Killing plants for fun and science

In getting ready for our trip to the Open Hardware Summit, we wanted to create some short wizbang experiments to show what Photosynq can do. These experiments are focused around measuring pulse modulated fluorescence (summary), which can distinguish between stuff that’s has active photosynthesis (like plants and things that are alive) from stuff that just fluoresces (like white paper and laundry detergents). We also want to create more experiments to show off the CO2 sensor and other stuff, but this is what we’ve got so far.

(Sebastian Kuhlgert and Kent Kovac from Kramer Lab came up with experiments, so thanks!)

Surprise – it’s alive!

Fruits and Veggies… alive or dead?

We wanted to see if fruits and veggies were 1) photosynthetically active at all (meaning, they were absorbing light and doing something with it), and 2) active even after they’ve been picked (sometimes many days after. Sebastian went and got an old pepper and a fresh pepper, an apple, some grapes, and some spinach and we ran some tests. The ‘how alive is it?’ value is called Phi(II) (this is the proportion of light absorbed by PSII which is used for photochemistry – you can read more in depth about chlorophyll fluorescence and ways to quantify it here). The ‘amount of fluorescence’ value is the absolute fluorescence response, which relates to the quantity of chlorophyll (it’s a relative value, not absolute, and it’s not perfect and we’re working on making it better – but let’s work with it for now). Here’s the results:

Blue bars are Phi(II) (ie how efficient is photosynthesis) and red lines are how much chlorophyll is there (these are relative values only)
Blue bars are Phi(II) (ie how efficient is photosynthesis) and red lines are how much chlorophyll is there (these are relative values only)

As expected, the new pepper was more photosynthetically efficient than the old one… but the old one had a higher chlorophyll concentration which was surprising. The grape was reasonably efficient, but didn’t have much chlorophyll (you could have guessed that – grapes aren’t very green and they are mostly water). I was surprised that the spinach didn’t have the highest levels of chlorophyll – it was certainly very green, though it is relatively thin compared to a pepper skin so perhaps that accounts for the difference.

Simple test and simple results, but they raise a lot of questions that even hardened plant scientists will hem and haw about!

Spinach is alive, but chlorophyll by itself is just another protein

Most of plant fluorescence comes from chlorophyll, which is the main component of the antennae which gather light. However, just because something fluoresces doesn’t mean it’s alive! In this experiment, we took a spinach leaf and measured Phi(II) and our relative measure of chlorophyll concentration. Then we mashed up the leaf and mixed it with 80% ethanol (you could use rubbing alcohol or nail polish remover). Then we poured the mixture through a paper towel (coffee filter would also work) so that only a clear green liquid remained. The alcohol removed the chlorophyll and the sieve prevented any whole cells from getting through – so we’re left with a chemical soup containing lots of chlorophyll.

So – what happens if we compare Phi(II) and chlorophyll concentration on the original leaf versus the alcohol/chlorophyll solution?

The spinach leaf is still using the photons to do useful work... the chlorophyll isn't, though they both produces roughly the same fluorescence
The spinach leaf is still using the photons to do useful work… the chlorophyll isn’t, though they both produces roughly the same fluorescence

As you’d expect – the spinach leaf is still using the photons to do useful work (high Phi(II))… but the chlorophyll/alcohol solution isn’t (near zero Phi(II), though they both produces roughly the same fluorescence (red line). This is relevant because fluorescence measurements from satellites can measure absolute fluorescence values, but not photosynthetic efficiency – so it’s valuable to have something like Photosynq to provide additional information on the ground.

Hearing plants cry “Heeeeeelp!”

Plants can’t talk, so when they are stressed (not enough water, too much salt, too hot, too cold, etc.) it’s sometimes hard to tell. Photosynthetic efficiency (Phi(II)) is a good indicator of stress (there’s many papers describing this, see here, here, and here for a few), so it’s a good identified of when plants are in trouble.

So we picked two prickly lettuce plants from the garden and stuck them in water. We added a bunch of salt to the water of one plant, and measured Phi(II) and relative chlorophyll content in both plants over a few days. Here’s the before and after:


In the beginning, there were two prickly lettuce plants…

but Greg and Sebastian smote one with salt!

The results show that not only was the plant wilting, but it’s photosynthetic efficiency was dropping too. It’s like if someone made you drink a bottle of absynthe and then go play in a soccer game – your performance would suffer!

The salt-treated plant (blue line) sees a big drop in photosynthetic efficiency over a couple days
The salt-treated plant (blue line) sees a big drop in photosynthetic efficiency over a couple days

In some ways, this experiment could be a lot more interesting – you can clearly see the plant leaves wilting, so what’s the point of taking the additional measurement with Photosynq, right? In this case that’s true, but out in the field it’s not always obvious that plants are under stress and the visual effects of stress (wilting, loss of color, slow growth) may be delayed by several days. Measuring photosynthetic efficiency directly will identify stresses much sooner.

We’ll have all these experiments plus (if we can get our act together) a few more by OHS on Friday. If you are going, come visit our demo!

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