Effects of Sulfur on Key Enzyme Activities Involved in Nitrogen and Sulphur Assimilation in Flag Leaves and Grain Yield under Different Nitrogen Levels in Winter Wheat (Triticum aestivum L.) Effects of water stress on activities of nitrogen assimilation enzymes and photosynthetic characteristics of rice seedlings THE EFFECTS OF CALMODUL IN ANTAGONIST ON THE ACTIVITIES OF NITROGEN ASSIMILATION ENZYMES AND THE ACCUMUL ATION OF DRY MATTER IN WHEAT SEEDL INGS Research Progress on the Impact of Salicylic Acid on Plant Photosynthesis