A method of detecting the onset of stress in chlorophyll-containing matter caused by said chlorophyll-containing matter being exposed to a stress inducing environmental condition, the method comprising: (a) exposing the matter to light to cause chlorophyll in the matter to fluoresce and emit a fluorescence signal, wherein exposing the matter to light comprises exposing the matter successively to at least three different levels of photon flux, each different level being below that required to stimulate a maximal fluorescence signal, (b) detecting the emitted fluorescence signal, (c) measuring the value of a parameter based on the detected fluorescence signal, wherein measuring the value of said parameter comprises measuring the intensity of the fluorescence signal at each of at least three different levels of photon flux to which said chlorophyll-containing matter is exposed, each different level of photon flux being below that required to stimulate a maximal fluorescence signal in said chlorophyll-containing matter, and wherein the value of said parameter is derived from the relationship between the level of photon flux to which said chlorophyll-containing matter is exposed and said at least three measured fluorescence intensities each measured at a different level of photon flux below that required to stimulate a maximal fluorescence signal in said chlorophyll-containing matter, (d) monitoring the value of said parameter by periodically measuring the value of said parameter for the same chlorophyll-containing matter, wherein, during said monitoring, said chlorophyll-containing matter is exposed to said stress inducing environmental condition, (e) detecting changes in the monitored value of said parameter, (f) comparing changes in the monitored value of said parameter with a predetermined level of change in the value of said parameter, and (g) interpreting a change above said predetermined level of change in the monitored value of said parameter as an onset of stress in the chlorophyll-containing matter caused by said stress inducing environmental condition. (a) exposing the matter to light to cause chlorophyll in the matter to fluoresce and emit a fluorescence signal, (b) detecting the emitted fluorescence signal, (c) measuring the value of a parameter based on the detected fluorescence signal, wherein measuring the value of said parameter comprises measuring the intensity of the fluorescence signal at each of at least three different levels of photon flux to which said chlorophyll-containing matter is exposed, each different level of photon flux being below that required to stimulate a maximal fluorescence signal in said chlorophyll-containing matter, and wherein the value of said parameter is derived from the relationship between the level of photon flux to which said chlorophyll-containing matter is exposed and said at least three measured fluorescence intensities each measured at a different level of photon flux below that required to stimulate a maximal fluorescence signal in said chlorophyll-containing matter, (d) monitoring the value of said parameter by periodically measuring the value of said parameter for the same chlorophyll-containing matter, wherein, during said monitoring, said chlorophyll-containing matter is exposed to said stress inducing environmental condition, (e) detecting changes in the monitored value of said parameter, (f) comparing changes in the monitored value of said parameter with a predetermined level of change in the value of said parameter, and (g) interpreting a change above said predetermined level of change in the monitored value of said parameter as an onset of stress in the chlorophyll-containing matter caused by said stress inducing environmental condition.