Aplant without enough water may wilt and close its stomata (leaf openings). this decreases the rate of photosynthesis because

the plant no longer is allowing the necessary carbon dioxide required for photosynthesis to enter through the stomata. The plant also needs to be able to release oxygen,the byproduct of photosynthesis through the stomata. The stomata are also responsible for theamount of water inside the plant. By closing when there is not enough water, the stomata prevent loss of additional water due to evaporation.

The higher you go up the less oxygen there is 

Regular or uniform type