Time-scale for accurate measurements in chambers
Technical Support and Questions
Saket Navlakha
Mar 2016
The last of my current barrage of questions :)
We are doing growth experiments in chambers. I assume the most accurate measurements will come if we measure The One (Phi2, PhiNPQ, PhiNO) while the plant is in the chamber. But let's say we remove the plant from the chamber, and then measure these values. How long can we wait before we start to get highly different measurements? Would this happen within seconds? Minutes? Perhaps some of this is plant-dependent; we are mostly working with tomato and tobacco plants.
Thanks again.
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Mar 2016
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Greg Austic
Mar 2016
No worries, Saket, they are good questions!
Yes, The One v3.0 is the best for reasonably quick but accurate photosynthesis measurements and Phi2, PhiNPQ, and PhiNO are the parameters to watch. We see they often correlate to yield or stress.
In terms of chambers, there are a few things to be aware of:
Due to the fact that the light source is relatively close to the plants, even a small change in plant height can produce a change in photosynthesis. In the examples I'm aware of (Jesse Traub's bean trials), a leaf which was a few inches higher than another produced a difference of 50uE in PAR... which of course produces a corresponding difference in photosynthesis. So it's important to understand the effect of plant height on photosynthesis in chambers. This can cause relationships between your samples in your experiment which are not truly based on photosynthesis. This is important to note and try to design around.
PhiNPQ and Phi2 will change very very quickly (in milliseconds) so removing the plants from their native environments is not preferable. Furthermore, the measurement takes the ambient PAR value and recreates that ambient intensity in the MultispeQ clamp in order to properly measure phi2 and the other related parameters. So if you take the leaf out of the chamber, PAR will drop and the measurement will be affected for this reason as well. So if at all possible, you should measure them in their normal location and leaf position. If that's not possible, then you could measure Fv/Fm, and simply always dark adapt the plants (in the complete dark) for 10 minutes before taking the measurement. This type of measurement is potentially interesting and in some ways easier to analyze (since light acts as a confounding factor which needs to be accounted for later using linear regression or similar). However, it's not what we typically do as the general use case in the field where dark adaptation isn't desirable.
I hope that's helpful!
Saket Navlakha
Mar 2016
Very helpful, thank you!
Might the SPAD III (Chlorophyll Content) measurements be less sensitive to changing light? Has this measurement also been shown to correlate with yield or stress?