Temperatures Too Cool
As a rule, when plants are exposed to temperatures that are too low, the leaves curl down and around themselves. The most recently formed leaves may become colorless. Because of reduced nutrient availability to the slow-growing root system, the old leaves may turn purple. Raising the temperature, particularly at night, is the only way to correct the problem.
Temperatures Too Warm
Generally, plants grown with too much warmth do not maintain a balance between sugars gained from photosynthesis and lost to respiration. Insufficient sugar slows root development, and water and nutrient uptake are reduced. Flower production slows, followed by a rapid loss of the oldest foliage and a paling of the surviving foliage. Lower temperatures, especially at night, restore growth and, eventually, flowering.
Most plants are not sensitive to the temperatures of liquids applied to their foliage or growing media. Although the temperature of a liquid may abruptly shift from that of the plant tissues, the plant temperature will rapidly return to the ambient level without any visible damage. However, some plants are sensitive. African violets often have adverse reactions to the temperature of liquids. When the temperature of a liquid is 10º F. higher or lower than that of the leaf, the chlorophyll is permanently damaged. The plant cells retain their structure, but the leaves are permanently marked with colorless sections.