A solution for bulk photosynthetic efficiency screening

Sometimes strange ideas come into my head. Most of the time, being too busy to act on them, I shoo them away and get on with my life. On this one occasion, though, we had a continuous metabolic measurement system already set up, and were about to use it for measuring the metabolic rate of a rat. (There's an interesting story behind that, but that's for another time.)  So, for once, with the help of Thomas Förster (Senior Research Scientist at Sable Systems, which is to say he actually made the measurements and did all the hard work), I thought: What about using the system to measure photosynthesis?

Well, the system wasn't specifically designed for measuring photosynthesis, but adapting it to do so was trivial. Because it's a pull system, it doesn't require a sealed chamber around the plant, so we could simply go down the road, purchase a few small decorative potted plants, cover them with simple transparent plastic bags, and place the input port of each channel of the system close to the top of each bag. You can see the tubing lengths inside the plastic bags in the photograph to the left. Because the system operates at a high flow rate of 2 L per minute, air is drawn up from the bottom of the bag, and all of the air that has been in contact with the plants is drawn into the tubing. From there, it is pulled into an accurate mass flow controlled pump, and then a subsample of the air is directed into an analysis chain, comprising a water vapor analyzer, a CO₂ analyzer and an O₂ analyzer. Above the plants, we installed a fluorescent grow light that was attached to a timer which turned it on and off at a fixed interval. We were measuring light levels, so it was easy to tell from the recordings when the light was on or off.

As you can read here, the respirometry system that we were using provides essentially continuous, uninterrupted measurements on multiple channels simultaneously.

And here are the results! The top trace shows CO₂, with brown denoting CO₂ production and green denoting CO₂ consumption, i.e. photosynthesis. You can see that the system rapidly shifts from CO₂ production to CO₂ consumption when the light is turned on, and also very rapidly changes from CO2 consumption to CO₂ production when the light is turned off. The dark periods are shaded. Data from just one plant are shown. Click on the graph^* to enlarge it.

The water vapor data are particularly interesting. They are on the bottom half of the graph. As you can see, there is a  strong inverse relationship between CO₂ consumption and water vapor output, caused by the fact that the plant's stomata were gradually opening wider and wider throughout the period of illumination. The effect was not caused by a slow time constant, as you can see from the immediate and drastic decrease in CO₂ production, switching to CO₂ consumption, as the light phase began. O₂ (not shown) obediently followed the inverse kinetics of CO₂. Needless to say, we used no chemical or thermal desiccants; all water vapor dilution compensation was performed mathematically.

Of course, using this system to screen plants for photosynthetic and water use efficiency in practice - as opposed to for amusement, as here - would require knowledge of the plants' leaf areas, and of the luminous intensity to which they are exposed. Neither is difficult to achieve.

Just for fun, I included this graph in a talk that I gave at the Society for Integrative and Comparative Biology in New Orleans earlier this year. A plant physiologist happened by accident to be in the audience, and afterwards, we had some interesting conversations. The ability to obtain data of this quality from essentially unlimited numbers of plants in real time and effectively without interruptions is, shall we say, not without interest to certain parties.

All of which goes to show that indulging in unbridled curiosity and curious whims can lead to interesting places. I commend it for your consideration.

If you have any questions regarding this post, feel free to contact me directly.

 * Thanks to Thomas Förster, Ph.D., for making the measurements and creating the graph.