Abstract: An apparatus and method of determining the optimal plant canopy temperature of a plant by measuring chlorophyll a variable fluorescence is described. Leaf samples taken from the plant are placed on a temperature gradient device, exposed to light for an amount of time, and the variable fluorescence emitted from the leaves is measured along with the temperature. Calculations of Fv/Fo over a period of time are used to determine the optimal plant canopy temperature for a plant or a crop. The apparatus and method can be used to compare specific cultivars, to assess the results of plant breeding programs, and to assist in crop management procedures.