Introduction:

This study aimed to evaluate the effects of defoliation, accelerated seed aging, and seed density on the yield and yield components of the Kareem wheat cultivar in the Gonbad region. Defoliation, seed quality, and planting density are among the most critical factors directly affecting wheat productivity. The experiment was conducted during the 2017-2018 cropping season at the research farm in Gonbad.

Method and Materials:

The study employed a factorial design based on a randomized complete block layout with three replications. The experimental treatments consisted of three factors: defoliation at four levels (no leaf removal, removal of all leaves, removal of all leaves except the flag leaf, and removal of all leaves except the flag leaf and its penultimate leaf), accelerated seed aging at two levels (non-aged and aged seeds subjected to 72 hours at 45°C), and seed density at three levels (350, 400, and 450 plants per square meter).

Results:

The results demonstrated that defoliation significantly impacted wheat yield. Reducing leaf area, particularly the leaves closest to the spike, decreased dry matter production and grain yield. Complete leaf removal reduced grain yield from 4125 kg/ha in the control to 3670 kg/ha. This decline was attributed to reduced photosynthetic activity and the inability to produce sufficient assimilates for grain filling. The flag leaf, playing a pivotal role in photosynthesis, had a profound influence. Removing the flag leaf and its penultimate leaf significantly decreased the number of grains per spike and the thousand-grain weight.

Further analysis revealed that lower canopy leaves had a relatively minor impact on yield reduction. These leaves are predominantly active during the early stages of growth and contribute less to dry matter production during the grain-filling period. The findings underscored the critical importance of preserving the flag leaf and its penultimate leaf to maximize wheat performance.

Regarding the accelerated seed aging treatments, seed quality was shown to play a vital role in determining wheat yield. Seed aging significantly reduced performance. Under aged seed conditions, grain yield decreased to 3423 kg/ha compared to 4244 kg/ha in non-aged seeds. This decline was primarily due to reduced germination rates, weaker initial growth, and underdeveloped root and shoot systems. Aging also diminished seed reserves and enzymatic activity, adversely affecting germination quality and seedling vigor.

Seed density also exerted a significant influence on yield. A density of 400 plants per square meter resulted in the highest yield. Lower densities (350 plants/m²) reduced dry matter production due to fewer plants per unit area. Conversely, higher densities (450 plants/m²) increased inter-plant competition for resources, leading to a decline in grain yield. A density of 400 plants/m² provided a balanced trade-off between resource utilization and competition reduction.

The results further highlighted that excessive plant density amplified competition for light, water, and nutrients, suppressing spike growth and development. Meanwhile, insufficient density limited total dry matter production due to fewer plants per unit area. These findings emphasize the importance of optimizing plant density to enhance wheat yield.

The interaction effects of treatments revealed that a combination of severe defoliation, accelerated seed aging, and inappropriate plant densities produced the most detrimental impact on yield. For instance, the combination of complete leaf removal and seed aging resulted in the greatest yield reduction. However, an optimal density of 400 plants/m² mitigated these negative effects to some extent.

Evaluating various treatment combinations underscored the necessity of preserving the flag leaf and its penultimate leaf due to their critical roles in photosynthesis and the provision of assimilates essential for grain filling. Additionally, the use of high-quality, non-aged seeds and appropriate planting densities can minimize the adverse effects of other factors.

Conclusion:

This research demonstrated that the flag leaf plays a crucial role in maintaining yield, with its removal causing a significant decline in productivity. High-quality seeds and a planting density of 400 plants/m² are recommended for this region to optimize wheat yield.