Leave Freeze Test

[mikkel]

just about 2 days ago I did a frost test with different citrus leaves. It is a first test and was very spontaneous without much theoretical forethought, simply because the weather outside was just frosty and the leaves in good condition for such a test.... I want to repeat the test again with more preparation. Ilya send me kindly a paper about this.
All leaves came from outside (around 1°C or maybe even 0°C) into the freezer at -15°C for 15 min. Than they were slowly thawed and brought to room temperature.
I did not hope for much, but I was surprised that the leaves were not really mushy the first day after thawing. Now after 1.5 days they are slowly drying up.
I think I need to compare this once with orange leaves or other more delicate leaves.

Here are some pictures:

after the freeze and 1-2 h after they were brought to room temperature again, barely noticeable damage.

Pt - I P - a Citrange seedling - unknown polyploid rootstock - unknown hybrid

1,5 days after the test

[mikkel]

surprisingly, the citrange leaf still looks pretty good right after the test, which surprises me because the plant froze back outside at -10°C last winter

[till]

Mikkel, I wonder why you froze them for only 15min. That is really not a long time. Can't you repeat the test with freezing of at least 10h? I have not much theory about that but I would have expected that almost any Citrus leave can withstand freezing of -15°C for 15min.
ArgumiLenzi, a nursery in Italy, ascribes to most citrus types a freezing tolerance around -10°C which sounds somewhat fantastic for any citrus grower in the north but may reflect experiences from what Agrumi Lenzi calls short periods in sheldered position. Since he has a pretty impressive catalog and good plants I would at least wonder if his data were completely without a real basis in experience.

[mikkel]

15 min were simply due to my impatience and as I said it was a short-term decision, because the plants were just outside already very cold and thus well prepared.
Probably nothing can be said about the general frost resistance via a short test, my idea was rather to find a relative comparison between the varieties. So which plant tolerates more compared to the others....
The leaves were completely frozen, so I think that even after 15 min all cells were destroyed, which do not tolerate -15°C. Now after 2 days all leaves have dried up, which I attribute to the destroyed cells, the unfrozen comparison leaves have not yet dried up.
But I will perform the test once or several times and better prepare, it remains a little longer frosty. I will also do a long-term test.
Ilya has sent me a paper, I'll gather some more inspiration.
It would also be good if I could adjust the temperature a bit, but for that I would have to talk to our food reserves first....

[till]

Ah, I understand.