While different cactuses may have specialized photosynthetic techniques, in general, photosynthesis occurs in a cactus' stem or trunk. The concept of photosynthesis in a cactus or succulent plant may seem strange considering that most of them do not have obvious leaves like many other plants, but their stems or trunks serve the same photosynthetic function as some other plants' leaves.
The saguaro cactus is a huge, treelike cactus with iconic branches that stem off its main trunk. Though it doesn't have leaves, these armlike branches and the body-like trunk perform photosynthesis in addition to storing water, providing essential resources for the plant's survival. Though other cactuses may have different parts than the saguaro, the concept is the same across most members of the cactus family.