New CO₂ microsensor reveals old secrets of plants

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.