The water-deficit stress of plants or crops is managed using plant temperatures to maximize product quality. The temperature of the target plant is repeatedly measured over a period of interest, and after each measurement, the plant temperature is compared with a predetermined plant setpoint temperature associated with a desired water-deficit stress level, above which temperature the plant is deemed to be in an undesired metabolic state. If the measured plant temperature is greater than the setpoint temperature, and humidity is not restrictive to plant cooling, then irrigation can be triggered to retain the plant within the desired stressed level, as determined by the temperature of the plant. By this method, irrigation can be withheld to increase plant temperature or applied to reduce plant temperature. The temperature of the plant is thus continuously increased and decreased relative to a desired temperature value that is indicative of a desired water-deficit stress.