The response of photosystem I to fluctuating light is influenced by leaf nitrogen content in tomato

Fluctuating light and nitrogen (N) deficiency can occur synchronously under natural growth conditions. How-ever, little is known about the photosynthetic regulation under fluctuating light in plants grown with N deficiency. In the present study, we examined the effect of N supply on the response of photosystem I (PSI) to fluctuating light in tomato. Plants grown under high N concentration (HN-plants) had higher leaf N content, chlorophyll content and saturating CO2 assimilation rate than plants grown under middle and low N concentrations (MN- and LN-plants). After an abrupt increase in illumination for 10 s, all plants could not generate a sufficient proton gradient (Delta pH). Meanwhile, PSI was over-reduced in HN-and MN-plants but was highly oxidized in LN-plants. In LN-plants, the smallest PSII electron flow avoided an over-reduction of PSI under fluctuating light. After transition from low to high light, CEF gradually increased to the peak in 30 s in HN-plants but rapidly increased to the peak in 10 s in MN-plants. Such delayed activation of CEF in HN-plants accelerated the over-reduction of PSI. After fluctuating light treatment, HN-plants displayed the greatest PSI photoinhibition, followed by MN-and LN-plants. These results indicated that leaf N content significantly affected the response of PSI to fluctuating light in tomato.