Enhanced Stomatal Conductance Supports Photosynthesis in Wheat to Improved NH₄⁺ Tolerance

The impact of ammonium (NH₄⁺) stress on plant growth varies across species and cultivars, necessitating an in-depth exploration of the underlying response mechanisms. This study delves into elucidating the photosynthetic responses and differences in tolerance to NH₄⁺ stress by investigating the effects on two wheat (Triticum aestivum L.) cultivars, Xumai25 (NH₄⁺-less sensitive) and Yangmai20 (NH₄⁺-sensitive). The cultivars were grown under hydroponic conditions with either sole ammonium nitrogen (NH₄⁺, AN) or nitrate nitrogen (NO₃^-, NN) as the nitrogen source. NH₄⁺ stress exerted a profound inhibitory effect on seedling growth and photosynthesis in wheat. However, these effects were less pronounced in Xumai25 than in Yangmai20. Dynamic photosynthetic analysis revealed that the suppression in photosynthesis was primarily attributed to stomatal limitation associated with a decrease in leaf water status and osmotic potential. Compared to Yangmai20, Xumai25 exhibited a significantly higher leaf K⁺ concentration and TaAKT1 upregulation, leading to a stronger stomatal opening and, consequently, a better photosynthetic performance under NH₄⁺ stress. In conclusion, our study suggested stomatal limitation as the primary factor restricting photosynthesis under NH₄⁺ stress. Furthermore, we demonstrated that improved regulation of osmotic substances contributed to higher stomatal conductance and enhanced photosynthetic performance in Xumai25.