Very cold temperatures will freeze the water inside plants so cells will burst from the expansion of water causing the plants to brown, wilt, and die. Some plants are better at surviving cold than others. Decidious trees will lose their leaves before winter to protect the plant from cold weather. Evergreen trees produce a special anti-freeze chemical that keeps them from freezing during cold weather. Tropical plants do not produce this chemical and many tropical plants will not lose their leaves (some lose their leaves during the dry season because of a lack of water though) and thats why they cant handle cold weather. Many plants are suceptible to freezing weather while they are actively growing. Even trees in cold climates can be killed if they see freezing weather during the summer months when they are actively growing.Hot weather can also kill plants. Some plants such as cactus retain a lot of water and thats how they can handle a lot of dry weather and hot weather. They retain water by keeping all the water in the center of the plant. They cant afford to grow leaves because that exposes the water to more heat. Plants with leaves undergo a process called transpiration. During hot weather plants will close their "air holes" as tight as possible to keep the water inside of them. When this method fails, plants will wilt which you see often during dry weather. If plants become severely dehydrated they will die.