The climate in deserts is extremely hot and dry. Desert plants are adapted to their environment in the following ways:
1) The leafs of the desert plants are modified into spikes to prevent water loss through leaf (by transpiration). There is already scarcity of water in the desert areas that is why plants can't afford to loose water thorough leafs. So, they modify their leafs into spikes/spines which have less surface area. Less surface are exposed towards sunlight leading to lesser water loss. This is a mechanism to conserve water in hot climate.
2) Desert plants, instead of having pores have thick waxy cuticle in their leafs. This waxy cuticle prevents water loss to the environment and conserves water for the plant.
3) Plants in deserts, instead of opening their stomata during daytime open them in night. Because there is no sunlight and therefore less heat so at that time water evaporation is less. Less water tends to evaporate during that time leading to conservation of water.