الفهرس 
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Abstract
Two experiments were conducted at the Experimental Farm of Agronomy Department, Faculty of Agriculture, Mansoura University, Egypt during 2007/2008 and 2008/2009 seasons. The aim of the first experiment was to study the effect of harvesting dates (45, 55, 65 and 75 days from 50% heading) under three classes of wheat seed (foundation, registered and certified) on yield and its components (c.v. Giza168). The second experiment was conducted at the laboratory of Seed Technology Research Unit, Mansoura, Dakahlia Governorate and aimed to study the effect of harvesting dates and wheat seed classes on seed quality during different storage periods, i.e. (0, 3 and 6 months after harvesting) as well as, their interaction on yield and its components and seed quality. *The most important results obtained from this investigation can be summarized as follows: The results revealed that, harvesting dates had significant effects on grains moisture content, number of spikes/m2, spike weight, number of spikeletes/ spike, number of grains/spike, grains spike weight, 1000-grain weight and grain yield. Harvesting wheat plants after 65 day from 50% heading produced the highest grain yield and its components i.e. spikes number/m2 and 1000-seed weight in both seasons. Result showed insignificant differences among seed classes on growth characters, yield and its components. In general, number of spikes/m2, spike length, number of spikeletes/spike, number of grain/spike and grain yield recorded the maximum value from foundation seed followed by registered and certified seed category. Increasing storage period had a significant effect on all studied traits. While prolonging the storage periods from 0 to 6 months lead to a reduction in germination percentage, speed of germination, germination energy and germination rate and decreased seedling vigor traits in terms of seedling length and seedling fresh and dry weight and carbohydrate percentage in both seasons while the increase in storage period increased mean germination time, protein content, insect infection percentage and loss in seed dry weight in the first and second seasons. Harvesting dates significantly affected germination energy, germination rate, seedling fresh weight, seedling dry weight, insect infection percentage, loss in seed dry weight, protein content and carbohydrate percentage in the first and second seasons. While it had significant effect on speed of germination only in the first season. The interaction among storage periods, harvesting date and seed classes had a significant effect on germination energy and seedling fresh weight in the first and second seasons and seedling dry weight only in the first season.