In October 2016 I went to Australia looking for the native citrus species.
I found trees of Citrus glauca near Broken Hill in New South Wales where annual rainfall is about 240mm - less than 10".
Details and photos at
http://www.homecitrusgrowers.co.uk/australia2016/australia2016glauca.html

I have spoken to several nursery growers (mainly European) about this.
It appears that they have two approaches to rootstocks.
Firstly they use whatever they have found quickly produces a visually appealing tree; secondly they use whatever is being used locally for in-ground commercial citrus production.
There is little scientific research into rootstocks for long-term container growth because there is no commercial driver for such research. Further, the container growing mix and growing environment is likely to be much more variable than for a commercial fruit grower, so comparisons are difficult.
The producers aim is to have a good-looking plant ready for sale in the shortest possible time. Fruit quality or plant growth after the first few years is of no interest to them. In fact some producers have admitted to me that it does them little harm if a plant dies or gets thrown out after a couple of seasons. They then have an opportunity for another sale.
The number of buyers who - unlike members of this forum - have any knowledge of rootstocks must be very small.

Cara cara is thought to be a chimera and is known to produce branches which revert back to the Washington navel.
On the CVC website there is a note about distributing Cara Cara budwood which acknowledges this possibility:
.... this is a chimera and could revert to non-pink fruit through inadvertent selection of mutant bud. We probably should not release from any trees which have not fruited.
See http://www.citrusvariety.ucr.edu/citrus/caracara.html

The fruit is always the type that you graft or bud with on to the rootstock. The top variety is technically called the scion.
The rootstock may make subtle differences to the fruit produced, such as a little change in size/sweetness/acidity/peel thickness and so on.
But basically if you graft, say, a lime on to a rootstock - the fruit will always still be a lime and always taste like a lime.
The rootstock's fruit flavour is in no way transferred to the top variety.

The following advice about botrytis is from the RHS and applies to home gardeners in the UK. Regulations may be different for Germany and elsewhere.
Non-chemical control
Hygiene is very important, especially under glass. Remove dead and dying leaves, buds and flowers promptly
Do not leave dead plant material lying around
Reduce humidity by improving ventilation and do not overcrowd plants
Chemical control
No fungicides are approved for use against grey mould by gardeners.

See https://www.rhs.org.uk/advice/profile?PID=165

To me they look like very small ants.
A bit of research suggests they may be Argentine Ants which are apparently widespread in California.
Although most images don't seem to show the greenish abdomen which yours have, I think this photo is similar:




Perhaps your citrus has scale insects that the ants are collecting honeydew from?
More details athttp://drkaae.com/InsectCivilization/assets/Chapter_18_Bees_Wasps_and_Ants.htm

This is not due to cold damage on the flight. I don't know if your figure of 7C in a plane hold is always true, but it certainly isn't allowed to freeze. There is never a problem on a short flight from Italy to UK for any citrus - I've often imported them this way.
It has been far too hot in a greenhouse in the UK for the past week, unless it is fully shaded. Although pummelos like tropical conditions, temperatures over 45C will be too high unless you continuously damp down the greenhouse floor. I remove all my citrus trees from the greenhouse from mid-May to mid September and they do far better outside.
I suspect your leaf problem is due to excess sun and heat. Tender new growth and recently fertilised nursery plants are likely to be worst effected. Those leaves won't recover, but the plant should be OK if you keep it outside and shaded for a few days.
Citrange/Mike - Buckinghamshire, England

I see you are in Stockholm which means growth will probably be rather like here in England if kept outside year-round. Very slow.
I have been growing in-ground FD from seeds for well over 15 years and still have not produced a single mature fruit.
However, right now I have one plant that is currently covered with hundreds of flowers, so I am expected a first harvest this year!
That is provided we don't have a late hard frost, an invasion of climbing snails, or a hail storm! All these have destroyed flowers or immature fruits in the past three years.

I have the species C.australis, C.australasica, C.inodora ,C.garrawayae, C.glauca. All potted plants, and some quite small and struggling to grow.
Various named hybrids: xSydney (virgata?), Eremolemon, Eremoorange, Blood Lime, Sunrise Lime, Faustrime, Faustrimedin.
Varieties is more complicated. From various discussions, I tend to think that the naming of many of the Finger Limes is of doubtful value.
Various individuals and growers have collected specimens from the wild and given them names for commercial reasons, but there has been no independent comparison to see if there are significant differences between them. Yes, colours vary and fruit size varies and growth habit varies. But no-one knows if there is really any difference between Lovely Green, Tasty Green, Wonder Green and Green Beauty. (My made-up names!)
I have types with peel of red or red/brown, black, green, yellow. Flesh of green, yellow, pink/red. Internal and external colour varies with growing conditions.
I also have lots of seedlings of various sizes that may one day fruit. I'm hoping for a nice blue one!
Mike/Citrange

I think most people on this forum are interested in growing the 'normal' citrus varieties, but I have become fascinated by the Australian species - even though they are not really very eatable.
A few weeks ago I returned from a trip to Australia to see if I could locate and photograph all the six native citrus species growing wild.
You can see the results on my website - follow the links at the bottom of the page starting at
http://www.homecitrusgrowers.co.uk/australia2016/australia2016.html
Mike.

Not many people seem to know about the incredibly useful 'Wayback Machine' which attempts to archive the whole internet.
For instance on 7th June 2009 it recorded one of the grafting tutorials. Find it now at
https://web.archive.org/web/20090607012756/http://citrus.forumup.org/about500-citrus.html
The other one is at
https://web.archive.org/web/20090606045806/http://citrus.forumup.org/about1762-citrus.html
However, I haven't checked whether all subsequent pages and images have been archived. Sometimes it is not complete. If so, then it's worth looking at captures on other dates.
The homepage of The Wayback Machine is at www.archive.org
Interestingly (for me) it has a copy of my own website www.homecitrusgrowers.co.uk going back to 2001! On occasion I have used it to recover photos I have lost due to hard drive failures.

I have seen them do this in Italy - they dig up older less productive or less desirable trees and plant them in large containers, having removed virtually all the top growth. After a couple of years the roots have recovered from the shock, new topgrowth is cut back again, and a new variety grafted on the trunk.

Trade it for a real lemon.

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Riverland - I was thinking exactly that - but I wasn't brave enough to say it!

A 'real lemon' has such an intense lemony zing. Meyers Lemon by comparison has a sweetened and watered down taste. Not really lemon and not really orange - which isn't surprising as it's thought to be a hybrid of the two.

I once wrote on a citrus forum that Meyers Lemon was far inferior to a true lemon, and I received a stream of abuse in response.
There seems to be a strong pro-Meyers, anti-lemon lobby around. My flak-jacket is standing by!