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Abstract Diallel cross excluding reciprocals was used to study the performance of six bread wheat varieties (Triticum aestivum L.) and their hybrids under water stress conditions. These varieties were used as parents (Giza 168, Giza 170, Gemmeiza 9, Prl x Toni, Rabi x Weaver and Pastor) of diallel cross mating design. During 2003/04 season; grains were sown in the crossing blocks of wheat research section. All F1 seeds of diallel mating design among the six parents were obtained. During 2003/04 season, the parental genotypes were sown and re-crossing for obtaining addition F1 grains to insure sufficient hybrid grains. 2005/06 season, the parental genotypes and resultant 15 F1 hybrids were grown under three irrigation treatments: W= 6 irrigations, W2 = two irrigations and W1= one irrigation. Each treatment was conducted as a separate experiment by using Randomized Complete Block Design replicated four times. At harvest, the bordered plants in each plot were considered for collecting data. The studied characters were; plant height, biological yield, number of spikes/plant, spike length, number of spikelets/spike, number of grains/spike,100-grain weight, grain weight/spike, grain yield/plant, peduncle length, flag leaf area, number of stomata- up, number of stomata- low and days to heading. The results of parental genotypes mean performance recorded increasing in the % of reduction between two irrigations and normal irrigation treatments for all studied traits except number of spikelets/spike, 100-grain weight, peduncle length and number of stomata- up indicating that these traits could be used as selection criteria under water stress conditions. According to drought susceptibility index, the present parental genotypes and cross combinations varied significantly in their tolerance or susceptibly to water stress conditions. But it seems that the most productive genotypes under normal irrigation are less susceptible (moderately tolerant) under water stress, also the hybrids which revealed tolerance behavior have at least one tolerant parental genotype. Therefore, using proper breeding program may enable combining high yielding ability with reliable drought tolerance. According to the heterotic effect, it could be concluded that different values of heterosis may be due to the genetic diversity of the studied parents with non-allelic interactions which increase or decrease the expression of heterosis. According to the ratio of additive to non-additive gene effects as an indication of the relative importance of both effects, the results showed that it exceed the unity only for (spike length, number of spikelets/spike, 100-grain weight and grain weight/spike) under normal irrigation treatment (W). Moreover, under water stress, the ratio of all studied traits exceeded unity except for (biological yield, number of spikelets/spike and grain yield/plant). Thus, additive gene effects appeared to be predominant only for water stress attributes. However, under normal irrigation, most of studied traits may be influenced by non-additive gene effects than additive ones. Specific combining ability, classified the genotypes into three categories: first one included the tolerant x tolerant genotypes which may be described as greatly favourable for improving the performance under water stress (W1) and none stressed (W) conditions. The second one comprised tolerant x susceptible genotypes which are favorable for intermediate condition of water stress (W2). The last category included susceptible x susceptible to water stress which exhibited good performance under normal irrigation (W). From the obtained results regarding to the specific combining ability effects, it s difficult to draw general view for inclusion of various parents in the wheat crossing program. The first reason rose from the purpose of conducting the hybridization in wheat. The main objective of crossing wheat isn t to search for the best cross combination only but to select high yielding and better adapted varieties to increase cross productivity. The second reason appeared from the recorded specific combining ability effects results from present studies, which proved great variation from combination to another and also between traits. Such findings based on the fixable evaluation of parent potential as general combining ability obstacle the recommendations for choosing the tolerant parents in the cross-breeding program. However, the utilization of some tolerant genotypes could be recommended as parents for improving specific traits under water stress conditions or drought. It could be concluded that crossing of different parental genotypes that having variable reactions to water stress widen the performance of resultant combinations. Generally, the water stress tolerance of wheat was strengthened by crossing and inclusion of the superior combinations in breeding program. The better cross combinations may be subjected to isolated intercrossing together for developing elite gene pool. Such gene pool may be used for recurrent selection program or used directly as a source of drought tolerance. Regarding to the effect of water stress on anatomical studies, it was obvious that this application induced thinner leaves from the prominent reduction in the thickness of mid-vein and lamina, but the reduction in the tolerant cross was less than the reduction in the susceptible one. |