blocks arrangement with 3 replications was used, where main plants were devoted to irrigation treatment (well watering (WW) and water stress (WS)), sub-plots to plant densities (20, 30 and 40 thous and plants/fed) and sub-sub plots to maize genotypes. Some newly developed maize hybrids that characterized by more ears/plant (EPP), narrow leaf angle (LANG), short anthesis silking interval (ASI), less barren stalks (BS) and less plant height (PH) and ear height (EH) gave very high grain yield(GYPF), reaching 51.16 ard/fed in the cross L20{u00D7}L53 when grown under high plant density (40,000 plants/fed) and were given well watering. Moreover, some crosses L53{u00D7}Sk5, L53{u00D7}Sd7, L20{u00D7}L18 Sk5{u00D7}L28) gave >46 ard/fed under 40,000 plant density. GYPF of all studied genotypes showed a quadratic response of increase to the six environments from low to high levels, except L18, L28, Sd7 inbreds, which showed a quadratic response of decrease |