The Effects of Different Nitrogen Forms on Chlorophyll Fluorescence and Photosystem II in Lonicera japonica

Plant nitrogen (N) utilization determines the photosynthetic capacity and functionality. This study investigated the response of Lonicera japonica plant growth parameters, leaf gas exchanges, light response curve (P-N/I), chlorophyll fluorescence, and chlorophyll a fluorescence transient (OJIP) to different N sources: ammonium (NH4+), nitrate, and mixed N sources through hydroponic experiment. Furthermore, the commonly used mathematical models and jip-test were adopted to analysis the P-N/I and OJIP curves, respectively. Here, plant biomass and photosynthetic rate significantly reduced under NH4+ treatment. The photoinhibition was more pronounced under treatment NH4+ compared with the other treatments, evidenced by simultaneous decrease of light-saturated point, photochemical quenching, and photochemical efficiency. Together with the result from P-N/I and OJIP curves, we found that: (1) a rapid decrease of photosystem II effective light absorption cross-section and pigments in the lowest excited state was observed in NH4+ supply with the increase of light intensity, and thereby the decreased light absorption resulted in downregulated electron transfer efficiency. (2) The photoinhibition appeared as damages to the function of oxygen-evolving complex and electron transfer beyond plastoquinone pools, resulting in the decreased photochemistry activity and upregulated heat dissipation efficiency. In conclusion, NH4+-N supply reduced photosynthetic performance and induced the photoinhibition of L. japonica, which can be partly interpreted by the decreased light absorption ability, the damage of oxygen-evolving complex, and the blocked electron transfer during the photosynthetic light reaction process.