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 (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 (phi(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 (H2O2) and superoxide anion radicals (O-2(center dot-)), 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.