Objective The photosynthetic response and water metabolism characteristics of P. alba var. pyramidalis leaves were studied, and the correlation between photosynthesis and water metabolism of poplar under pathogen infection was discussed in this study, so as to provide theoretical and experimental basis for the occurrence and control of the poplar Valsa canker disease.

Method Using a girdling-inoculation system, we evaluated gas-exchange and chlorophyll fluorescence characteristics, concentrations of non-structural carbohydrates in root and midday water potential of one-year poplar saplings with Valsa canker disease, and analyzed the relationship between transpiration rate, midday water potential and stomatal conductance, vapor pressure deficit.

Result Compared with girdle control, Valsa canker significantly reduced the net photosynthetic rate (62.45% to 91.05%), stomatal conductance (64.19% to 87.43%), the maximum photochemical efficiency (19.13% to 42.79%), actual photochemical efficiency (4.04% to 69.93%) and electron transport rate (52.58% to 68.03%); also decreased the maximum net photosynthetic rate (76.94%), light saturation point (40.40%) and apparent quantum efficiency (46.09%), and increased dark respiration rate (82.14%) and light compensation point (242.42%). Valsa canker infection significantly decreased soluble sugar (35.06% to 44.50%, 20-30 day) and starch content (35.77% to 58.39%, 10-30 day) in roots. The fungi inhibited leaf transpiration rate (57.36% to 80.49%), water use efficiency (24.92% to 70.55%), and increased water vapor pressure deficit (13.59% to 33.65%) and midday water potential (39.74%, 20 day). The results of correlation analysis showed that transpiration rate was positively correlated with stomatal conductance and negatively correlated with water vapor pressure deficit, and there was no linear relationship between midday water potential and stomatal conductance and water vapor pressure deficit. Stomatal closure caused by Valsa canker infection was not related to leaf water status.

Conclusion The main reasons for the decrease of net photosynthetic rate of poplar leaves were that leaf light energy conversion, photosynthetic electron transport and light energy utilization were hindered. Valsa infection did not cause water stress, even had some improvement. And had an important effect on the carbon accumulation of host roots, leading to the content of non-structural carbohydrates in roots was always at the initial level of infection.