How hot do cacti get?

[ralphrmartin]

I've been playing with a thermal camera in my greenhouse. Its a device which takes pictures, but in which the colour indicates the temperature. It also tells you how hot the hottest point in the picture is, and how hot the centre spot is (accuracy is 2 degrees ± 2%). These cameras have quite a low resolution (unless you pay an arm and a leg), but I thought you might like to see these results, taken today, in the afternoon.

A view inside the seed propagator, where its nearly 50C in the shade, and over 60C in the bright sun. It's amazing what a roasting seedlings can take.

A Rebutia, where the crown of the plant is well into the 50s.

A large Copiapoa krainziana, also well into the 50s.

[el48tel]

OOOOOH that looks a nice bit of kit .... do tell us more

[Stuart]

It would be interesting to compare the greenhouse results with results of plants taken in habitat.

Stuart

[Cidermanrolls]

Interesting pics Ralph, but I wonder how trustworthy this camera really is.
For example, why is the stone around that Rebutia quite so hot? 60c is decidedly toasty and that stone looks pale in colour. I wouldn’t expect it to be so hot.
I don’t know the conditions in your greenhouse, but I would be looking at those images with a dose of scepticism and asking what interferences could give skewed results. For example, could spines that reflect IR light fool the camera?

[Phil_SK]

I'm struggling to understand the cool buds on the middle photo, making me a little wary of over-interpretation.

[Pattock]

For example, could spines that reflect IR light fool the camera?

That was my thought. Not just spines but the surface of the stem is probably also selected to reflect IR effectively.

I had an IR thermometer and it gave some very strange readings when pointed at complex surfaces.

It would be easy to check by putting the cactus in a black box so there is no IR illumination.

[ralphrmartin]

It would be interesting to compare the greenhouse results with results of plants taken in habitat.

I'll take the gadget with me, if and when I next go...

Its a UTi260b

[ralphrmartin]

I certainly believe the 60C in the propagator as last year I put a thermometer in there (in the shade) and it read over 60C.

You can adjust it for emissivity of different surfaces, but I would expect the metal surfaces to be hotter than the plants.

Basically, these devices are arrays of infrared sensors.

I dont think spines will fool the camera. Its pixels are are far too big to be picking up individual spines.

[ralphrmartin]

Flowers might be cooler, if they are losing water (although I'd imagine all stomoata ought to be shut at this time of day).

[Pattock]

The nature of NIR reflectance varies for different
plant species. The magnitude of the increase in
reflectivity and the wavelength at which the increase
occurs both vary for different vegetation types.
Deciduous vegetation on Earth causes an increase in
reflectance in the NIR from approximately 5% to
50% when observed alone. For non-vascular plants
like mosses, this reflectance peak can be as low as
20%, whereas for desert plants like cacti, it can be
>80% (see Fig. 2).

Note that the reflective spines on some cacti can scatter
light into the column of light in which reflectance is being
measured, giving the appearance of >100% reflectance
when observed alone.

Jack T. O'Malley-James and Lisa Kaltenegger "The Vegetation Red Edge Biosignature Through Time on Earth and Exoplanets" Astrobiology 18(9): 1123-1136
https://arxiv.org/ftp/arxiv/papers/1809/1809.08832.pdf