Photon emission is inherently associated with fundamental biological processes such as cell division, photosynthesis, stress or death of organisms. For demonstrating the potential use of photon emission for monitoring stress effects on plants, we designed and performed a series of experiments about the physiological effects of ozone. The photon emission of wheat (Triticum aestivum cv. Perlo) was recorded by means of a portable single photon counter. The results showed that photon emission of green plants acts as an indicator of ozone stress response and that a portable photon counter can be used successfully for detailed and sensitive in situ investigations of photon emission from stressed plants. This analytical method may complement other techniques used in plant physiology, especially those for studying effects on photosynthesis