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

No single mechanism can provide an adequate explanation for the inhibition of photosynthesis when plants are supplied with ammonium (NH 4 + ) as the sole nitrogen (N) source. We performed a hydroponic experiment using two N sources [5 mM NH 4 + and 5 mM nitrate (NO 3 − )] to investigate the effects of NH 4 + stress on the photosynthetic capacities of two wheat cultivars (NH 4 + ‐sensitive AK58 and NH 4 + ‐tolerant XM25). NH 4 + significantly inhibited the growth and light‐saturated photosynthesis ( A sat ) of both cultivars, but the extent of such inhibition was greater in the NH 4 + ‐sensitive AK58. The CO 2 concentration did not limit CO 2 assimilation under NH 4 + 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 NH 4 + . As a result, NH 4 + nutrition resulted in a significant increase in the production of hydrogen peroxide (H 2 O 2 ) and superoxide anion radicals (O 2 •− ), but these symptoms were less severe in the NH 4 + ‐tolerant XM25, which had a higher capacity of removing elevated reactive oxygen species (ROS). Thus, NH 4 + 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.