Impaired electron transfer accounts for the photosynthesis inhibition in wheat seedlings (<scp><i>Triticum aestivum</i></scp> L.) subjected to ammonium stress

Abstract

No single mechanism can provide an adequate explanation for the inhibition of photosynthesis when plants are supplied with ammonium (NH₄ ⁺ ) as the sole nitrogen (N) source. We performed a hydroponic experiment using two N sources [5 mM NH₄ ⁺ and 5 mM nitrate (NO₃ ^- )] to investigate the effects of NH₄ ⁺ stress on the photosynthetic capacities of two wheat cultivars (NH₄ ⁺ -sensitive AK58 and NH₄ ⁺ -tolerant XM25). NH₄ ⁺ significantly inhibited the growth and light-saturated photosynthesis (A_sat ) of both cultivars, but the extent of such inhibition was greater in the NH₄ ⁺ -sensitive AK58. The CO₂ concentration did not limit CO₂ assimilation under NH₄ ⁺ nutrition; though both stomatal and mesophyll conductance were significantly suppressed. Carboxylation efficiency (CE), light-saturated potential rate of electron transport (J_max ), the quantum efficiency of PSII (Φ_PSII ), electron transport rate through PSII [Je(PSII)], and F_v /F_m were significantly reduced by NH₄ ⁺ . As a result, NH₄ ⁺ nutrition resulted in a significant increase in the production of hydrogen peroxide (H₂ O₂ ) and superoxide anion radicals (O₂ ^•- ), but these symptoms were less severe in the NH₄ ⁺ -tolerant XM25, which had a higher capacity of removing elevated reactive oxygen species (ROS). Thus, NH₄ ⁺ N sources might decreased electron transport efficiency and increased the production of ROS, exacerbating damage to the electron transport chain, leading to a reduced plant photosynthetic capacity.