Effects of combined application of nitrogen and potassium on absorption of N and K,grain yield and quality of weak gluten wheat Effects of Sulphur and Nitrogen Combined Application on Kernel-Starch Characteristics in Wheat Varieties with Weak Gluten Activities of Enzymes Involved in Sucrose and Starch Synthesis during Grain Filling and the Relation to Nitrogen Metabolism in Strong- and Weak-Gluten Wheat Cultivars