Australian Journal of Basic and Applied Sciences, 3(1): 206-217, 2009 ISSN 1991-8178 © 2008, INSInet Publication Stability of Wheat Genotypes under Different Environments and Their Evaluation under Sowing Dates and Nitrogen Fertilizer Levels K.A. Hamam and Abdel-Sabour. G.A. Khaled 1 2 Department of Agronomy, Faculty of Agriculture, Sohag University, 82786 Sohag, Egypt. 1 Department of Genetics, Faculty of Agriculture, Sohag University, 82786 Sohag, Egypt. 2 Abstract: The current study aimed at assessing the heat tolerance of twelve wheat genotypes under six environmental conditions (two locations and three years). Wheat genotypes were sown in two locations (Sohag and Assiut, Egypt) at two dates: November (favorable) and December (heat stress) during winter seasons of 2005/2006, 2006/2007 and 2007/2008. The combined analysis of variance showed that the flag leaf area, days to heading, plant height, spike length, 1000-kernel weight, biomass and grain yield were significantly influenced by years, locations, sowing dates, nitrogen fertilizer levels and genotypes. The results showed that sowing at the favorable date using 100kg/fed Nitrogen fertilizer increased all studied traits. The stability analysis revealed that four and three genotypes were high and intermediate yielding and stable for yield, respectively. The temperatures were decreased up to 2.50 to 6.39 º C at sowing late date than at the favorable sowing date. However among wheat genotypes a wide variation was found in response to heat tolerance. The results indicated that the 1000-kernel weight and grain yield traits are stable under heat stress. Some wheat genotypes conferred productive and adaptive advantages where they expressed high yield and yield stability when compared to other genotypes. This study indicated that higher 1000-kernel weight and days to heading are the two important traits which could be considered as potential selection criteria for yield under heat stress. Key words: Wheat genotypes, sowing date, nitrogen fertilizer, years, locations, stability. INTRODUCTION Terminal heat is a major abiotic stress affecting yield in wheat. Under heat stress, the photosynthetic process is affected especially during grain filling stage when demand for assimilates is the greatest (Kumari et al. 2007). Stay-green character is an important trait that allows plants to retain their leaves in active photosynthetic under stress conditions (Rosenow et al. 1983). Terminal heat stress can be a problem in 40% of the irrigated wheat growing areas of the world (Fisher and Byerlee, 1991). In the rice-wheat cropping system, crop damage due to heat stress under late planting conditions has become an important factor limiting wheat yields (Aslam et al., 1989). Yield reduction in wheat under heat stress could be caused by accelerated phasic development (Frank and Bauer, 1997), accelerated senescence (Kuroyanagi and Paulsen, 1985), increased respiration (Berry and Bjorkman, 1980), reduced photosynthesis (Conroy et al., 1994) and inhibition of starch synthesis in developing kernels (Jenner, 1994). High temperatures during early crop development and particularly after anthesis may limit yield (Hunt et al., 1991). Temperature fluctuations during grain filling were found to cause deviations from expected dough properties (Blumenthal et al. 1991). The rise in daily average temperature, up to about 30 ºC, increased dough strength, while temperatures above this threshold value (35 - 40 ºC), even for periods of only few days, tended to decrease dough strength (Randall and Moss, 1990; Borghi et al., 1995 and Corbellini et al., 1997). Heat stress tolerance was evaluated as relative reduction in grain yield from normal environment to heat stress under full irrigation (Shpiler and Blum, 1986). The phenotypic performance of a genotype is not necessarily the same under diverse agro-ecological conditions (Ali et al., 2003). Genotype-environment (GE) interactions are extremely important in the development and evaluation of plant varieties because they reduce the genotypic stability values under diverse environments (Hebert et al., 1995). The concept of stability had been defined in several ways and several biometrical methods including univariate and multivariate ones (Crossa, 1990). The most widely used one is the regression method, based on regressing the mean value of each genotype on the environmental index or marginal means of environments (Tesemma et al., 1998). A good method for measuring stability was previously proposed (Finlay and Corresponding Author: K.A. Hamam, Department of Agronomy, Faculty of Agriculture, Sohag University, 82786 Sohag, Egypt. E-mail: khalafhamam@yahoo.com 206