Each plant is adapted to a range of temperatures. When the temperature gets too high for its tissues, they are stressed, then finally die.
How Plants Heat Up
Plants get their heat from two sources, usually both at once. The first source of heat is the air that surrounds them. Air temperature is a good indication of plant temperature, but leaves can be warmer or cooler than air temperatures. The second source of heat is the radiation they receive. Plants in the sun can absorb enough heat from the sun’s rays to warm them to a temperature much higher than the air.
Heating from the sun is very local and specific. Leaves that are perpendicular to the sun’s rays heat up more than those at an angle to it, and the part of a curved leaf that is most perpendicular heats up most. When leaves burn from too much sunlight, it is usually due to the heat carried by the sunlight, and not from the light it carries.
Too much light can bleach the chlorophyll from a leaf not adapted to full sun, but usually doesn’t kill the tissues. The tissues are killed by overheating from the energy in the sunlight. The injured spot is usually in the part of the leaf that faced the sun late in the afternoon, when the air temperature is highest. It is usually on the southwest side of the plant, where the sunlight is at that time of day.
Radiant heat can also be used to warm plants in greenhouses, or outside when early or late frosts threaten. Even a single light bulb hung in a tender tree might keep it from freezing on a cold night.
How Plants Keep Cool
Plants keep cool by transpiring water. Almost all the water that enters their roots transpires from their leaves; only a small amount is incorporated into the plant. Transpiration cools a plant just as wearing a wet shirt cools a human. This is why it’s so important to keep plants watered on hot days. A plant can go without water for several days in cool weather, but can be killed from overheating in just a few hours on a hot day.
The American Horticultural Society has published a map of heat zones for the United States. For more information, see The Heat Zone Map.