Abstract: In the present study，Effect of infection by Cylindrocladium canadense on behaviors of
photosystemⅠ（PSⅠ）and photosystemⅡ（PSⅡ）in tree peony（Paeonia suffruticosa‘Zangzhihong’）
leaves was estimated by simultaneously measuring their Chlorophyll fluorescence transient，light
absorbance at 820 nm，gas exchange and hydrogen peroxide（H₂O₂）content. The data showed that the net
photosynthetic rate（P n），stomatal conductance and chlorophyll content were significantly decreased
compared with that of the control after infection by C. canadense，whereas intercellular CO₂ concentration
was significantly increased. These results indicated that the decreased photosynthesis in infected leaves
was resulted from non-stomatal factor. Furthermore，significantly enhanced H₂O₂ content，reduced Fv/Fm
and PIABS，as well as the changed chlorophyll fluorescence transient was also observed in leaves of tree
peony infected by C. canadense. The donor sides（Oxygen evolution complex），reaction centers and
acceptor sides of PSⅡ were significantly inhibited by C. canadense infection. But the acceptor sides were
more suffered than the donor sides. At the same time，the activity of PSⅠ was also significantly reduced.
Taken together，the data presented here indicated that C. canadense infection resulted in the accumulation
of H₂O₂，which in turn damaged the functions of PSⅠand PSⅡ. The decline of PSⅠ activity inhibited
transportation of electrons from PSⅡ to PSⅠ，which then led to the accumulation of excessive excitation
energy and reactive oxygen species（ROS）. The increased ROS Inhibited synthesis of protein D1，
accelerating damage to PSⅡ. It was the main reason for inhibition of photosystems in tree peony leaves
after infected by C. canadense.

Key words: tree peony, Cylindrocladium canadense, PSⅠ, PSⅡ, reactive oxygen species