Steady-state chlorophyll fluorescence can show how well plants are photosynthesizing and dealing with drought. When plants are stressed by lack of water, their fluorescence changes, especially at midday. By measuring this fluorescence, we can tell how stressed the plants are and how much water they need. The ratio of fluorescence to dark-adapted intrinsic fluorescence is linked to how well the plant is using light and carbon dioxide. This ratio can be measured remotely to quickly detect water stress and help decide when to water plants.