الفهرس Only 14 pages are availabe for public view
 |  | from | 176 |  |  |
| | | from | 176 | | |
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.