Millet, thank you, your support is invaluable as always.
You gave us nice decription of cold tolerance for citrus in general showing that many parameters are involved.
That is the starting point which should be refined. Method usually used dealing with composite problems is 'Divide & Conquer' http://en.wikipedia.org/wiki/Divide_and_conquer_algorithm or one can call that simply 'Peeling An Onion' - dealing with one problem at a time while minimazing effects of other parameters as much as possible.

Lets take last mentioned article "Cold tolerance and freezing point of citrus seedlings" by L.F Jackson and J.F. Gerber as example of scientific report of targetted research, specifically of 'Pineapple' orange seedlings. One of the results is "...it was further reasoned that 19.5° +- 2° F. should kill 95 percent of the leaves found in the lot of plants used." That result do not falls from sky, they gave detailed info how they got to that.

On contrary, on many places one can read "Meyer Lemon is cold hardy down to -5C", a number like fallen from sky. I'm not aware of any such 'place' which backups claim with reference to scientific research.

Why I need that?
-------------------

Let me first to say more about my motivation and lets go back to the thread "50y old lemon trees of unknown variety"
link
where i wrote:
"Cold Hardiness
Owner says that the trees should be cold hardy down to -7 degree Celsius.
"
Current owner is not the first owner. He got the trees 10 years ago when the first owner became too old to take care of them. The first owner passed away several years later. I had no oportunity to meet him. "-7C" is information verbally passed to current owner.

*** If so, that is the hardiest lemon tree known. ***

This thread is devoted to research of Cold Tolerance Of Siscia Lemon (ex XXX Lemon, name has been recently announced here: link )

This research is not 'scientific' in the sense of above mentioned article, I like to think about it as 'Field Research'.
My task is to refine "-7C" as much as possible for single w/o professional equipment.
My hope is that observation and information gathered my way will be interesting enough to others to join me on this project one day.

And cold tolerance is not the only interesting characteristic of this lemon tree. It is a long term project.
Most cited articles, mother trees and myself have something in common - we are 50 years old. I like that.

Lets 'shake' perspective somewhat.
My roots belongs to Computer Sciences where we make heavy use of KISS principle, "KeepItSimple,Stupid!" http://en.wikipedia.org/wiki/KISS_principle

1st KISS application here is photo "Aftermath, March 2010" showing effects of cold on 2 branches, PT Siscia and Siscia Lemon branch.

2nd KISS application should be comparing "Statistic for the main period of cold attack" with something. That 'something' is missing here.
Having that 'something' for example MeyerLemon related, one could say "Wow, Siscia Lemon could be hardiest than Meyer L., officialy the hardiest lemon tree known so far. That is interesting, lets join efforts and refine that further..."

FieldResearch is more-or-less one man job, to make sense of that such work should be presented to others in 'acceptible form'. What 'acceptible form' means is not yet clear.

Several other conditions that affects what temperature a citrus tree will be damaged or killed by a freeze.

Insects: Trees affected by white-fly, scale or other insects suffer to a far greater degree in a cold period than those free from these pests. A rough estimation of the difference in favor of trees free from insects wold be four or five degrees: it may be more, it certainly is not less.

Food: Citrus trees are in much better condition to withstand cold when well supplied with food, so that they form healthy vigorous shoots and well developed leaves, than when they are starved or only poorly fed. Nitrogen sufficient to give healthy dark green color is essential. The trees must not be over stimulated, however, else the sappy immature wood suffers severely, but enough well balanced food should be available to keep the trees healthy.

DISEASE Diseased conditions, brought about by die-back, rot, or other disturbing factors, either of a physiological or a fungous nature, can reduce the resistant power of the tree as much as 5 or 6 degrees.

If the different kinds of citrus commonly in cultivation are arranged in order of hardiness, the sequence would be about as follows: (1) Trifoliate orange, (2) Satsuma, (2) Kumquat, (2) Calamondin, (3) Sour orange, (4) Mandarin, (5)Sweet Orange, (6) Grapefruit, (7) Lemon, ( Citron and Shaddock.

Miilet, that is nice add.
Are all those temperatures given in degrees Fahrenheit ?

No, this is sorting numbers.

Sylvain, is your answer based on the last sentence of Millet's post?
"... the sequence would be about as follows: (1) Trifoliate orange,..."
Posted question is not related to that.

The question is related to:
"...four or five degrees: it may be more, it certainly is not less. "
"...can reduce the resistant power of the tree as much as 5 or 6 degrees. "

It was about :
"(1) Trifoliate orange, (2) Satsuma, (2) Kumquat, (2) Calamondin, (3) Sour orange, (4) Mandarin, (5)Sweet Orange, (6) Grapefruit, (7) Lemon...".

Ivica, all temperatures are in F. Sorry, I should have made my post clearer. = Millet (747-)

Growth Season 2010
----------------------------

Inspecting a year old graft for incompatibilities...

Seems good to me.

June 2010, 2nd growth on the way


More grafts added

1,2 in May.
3,4 in June, using immature (not yet rounded) budwood.
'C' is a year old Calamondin graft.

New growth


Fertilization/Watering Issue
-------------------------------
A way to strong application of home-made fertilizer in July and even worse, summer watering with fish pool water (too high pH) resulted with nutrients uptake problem.
Symptoms: Siscia Lemon


Symptoms: Calamondin


Consequence for the host tree (PT Siscia) was:
not much of late summer growth and smaller fruits.

I was unaware of how high pH in fish pool water can be:
http://aquanic.org/management%20practices/documents/coppersulfatephosphateissue.pdf

*** I should buy multimeter! ***

2010-12-09
-------------
Weather will change today, it's time to check cold frame setup.
Still nice and pleasant morning, temp about 15C, so I took a few photos.

Growth season lasts here about 6 months. The tree had 3 growths and 4th one is like frozen-in-time since the first frosts (last week of October) when the lowest temp was -3C at 2m, -6C at 5cm. At that event the tree had partial cover (northern side and top only) by shading net.

End of 3rd growth suggests that nutrients uptake is back to normal.

Branching


Cover


The same old Heater again


And a couple of hours later, snow and thunder

Enjoying too much pleasant weather I had to finish work on tree protection under blizzard conditions and catched cold, ciha.

Heated frame in this setup can rise inner temp wrt outer for about 7 degrees Celsius. The lowest temperature so far was -12.5C (at 2m level).
No cold-hardiness tests planed for this winter. I want productive tree, ASAP.

The best read found so far:
[1] "Biology Of Citrus" by Pinhas Spiegel-Roy and Eliezer E. Goldschmidt
http://books.google.hr/books?id=SmRJnd73dbYC&printsec=frontcover&dq=Biology+Of+Citrus&source=bl&ots=vFFPMFhg1Z&sig=KlmOqME5bYWG0g5OZ4f6gLheJB4&hl=hr&ei=8P4cTeepF8eb8QP-kLTYBQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCEQ6AEwAA#v=onepage&q&f=false

http://www.ebook3000.com/The-Biology-of-Citrus--The-Biology-of-Horticultural-Crops-_58253.html

I had this completely forgotten:
Joe Real's opinion on "How is plant hardiness defined?"
link

First mild day with sunshine on my face was enough to change my mind about skipping cold-hardiness test this winter.
Rearranging the frame took me a few days. I had to do a lot of autumn/winter pruning, I hope that the host tree will forgive me that.

Frame has 2 sections now, non-heated section contains grafts #1 and calamondin.


Wall between sections: 4 layers of plastic used as frame cover year ago.


Termometer for calibration data with respect to official meassurement 50 m away.


It's up to winter now.
Catching temperature at which defoliation occures would be perfect.

Closely related term to Cold Hardiness, or maybe even substitute, is Cold Tolerance (Resistance, Chilly-Freezing Tolerance).
'Precursor' to above is Cold Acclimation (Acclimatization, Preconditioning) where temp just above 0C (32F) play important role.

How ammount of light (energy input) affects citrus metabolism at low temps?
Possibly interesting discussion is here: "Citrus with fruits in the full darkness"
link

Good read for long, cold, scary nights
Jan et al., 2009., "Cold resistance in plants: A mystery unresolved"
http://www.ejbiotechnology.info/content/vol12/issue3/full/3/index.html

I did some meassurement this morning,
Time: 8:00 AM
Temp 2m: -4.6 C

Section 1 (heater not used, about 1 square meter of soil surface act as heat source): temp -3.7 -0.5 C = -4.2 C
0.5C above stands for error of remote digital termometer used there.

Section 2: temp -4.7 C
This section has no significant heating from soil radiation, that is deflected to section 1.

It is good to know that even now, when soil is quite cold, 1 m^2 can still contribute with 0.5 C.

19. was the coldest day in December with -12.5 C. Calibration factor was about 7 what leaves about -5.5 C of internal temp inside frame. Mild period with min temp above -6 C followed lasting till end of February. Such weather pattern gave me oportunity for not planed observation.

Events with min temp of around -5 C:
Event Date Minimal Temp 2m [C]
------------------------------------------
2010-12-19 -5.5 (-12.5 +7 = -5.5 inside frame)
2011-01-25 -5.5 *
2011-02-02 -5.1 *
2011-02-03 -4.8 *
2011-02-04 -5.6 *
2011-02-05 -4.0 *
...
(*) Heater was off and calibration factors was not meassured.

A couple of centimeters long young tip of graft G2 (i.e. autumn growth, seasonal growth #3) was exposed to the first two events and survived!
Photos of the tip, 2010-12-09...2011-04-12


The only damage are blackened a milimeter long tips of leaves, photo 2011-04-03


A couple of milimeters long young autumn shoots (growth #4 of the oldest graft) survived too, with no apparent damage, and now continuous to grow.

A centimeter long young tip of graft G1 (i.e. autumn growth, seasonal growth #3) was exposed to all events and gave up after 5. event, 2011-02-04!

Graft G1 is also the subject of the main experiment what I'll report later.

Relaxation item: http://www.youtube.com/watch?v=nd80Mqj5qrM&feature=related

At -6 there were no damage as I see , good to know.

Thanks Ivica