Radoslav, I would think that the trees in Petr's greenhouse would not suffer at all from low light, even with the snow. Snow comes and goes, it is all most never that every inch of a greenhouse's glazing is covered by snow cover. I grow greenhouse citrus in Colorado, a state that receives a lot of snow every winter, and I can tell you snow is not much of a problem as far as the trees receiving enough light. Citrus trees originated as understory trees, growing in the shade of taller vegetation. Citrus use light in the process of chlorosynthesis, to manufacture sugars which the tree uses as their food source. For citrus this process maximizes at only 650 PAR which is just 1/3 of full sun light. Therefore, when a citrus tree receives higher levels of sunlight radiation the tree will not (cannot) produce more sugars. Millet - (725-)

Detrich Bonhoeffer (February 4, 1945 - April 9, 1945)

Radoslav , Petr got some beautiful trees over there , but still he doesn't keep them in full darkness as this topic is all about.

How to take care of them in the house have been allready discused in this forum , but how to keep them at lower temp and full darkness without loosing any of the leaves is a big thing to be done.

As far as I know from Peter, if there is enough snow (and in his place, snow stay for a long time and sometime there is more than 1m of snow)
he used to use this snow from garden to cover all surface of greenhouse - it works like isolation and sometime temperature is very low inside greenhouse, in section of greenhouse for more cold hardy citrus plants, some degrees under 0 C - upper surface of soil in pots frozen and some fruits - close to greenhouse wall, are demaged by frost.
Yes, in that case plants usually lost most of their leaves and some twigs, especially at the beginning of spring, but plants recover their habitus in spring and summer.

Petr is member of this forum. Hopefully he'll find this topic and share his experiance with us.

I hope so , because in a central heated house is like a battle with the heat and keeping alive all my trees , it will be much better if we could just put them on "stand-by" for 3 months

Johann Chr. Volkamer, writing in 1714, recommended keeping citrus roots warmer. Volkamer, who lived before modern conservatories, set his trees in glass houses where the pots were completely surrounded by horse and cow manure. This manure, if kept moist, started to rot and this rotting process released heat, which kept the roots warm, besides also feeding the trees even in winter. Winter Leaf Drop was avoided during the 18th century by manure packs around the container, and a more shaded location (because glass was not as transparent as it commonly is today). Volkamer has even written of heavy fruit wilting that can be avoided by the manure packs, which illustrates what cold stress does to a citrus tree. Cold sensitive varieties like citrons, limes and some lemon varieties are more prone to WLD than sour oranges, oranges and mandarin trees. - Millet (723-)

Also consider this:
"...The grafted citruses also have improved hardiness, but during the winter, they need to be kept dormant in temperatures best ranging 0°C to 8°C..."
and later
"...If the citrus with Poncirus as a rootstock is kept in a flat, it should be put into a cold room with the temperatures of about 8°C..."
from http://www.exotickerostliny.cz/en/katalog-rostlin-othermenu-95/poncirusy-a-hybridy-othermenu-91/29-poncirus-trifoliata.html

I don't know is the source of claims above author's personal observation or something else but those are close to my own observations over several years.
Simply said: My flat is not citrus friendly, that is. Controlled greenhouse conditions are not available option.
My observation:
Keeping citrus in low-light non-heated area result in no significant WLD.
Proper explanation needs much more observations/experiments to be done.

/LoudThinkingOn
Something very interesting is going on here, roughly speaking like a sort of hysteresis...
We know that citrus is C3 plant and as such suffer from Saturation at high temps under high light. That further leads to Photoinhibition.
What happens on other side ?
Low temp and high light leads to Photoinhibition too, can we talk about role of ratio Temp/Light ? After all both are manifestation of EM energy...
/LoudThinkingOff

danero2004, if you are keeping your citrus in low-light low-temp conditions already:
Please, be very, very carefull with watering. Check twice before doing that. Most of my citrus will not need watering this month.

When we are talking about WLD, I would say that the ratio between temperature to light is not a direct ratio. Rather it is a indirect ratio, at least for containerized citrus trees. For container grown citrus trees, the temperature is related to the tree's root system, whereas light is related to the tree's foliage. One of light's causations, at least for citrus leaves is heat, which at a point needs to be reduced to avoid damage. While the causation of temperature to the tree's roots is the rate of activation of the root system. Now there is a direct ratio between the temperature of the root system, and the cooling process of the trees foliage through transpiration. The usable heat range for citrus roots is between 55.4F to 95F. I'm not sure about the lower light number for citrus, but its probably 400-PAR the upper usable light limit is 650-PAR- Millet (719-)

Let's skip over 'ratio' and use more general math term 'relation', that is much 'safer'. Seems to me that we both agree that some relation exists there. I expect that such relation can be expressed arithmeticaly, not exact - but good enough for our use.

Root temp in this context is significantly bellow 13C (55.4F). Can you give me more info about root activity in the range important here, 13C ... +0C ?

Please, can you elaborate more on 400 PAR as the lower light number for citrus?

That is very important here, when max number of photons which can be proccessed by metabolism at given heat/temp is exceeded we could expect ill effects, a sort of Photoinhibition and later maybe even abscission of leaves.

Related question is, what is minimal PAR needed for "maintaing minimal but safe activity" of citrus at given heat/temp?
Also, what happens when PAR is lower than min (over some period, how long?) ?

With a time experiments should be conducted, sort of TrialAndError. It's a long term game w/o shortcuts. We've no reason to hurry.

NOTE: I'm missing proper term for "maintaing minimal but safe activity", something like "cold/passive standby operation/regime" is maybe better.

NOTE: Photosynthetically Available Radiation (PAR)
Sometimes called Photosynthetically Active Radiation. Photon flux density (photons per second per square meter) within the visible wavelength range (usually 400 to 700 nm). It indicates the total energy available to plants for photosynthesis, and is thus a key parameter for biological and ecological studies.
http://www.hobilabs.com/cms/index.cfm/37/1288/1301/1407/3241.htm

NOTE: Photoinhibition
http://en.wikipedia.org/wiki/Photoinhibition

NOT FULLY OFF-TOPIC
http://genome.jgi-psf.org/chlpb/chlpb.home.html
"...Black Sea chemocline reach 0.001 µmol photons m-2s-1 during winter, corresponding to 0.0007% of surface light intensity...

...Each bacteriochlorophyll e molecule present would absorb on average one photon every 8 hours at the light intensities present in the Black Sea chemocline. The ecological situation of phototrophic organisms in the Black Sea chemocline is comparable to that of plants grown at a distance of 50 m from a little candle in an otherwise pitch black glass house..."

ivica asked: "Related question is, what is minimal PAR needed for "maintaing minimal but safe activity" of citrus at given heat/temp?
Also, what happens when PAR is lower than min (over some period, how long?) ? "

The minimal PAR needed for "maintaining minimal but safe activity, seems to be: CO2 exchange rate (CER) of field citrus at a solar photosynthetic photon flux density seems to be 50 umol m-2 s-1. This and other information that you are looking for can be found in the link posted below.

The photosynthetically active radiation (PAR) pretty much corresponds to the visible spectrum for humans. PAR is from 400nm to about 700nm. The spectral characteristics of chlorophyll make leaves strong absorbers of photons in the blue spectrum (400-500nm) and red wavebands 600-700nm (for citrus 600 - 650nm) of the solar spectrum. Absorption of green (500 - 600nm) is weaker and many photons of this wavelength are reflected or transmitted and scattered in the form of diffused radiation. In complete darkness (ie at night),a leaf's net CO2 assimilation is negative because there is only respiration but no photosynthesis, and the leaf's metabolism and continued sugar export depends on the break down of starch accumulated during the day.

http://books.google.com/books?id=xsobnlXZBwQC&pg=PA948&lpg=PA948&dq=Light+compensation+point+for+citrus&source=bl&ots=xao4vZcJAL&sig=uzsI8RfHAT-KoeN-56P4jF1L1t8&hl=en&ei=puBATYuZNM3UgAf6w_zPAg&sa=X&oi=book_result&ct=result&resnum=3&ved=0CB4Q6AEwAg#v=onepage&q=Light%20compensation%20point%20for%20citrus&f=false

Millet (719-)

Thank you Millet,
I have now enough information about leaves.
Roots are different story.

Linked book in your post (2nd Edition, 1999.) has a lot of opened questions, and urge for more experiments.
3rd Edition is published 2010. Not available as e-book.

Rao et all.,2006., "Physiology and molecular biology of stress tolerance in plants", page 147:
"To date, a complete analysis of phytohormonal involment in cold acclimation has not been performed."

One more source of (recent) information: e-print archive at Cornell University http://arxiv.org/

ivica, there is always more information about the above ground portion of plants, than the below ground (unseen) portion of plants. Thanks for the information on the 2010 3rd addition. I will look into it. However, technical books of this nature usually are quite expensive. We'll see. - Millet (716-)

well , long stoy short , is it possible to be keept in darkness at 8C during winter?

danero2004,
give us more info first.
1. What is "darkness" in your case ?
No ligth source, no window at all?
Window facing north maybe? How large window?
Something else?
2. What is "8C"?
Ambient temperature?
Max temp? Average temp? ...

For example:
Room without windows and with avg ambient temp of 8C
do not looks to me as good place for citrus.
That is my impression only, which I can not support by observational data because I don't have them.

My experiance with citrus overwintered in low-light low-temp conditions spans 8 years, and that works for me.
However, I took no real meassurements. Next winter should be the start...

Rough estimation of conditions used here:
Period: mid December ... mid February
Ambient temp: Min: around 0 C, Avg : 4 ... 6 C, Max: around 10 C
Light source: window facing north, 1.5 ... 2.5 square meters.

To help you to get a feeling of it I took a few photos for you today at 12:30. Containers are there since early December.
The coldest room in the house, tool room - basement:

From right to the left, (partialy) visible are W. Navel, Meyer L., U. Chahara, Calamondin...
Meyer L. soil temp: 4C

Something I don't like: In spite of such conditions, flower buds on Meyer L. and W. Navel



Both grafted on PT rootsock.

Non-heated upper staircase, 2nd floor:



Termometer at window sill reads 3C.

Nagami Kumquat:

Keeping Nagami in such conditions helped recovery from root rot a lot
link

You got the perfect spot there for you citruses, mine is in fact dungeon , with a very small window 3"x20" from where I get a little light , but the darkness is 95%.
Well the 8C is an average but it is constant during the winter and stays at 15C during summer , here is where I keep my bottles of wine .

Once winter comes I move inside all the : kiwi , punica granatum , cacti, dormant bulbs , gladiola . canna indica , and a few more who doesn't need light durin winter time.

I know that low temp could lead to a freeze status on a citrus tree , but what I don't know is that maybe they will like also the darkness since there is no root or other activities during this time

Thanks


PS I heard that trees grafted on PT are not doing so well , an this because the PT has a diferent behaviour and in opposite to meyer's behaviour.