Regression of Grain Yield of Maize Inbred Lines and Their Diallel Crosses on Elevated Levels of Soil-Nitrogen

Maize grain yield response to elevated levels of soil nitrogen is dependent upon genotype of the cultivar. Thus the optimum rate of N-fertilizer differs from maize genotype to another according to its nitrogen use efficiency (NUE). The main objective of this study was to determine the optimum N-rate for each studied inbred and hybrid that maximize grain yield. Six inbred lines of maize differing in their productivity under low-N were crossed in a diallel fashion to produce 15 F1ËŠs. Parents and F1ËŠs were evaluated in two seasons (2012 and 2013) using a split-plot design in randomized complete blocks arrangement with 3 replications. Main plots were allotted to four N-rates, i.e. 0, 80, 160 and 240 kg N/fed for N1, N2, N3 and N4, respectively. The sub-plots were assigned for the genotypes. Reducing N-level from 204 to 160, 80 and 0 kg N/feddan (fed) [one fed = 4200 m2] caused an increase in days to silking (DTS), anthesis silking interval (ASI), barren stalks (BS), economic NUEe and biological NUEb NUE and a decrease in the remaining studied traits including grain yield and its component. Maximum increase and decrease in traits occurred at N1 level (0 kg N/fed). The inbred lines L17, L18 and L53 proved to be tolerant (T), while L29, L54 and L55 inbred lines were sensitive (S) to N stress. The most tolerant crosses to low-N stress and the most responsive crosses to elevated levels of nitrogen were identified. Only two crosses (L18 × L53 and L18 × L55) showed high tolerance to low-N stress and responsiveness to high-N expressed in grain yield per plant (GYPP) and per fed (GYPF). The T×T group of crosses exhibited better performance in most studied traits than T×S and S×S groups of crosses under the low and high N levels. The three inbred lines L53, L18 and L17 and the three groups of crosses [inefficient responsive (IR), efficient non-responsive (ENR) and inefficient non-responsive (INR)] showed a quadratic response to the elevated levels of nitrogen with an optimum N level of 180 kg N/fed, while the three inbred lines L54, L29 and L55 and the efficient responsive (ER) group of crosses (L18 × L53 and L18 × L55) showed near linear response to elevated N levels.