Heterosis, combining ability and drought susceptibility index in some crosses of bread wheat (Triticumaestivum L.) under water stress conditions

Eight bread wheat genotypesof diverse origin were crossed in a half-diallel model in 2011/2012 season. The eight parents (Sahel-1 P1, Giza-168 P2, Misr-1 P3, Gemmieza-9 P4, Gemmieza-11 P5, Line-1 P6, Line-2 P7, Line-3 P8) and their 28 F1 hybrids were evaluated under normal and water stress conditions during 2012/2013 season in a two field experiments wereas designed a randomized complete block design (RCBD) with three replications in farm of NubariaAgricultural Research Center, Egypt. Days to heading, plant height,number of spikes per plant, number of kernels per spike, 1000-kernel weight and grain yield per plant were studied. The mean squares of all traits were significant or highly significant for all studied traits. Water stress caused significant reductions in most studied traits. Average reduction due to water stress was significant for all studied traits, except that of hybrids K/S. Positive significantheterosis over the better parent was detected in thecross 1 X 5 for S/P under non-stress only, the F1’s crosses 3 X 8 and 4 X 7 for K/S under non-stress and stress condition respectively, and cross 1 X 3for GY/P under non-stress and stress condition, respectively. Negative significant heterosis over the better parent was detected in thecross 2 X 5 for days to heading under non-stress and stress condition respectively,and cross4 X 7 for plant height under non-stress and stress condition, respectively. Estimate the general and specific combining ability effects and other genetic parameters related to yield and its components. Partitioning of genotypes mean squares to general combining ability (GCA) and specific combining ability (SCA) mean squares based on Griffing's method 2 and fixed model indicated highly significant differences among wheat genotypes in their GCA values for all traits. There were significant differences among crosses in their SCA values for all traits except for 1000 KW under non-stress condition. The high ratio of GCA to SCA mean squares implied the importance of additive gene effects in the appearance for all studied traits. P5 was a good combiner for heading date, plant heightand 1000 KW under both irrigation and GY/P under non-stress only, and P2 for GY under stress condition, P4, P6 and P7 for K/S, S/P and PH under both irrigation regimes, respectively. The highest desirable SCA effects were obtained with 4 x 7 for K/S under both irrigation,2 x 8 for S/P under non-stress, 5 X 7 for 1000 KW under stressconditions and 1 X 3 under stress conditions for GY/P. P1 had the lowest drought susceptibility index value followed by P4 and P2 and F1’s crosses 1 X 4 followed by 4 X 6, 1 X 3 and 6 X 8would be classified as drought resistance due to the lowest reduction in yield under water stress compared to non-stress conditions. Line-1 was a moderately drought tolerant, while Line-2 was moderately susceptible.