H. Mohammadi - Department of Agronomy, Faculty of Agriculture, Azarbaijan Shahid Madani University, Tabriz, Islamic Republic of Iran.
A. Ahmadi - Department of Agronomy and Plant Breeding, Faculty of Agricultural Sciences and Engineering, University of Tehran, Karaj, Islamic Republic of Iran.
J. C. Yang - Yangzhou University, Yangzhou, Jiangsu ۲۲۵۰۰۹, People's Republic of China
F. Moradi - Agricultural Biotechnology Research Institute of Iran (ABRII), Karaj, Islamic Republic of Iran.
Z. Wang - Yangzhou University, Yangzhou, Jiangsu ۲۲۵۰۰۹, People's Republic of China.
A. Abbasi - Department of Agronomy and Plant Breeding, Faculty of Agricultural Sciences and Engineering, University of Tehran, Karaj, Islamic Republic of Iran.
K. Poustini - Department of Agronomy and Plant Breeding, Faculty of Agricultural Sciences and Engineering, University of Tehran, Karaj, Islamic Republic of Iran.

Individual grain weight of wheat kernels differs with their positions on a spike. Cultivation practices (such as fertilizer nitrogen and plant growth regulators application) can be used to improve weight of basal and distal kernels. For this purpose, two experiments based on randomized complete block design were carried out with three replications. The aim of this research was to study the mechanisms related to sink strength as well as the regulatory factors affecting sink activity. One cultivar of a facultative wheat, cv Yangmai۱۵, was used. Treatments of the experiments included application of nitrogen (two levels) and plant growth regulators [abscisic acid (ABA) and Fluridone (inhibitor ofABA synthesis)] in basal and distal kernels, respectively. Results showed that nitrogen application increased grain yield and its components. Grain filling in basal kernels started earlier and its rate was higher than that of the distal kernels. Nitrogen fertilizer increased the individual kernel weight both in basal and distal kernels, and the rate of increment was higher than the control, even in distal kernels. The application ofABA resulted in increase in grain weight, whereas a considerable decrease in grain weight was observed in response to Fluridone compared to the control. Nitrogen application together withABA application enhanced the activity of SuSase, AGPase, SSS, and SBE in basal and distal kernels and the increment in the activity was higher in distal kernels. It is concluded that simultaneous application of nitrogen andABA enhanced grain weight by regulating the activity of key enzymes involved in starch synthesis.

Basal and distal kernels, Grain growth, Plant growth regulators, Sink activity, Wheat (Triticum aestivum L.)