Grain growth (by weight), grain-filling rate, and the duration of each grainfilling phase are crucial in determining the final grain weight. Two experiments were conducted to evaluate the effects of ZnSO4 on grain filling in the Sardari wheat cultivar. In the first experiment, the grain-filling rate and grain growth were assessed following foliar application of ZnSO4. The second experiment explored the effects of foliar Azospirillum brasilense, applied either alone or combined with ZnSO4, on grain-filling rate, grain growth, and the duration of both the lag phase and the linear phase of grain filling. Zinc sulfate was applied at a rate of 3 kg ha-1 at the anthesis stage, and only the main stem spike was used for assessing the grain-filling process. Richards equation was employed to simulate grain filling. The results indicated that, compared to foliar Azospirillum alone, Zn application was more effective in enhancing grain growth (by weight) and 1000-grain weight. Foliar application of ZnSO4 significantly increased the grain-filling rate, and ZnSO4, whether applied alone or combined with Azospirillum, extended the lag phase duration. Additionally, the ZnSO4 + Azospirillum treatment sustained higher relative water content in the flag leaves. While the time to reach the maximum grain weight was shorter in untreated plants, ZnSO4 combined with Azospirillum extended the lag phase and grain-filling rate. This combined treatment may benefit wheat production in semi-arid areas where soil drying during the grain-filling period often shortens this crucial phase.
