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

Abstract

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.

Keywords: ammonium stress; osmatic potential; photosynthesis; stomatal conductance; wheat.

Figure 1

Effects of ammonium stress on (A) net photosynthetic rate (A), (B) stomatal conductance (g_s), (C) leaf mesophyll conductance (g_m), (D) sub-stomatal CO₂ concentration (C_i), (E) transpiration rate (Tr), and (F) stomatal limitation (l) of wheat seedlings at 0, 1, 3, 5, and 10 days after treatment. NN, nitrate treatment; AN, ammonium treatment. YM20, Yangmai20 (NH₄⁺-sensitive); XM25, Xumai25 (NH₄⁺-less sensitive).

Figure 2

Effects of ammonium stress on the (A) carboxylation efficiency (CE), (B) electron transfer rate (J_e), (C) maximum carboxylation rate (V_max), and (D) the maximum electron transport rate (J_max,) at 0, 1, 3, 5, and 10 days after treatment NN, nitrate treatment; AN, ammonium treatment. YM20, Yangmai20 (NH₄⁺-sensitive); XM25, Xumai25 (NH₄⁺-less sensitive).

Figure 3

Effects of ammonium stress on (A) total chlorophyll content, (B) actual photochemical efficiency (Y(II)), (C) maximum quantum yield (Fv/Fm), and (D) non-photochemical quenching (NPQ) of wheat seedlings at 0, 1, 3, 5, and 10 days after treatment. NN, nitrate treatment; AN, ammonium treatment. YM20, Yangmai20 (NH₄⁺-sensitive); XM25, Xumai25 (NH₄⁺-less sensitive).

Figure 4

Effects of ammonium stress on the (A) relative water content (RWC), (B) leaf osmotic potential (ψs), (C) pre-dawn water potential (ψp), and (D) midday water potential (ψm) of wheat seedlings at 0, 1, 3, 5, and 10 days after treatment (DAT). NN, nitrate treatment; AN, ammonium treatment. YM20, Yangmai20 (NH₄⁺-sensitive); XM25, Xumai25 (NH₄⁺-less sensitive).

Figure 5

Effects of ammonium stress on (A) K⁺, (B) sucrose, and (C) ABA concentration of wheat seedlings at 5 and 10 days after treatment. Different letters indicate significant differences (p < 0.05) according to ANOVA. NN, nitrate treatment; AN, ammonium treatment. YM, Yangmai20 (NH₄⁺-sensitive); XM, Xumai25 (NH₄⁺-less sensitive).

Figure 6

Effects of ammonium stress on the expressions of genes encoding K⁺ channel, water channel, and proton transport protein in leaves and roots of wheat seedlings at 5 days after treatment, including (A) TaHA1, (B) TaAKT1, (C) TaKOR1, (D) TaKAT1, (E) TaTIP2.3, (F) TaPIP1.1, (G) TaAKT1. Data are expressed as means of three biological replicates, and different letters indicate significant differences (p < 0.05) according to ANOVA. NN, nitrate treatment; AN, ammonium treatment. YM, Yangmai20 (NH₄⁺-sensitive); XM, Xumai25 (NH₄⁺-less sensitive).

Figure 7

Principal component analysis (PCA) of photosynthetic parameters and other physiological traits of two wheat cultivars (Yangmai20 and Xumai25) under AN and NN treatments. NN, nitrate treatment; AN, ammonium treatment.