Effects of Nitrogen and Phosphorus Imbalance on Photosynthetic Traits of Poplar Oxford Clone Under Ozone Pollution

Ozone (O) pollution and the availability of nitrogen (N) and phosphorus (P) in the soil both affect plant photosynthesis and chlorophyll (Chl) content, but the interaction of O and nutrition is unclear. We postulated that the nutritional condition changes plant photosynthetic responses to O. An O-sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha; N80, 80 kg N ha), two P levels (P0, 0 kg P ha; P80, 80 kg P ha) and three levels of O exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) over a growing season in an O free air controlled exposure (FACE) facility. The daily change of leaf gas exchange and dark respiration (R) were investigated at mid-summer (August). Chl a fluorescence was measured three times in July, August and September. At the end of the growing season, Chl content was measured. It was found that Chl content, the maximum quantum yield (F/F), Chl a fluorescence performance index (PI) and gas exchange were negatively affected by elevated O. Phosphorus may mitigate the O-induced reduction of the ratio of photosynthesis to stomatal conductance, while it exacerbated the O-induced loss of F/F. Nitrogen alleviated negative effects of O on F/F and PI in July. Ozone-induced loss of net photosynthetic rate was mitigated by N in medium O exposure (1.5 × AA). However, such a mitigation effect was not observed in the higher O level (2.0 × AA). Nitrogen addition exacerbated O-induced increase of R suggesting an increased respiratory carbon loss in the presence of O and N. This may result in a further reduction of the net carbon gain for poplars exposed to O.