الفهرس 
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Abstract
Wheat (Tritium aestivum, L) is the most important strategic cereal crop for the majority of the world populations. It is the most important staple food for about two billion people (36% of the world population). It exceeds in acreage and production than other grain crops (including rice, maize, etc.). Wheat is an edible grain, one of the oldest and most important of the cereal crops in Egypt. The annual consumption of wheat grains in Egypt is about 18.6 million tons, while the annual local production increased from 1.74 to 8.8 million tons (1980 – 2012). The increase was achieved by increasing wheat area from 1.34 to 3.21 million fed/year and grains yield from 1.3 to 2.74 ton/fed, in the same periods. (Food and Agricultural Organization 2012).
Though grown under a wide range of climates and soils, wheat is the best adapted crop to regions with rainfall between 300 and 900 mm. World food productions is primarily limited by environmental stresses. It is very difficult to find ”stress free” areas where crops may approach their potential yield. It is also increasing the consumption of wheat crop and the resulting increase in population, one of the most important problems of development, which calls for adding new areas of reclaimed land to cope with this increase, and under the conditions of the new reclaimed land which is basically environment low input system. The availability of water for irrigation is the most important factor determining the productivity of wheat crop. Therefore, devising new strains of wheat with less water needs while maintaining the ability to plant a crop most is one of the main objectives of the Wheat Research Department, Agricultural Research Center (A. R.C), Ministry of Agriculture, Egypt. Abiotic environmental factors are considered to be the main source (71%) of yields reductions (Boyer, 1982). Drought is one of the most common environmental stresses that affects growth and development of plants through alterations in metabolism and gene oppression. Wheat production suffers from variability in yield from one year to another and from location to another. Plant species vary in their sensitivity and response to the decrease in water potential caused by drought, low temperature or high salinity. It could be assumed that all plants have encoded capability gene(s) for stress perciption, signaling and response (Bohnert et al., 1995). Drought stress may occur early in the season or terminally at grain filling and development. Productivity improvement of wheat cultivars under drought conditions becomes one of the important objectives in wheat breeding program. Breeding for drought tolerance wheat cultivars is a major objective in arid and semi-arid regions of the world. Most of the Egyptian newly reclaimed lands (West and East of the Delta and West of the Nile Valley in Upper Egypt) suffer from drought and salinity stresses. Therefore, there is a major need to increase drought tolerance for the Egyptian wheat cultivars to increase the Egyptian wheat production, especially in the new lands, to meet the increasing consumption due to the increasing number of population. So wheat breeders are always looking for genetic diversity in which drought tolerance and yield potential can be combined. At the same time, breeders are looking for such criteria that play role in the plant water relations for wheat under water stress conditions which could be used to screen germplasm for drought tolerance.
Water content in plants is not necessary to be important, what is the interest where if the water content of the plant is high, but in the form of bound water, so most of the water content is the water potential which is the ability of the water in the plant to perform fill certain, if the water potential is high, the amount of free water in the cells be high, leading to the good functioning of physiological processes, but when the voltage is low and the water here is not necessarily a low water content, it may be high, but it is linked to and based on the physiological processes that are not going as they should.
Diallel crossing analysis is an excellent tool providing the breeder with:
(1) The nature and amount of genetic parameter such as additive, dominance and interaction in additive potency ratio and inbreeding depression.
(2) General and specific combining ability of parents and their crosses, respectively which guide the breeders to build new breeding programs in order to obtain desirable plant.
The main objectives of this research were to study the performance of eight different genotypes of bread wheat (Triticum aestivum, L) and their possible crosses using half diallel crosses under water stress conditions and to determine heterosis percentages in the (F1) hybrids, general combining ability (GCA), specific combining ability (SCA) for yield and it’s components, some agronomic characters and drought susceptibility index for grain yield.