This requires a better knowledge of evapotranspiration rates and crop water regime.While, on a global scale water resources are still ample, serious water shortages are developing in the arid and semiarid regions as existing water resources reach full exploitation stated that water stress affect carbohydrate metabolism, protein synthesis and activities of many enzymes and the changes between enzymes activities reflect a change in the balance between rates of synthesis and degradation which, in turn decrease yield.The optimum soil moisture produce higher percentage of large sized bulbs, whereas water shortage during crop growing period led to higher percentage of smaller sized bulbs obtained higher total marketable, jumbo size and colossal yields with watered treatments.Deficit for higher yields per unit of applied irrigation water. Sulphur as a macronutrient has a positive effect on onion and other crops. It is essential for proper vegetative growth and bulb development, inadequate supply of sulphur nutrient causes slow crop growth at any stage resulting in yield reduction.This experiment included treatments, which were the combinations of three levels of irrigation water as well as control. These treatments were randomly arranged in a splitplot system in a complete randomized block design with three replicates.Drip irrigation system was used, each sub plot area had two dripper lines.The distance between the two single dripper lines of the double dripper line was cm, while the distance between the centers of each two double dripper lines was cm.The irrigation treatments were started after days from transplanting.Carbonate and bicarbonate were determined titremetrically using HSO, phenophthalein and bromocresol green as indicator.Field cured onion bulbs from each subplot with a sample of kg were stored under room temperature. The bulbs of each treatment were <a href="http://www.targetmol.com/compound/D-Cycloserine">Targetmol's
D-Cycloserine</a> weighed at days intervals, and decayed bulbs were excluded, then the cumulative weight loss percentage was calculated.Average bulb weight was significantly affected by application of sulphur. The same data clearly show that there were increases in bulb weight with increasing sulphur application in both seasons.The heaviest bulb weight was observed with application of kg S fed.These results might be attributed to the favorable effect of sulphur on reducing soil pH, increasing soil particles flocculation, thereby improving soil structure and increasing the availability of certain plant nutrients in the soil. While in the first season, there were significant effects owe to sulphur applications, without differences among all sulphur rates, except application of kg S fed.This result may be owe to increasing the availability of nutrients for onion plants.As for protein content of onion bulb, the obtained data reveal a significant effect due to sulphur application in both seasons.These results may be owe to the progressive effect of sulphur on soil properties which decreased soil pH and increased cation exchange capacity of soil which led to increase the availability of nutrients for plants. In this connection the low level of water irrigation in the second season.The increment in weight of all grades as well as total yield due to the increment in level of irrigation water could be due to its effect on increasing bulb diameter and average weight.