Figure 24 is another simple experiment illustrating the temporal evolution of fluorescence and PRI over time. The plants start off in the dark at hour zero. Then after an hour, the lights are turned on and there is an almost immediate response of fluorescence and PRI. They very strongly respond downward in correlation with light use efficiency. Then after the light has been on for some time, the experimenters turn the light off. And again, there is a relatively rapid, immediate response of both fluorescence and PRI to that change in light level as the physiology of the leaves are changing.
These experiments show that PRI could be useful for measuring rapid changes that might occur over the course of an entire day or as a plant canopy is going from sun to shade. As these or other environmental variables are changing, we would expect changes in light use efficiency that the PRI could possibly detect.
Using PRI for long-term changes
The idea of using PRI for long term measurements has always been around, but not as many researchers have pursued this area because measuring PRI over long time periods has been somewhat difficult. It usually requires advanced instrumentation to get those narrow wavelengths of light. However, recently there’s been more interest, and the results presented in Figure 25 are relatively new and still being explored.
What researchers have found so far is that over longer time periods, the PRI isn’t just sensitive to the xanthophyll cycle, it’s also sensitive to total carotenoid and chlorophyll content. It’s important to note that the xanthophyll (violaxanthin, antheraxanthin, and zeaxanthin) are a class of carotenoids. So when researchers measure total carotenoid content, oftentimes a large fraction of that is actually xanthophylls. It’s not entirely clear yet whether or not the PRI is responding to just the xanthophylls or total carotenoids. Further research will help clarify that. But researchers have found that there’s a really tight correlation between PRI and the carotenoid to chlorophyll ratio. The carotenoid to chlorophyll ratio can be linked with light use efficiency because the carotenoids are accessory pigments, and they can play a light harvesting role. But a large majority of them, like the xanthophylls, also play a photo-protective role. So when we compare the content of carotenoid to chlorophyll, we can look at the dynamics of that ratio over time and get a sense of how stressed a plant may be. So this could be a way of looking at light use efficiency or photosynthetic efficiency using the PRI but over a broader time period or over broader spatial areas.