Well, alcohol is classified as a depressant, based on how it works with the human central nervous system. A depressant is any drug that slows the reaction time, the processing time, between the stimulus and the response. That is why people should not drink and drive, because it increases the reaction time needed to operate the motor vehicle.
I would expect to see a similar situation involving the plant. The photosynthesis reaction involves the reaction of carbon dioxide with water with sunlight to produce glucose and oxygen as a waste product. I think the alcohol would slow down the reaction rate at which glucose was produced in the chloroplasts of the plant's cells. Depending on the alcohol used, the simple alcohols are immiscible in water. After pentanol (5 carbon atom chain), the influence of the carbon chain predominates, and the alcohol is not soluble in the water. My prediction would be it would either slow the rate of photosynthesis, or stop it altogether.