As hinted in other answers, the key problem with a pure oxygen environment would be the inability to photosynthesise caused by the absence of carbon dioxide.
The overall equation for photosynthesis (the process in which energy from the sun is harnessed to fix inorganic carbon dioxide into organic molecules that are of use to the plant) is:
6CO2 + 6H2O → 6O2 + C6H12O6
The carbon dioxide required for photosynthesis is obtained from the atmosphere by diffusion through stomata on the leaves. Therefore if the atmosphere contains no carbon dioxide (i.e. in the scenario you suggest) photosynthesis would not progress, meaning glucose used by the plant in respiration would not be replaced.
This would eventually lead to the cessation of cellular respiration due to a lack of reactant. This would cause the plant to die.