Water stress detection by monitoring fluorescence of plants under ambient light

Plant fluorescence is inherently weak, and it is difficult to detect and measure under broad–spectrum ambient light. The relative referencing method is a technique developed to acquire a plant’s fluorescence signal in the presence of ambient light. This study evaluated the method’s ability to detect early signs of water stress in freestanding tomato plants as the plants were continuously illuminated by fluorescent lamps. Water stress was initiated by imposing drought conditions, and the subjects were tested daily over a seven–day period. Transpiration, chlorophyll content, and water status data were taken to quantify physiological changes in the plants. The relative referencing method was able to monitor early signs of water stress, but the results differed from those previously reported. The percent overshoot parameter correlated well with the early onset of water stress, but the time to peak parameter correlated poorly. The main difference between this and the previous study was that freestanding plants were tested, which affected the amount of incident light on the leaves during testing. In addition, younger plants were used, which had an effect on the fluorescence response. In spite of these differences, the results showed that the relative referencing method was able to detect early signs of water stress in free–standing tomato plants.