CAM and C3 plants show contrasting CO2 and O2 dynamics in the leaf tissues, but the finer details had never been studied due to lack of suitable technology. Our new CO2 microsensor enabled us to unlock the doors to a secret chamber inside the leaves of two submerged aquatic CAM and C3 plants. Our study revealed unexpected oscillations in CO2 throughout a diel (24-hour) cycle.
CO2 levels in tissues of CAM plants have previously only been studied by discrete and destructive sampling of tissue gases. The reason is that CO2 in CAM plants (and also in C3 plants) cannot be measured directly, but is instead inferred from stomatal conductivity. In CAM plants, the stomata are typically closed during the day. Therefore, the technique relying on stomatal conductivity cannot be used. Researchers were left with the only option to extract gases from bulky leaves: using a syringe. This technique, however, offers poor temporal resolution, and also damages the tissues.
To address this problem, we constructed a new CO2 microsensor with a diameter as small as the thinnest of human hairs. We fixed the tiny CO2 and O2 microsensors on a micromanipulator in order to position both sensors in the same leaf on either the CAM or the C3 plant. In this way, we were able to assess the diel oscillations of both gases simultaneously.
In C3 plants, CO2 inside the leaves increased to ~3.5 kPa (75-fold CO2 in air) during darkness. For the CAM plant, CO2 was mostly below 0.05 kPa due to CO2 sequestration into malate. Upon darkness, the CAM plant had an initial peak in CO2, which then declined to a steady-state for several hours. CO2 increased again towards the end of the dark period. In light, leaf CO2 in the C3 declined and O2 increased, whereas both O2 and CO2 increased in the CAM as CO2 was produced from malate!
This new tool in plant biology will likely reveal many more secrets of the plants in the years to come. Similar to the use of O2 microsensors to resolve questions related to plant aeration, deployment of the new CO2 microsensor will benefit plant ecophysiology research. One field that I find particularly interesting to further investigate is what happens when respiratory CO2 accumulates in plant tissues. CO2 produced in cellular respiration during the night has a potential to drive photosynthesis in the early hours of dawn, as indicated by early morning peaks in O2 in rice, as well as in natural wetland plants. The quantitative importance of this CO2 source for photosynthesis can now be studied using the novel CO2 microsensor
About the author
Ole Pedersen is professor in experimental plant ecophysiology at the University of Copenhagen, Denmark. Ole is also an adjunct at the University of Western Australia and at the Florida Atlantic University. In addition to research question related to the ecophysiology of seagrasses, macroalgae and submerged freshwater plants, he works with flood tolerance of natural wetland plants in order to identify traits that may prove useful to incorporate into climate-smart crops.