Impact of reduced irrigation on physiological, photosynthetic, and enzymatic activities in wheat ( Triticum aestivum L.) exposed to water stress at varying plant densities

Drought stress is one of the most critical factors reducing the performance of crop plants in arid and semi-arid regions globally, and understanding underlying mechanisms is important to be able to implement measures for alleviating the negative impacts of drought on yield and yield quality. The present experiment aimed to investigate the effect of drought stress and planting density on the physiological characteristics, photosynthetic pigments, and enzymatic activity of wheat plants exposed to water stress. The experiment was conducted in a factorial arrangement based on a completely randomized block design with three replications. The treatments included irrigation cessation at three levels (control, i.e., full irrigation, irrigation until flowering (IUF), and irrigation until the dough stage (IUD)), and plant density (300, 400, 500, and 600 plants m2). The results indicate that leaf relative water content, soluble sugars, cell membrane stability index, chlorophyll a, b, and carotenoids significantly decreased under IUF, while these parameters increased at higher plant densities. Additionally, the interaction of drought stress and plant density significantly affected leaf proline and flavonoid content, total chlorophyll, and catalase activity. The highest leaf proline content (3.88 mg g1 FW) was observed in IUF and a density of 600 plants m-2, representing a 192% increase compared to the control. Additionally, the highest total chlorophyll content (3.66 mg g-1 FW) was recorded at no-stress conditions at a density of 500 plants m-2. The activity of antioxidant enzymes increased under water stress. Overall, our results indicate that a density of 500 plants m-2 is optimal for maintaining stable growth conditions in wheat in the semi-arid to arid climate of Iran. These findings provide valuable insights to develop agronomic strategies for coping with drought in wheat cultivation, particularly in arid and semi-arid regions.