After reading countless posts about seedlings requiring specific leaf counts before flowering, while grafted trees do not, I am wondering how the tree "knows" when the count is reached, and why grafting circumvents this? Does it apply to just citrus trees or any sort of flowering plant, in general?

Has anybody ever made attempts to induce flowering in immature seedling? Or genetically engineer seeds that ignore node count? It'd sure be convenient to drop seeds in the ground and have a fruiting tree in a couple years.

I don't think you could force a seedling to mature. The tree itself knows the node count, and will not flower until then. The reason grafted trees can be smaller and flower is that the plant grafted to the rootstock is usually from an already mature plant, and it 'remember' the node count of the previous tree. It could have 20 nodes on it when it is grafted, but if they last tree had 900 nodes, it remembers, and acts as if it is still a 900 node tree.

If you want seedlings to flower in a couple of years, key limes flower and fruit by 2 years usually, and lemons only take a little longer. Both with the correct growing conditions, of course

There have been a lot of articles about this over the years. Here are a few random thoughts.
I don't think anyone really knows how this 'node count' is remembered.
And I don't believe it is really an exact number defined for every variety.
There have been theories (mostly not accepted) that after a cetain time a plant starts producing a flower-inducing substance or hormone called 'florigen'. But attempts to isolate it have failed, probably because it doesn't exist. Try googling.
Biological systems have genes that only operate after a certain time. So they know how old they are - think puberty onset, or hair turning white in humans. So why shouldn't a plant know its age and also know how far its growing tip is from its roots? So at some point the flowering gene is triggered. But this gene is not generally flowing through the plant, although some people have claimed that grafting a mature limb on a juvenile rootstock causes flowering in the rootstock. Again this is not universally accepted.

Citrange wrote..............."Biological systems have genes that only operate after a certain time. So they know how old they are"............. -

Mike, if maturity of a citrus tree is not really about the node count, and if the reason a citrus tree begins to bloom is because the tree knows how old it is, than why does an old citrus tree, even a very old citrus tree, never ever bloom when it is continually pruned to keep the tree from ever reaching its required node count ? - Millet

Looking around on google briefly, most of the links are to Millet's posts on various forums. The only lead was this book, parts of which seem to be available free online: http://www3.interscience.wiley.com/journal/119087646/abstract

Partial PDF:
http://www3.interscience.wiley.com/cgi-bin/fulltext/119087646/PDFSTART

Some more related links from the same abstract:
http://www3.interscience.wiley.com/journal/119087645/issue

Interesting article. Didn't read the whole thing yet but of the ideas given, the explanation that makes the most sense to me is that flowering is determined by the number of cell divisions, which closely correlates with node count, and would explain why grafted plants "remember", and why pruned plants never flower. No idea what the actual mechanism is, though.

A second thought. When Citrus seedlings are grown for use as a root stock, and upon attaining proper size, are grafted/budded with the desired scion variety, the rootstock portion remains juvenile its entire life, no matter how great of age the rootstock becomes, "due" to the stock never reaching its required node count. I believe your correct that the actual maturity process is not fully understood, and that hormones, or some yet unknown process, could be involved, but the node count explanation currently is the best method of explanation. Lastly, in seedling citrus trees, after the required node count is finally reached, ONLY nodes greater in count ever produce fruit. All the node lesser in count forever remain remain juvenile. - Millet

I'd love to look more into this to find out what the actual switch is, genetic, chemical, or other. If you know any other resources please let me know.

I have a C. macrophylla as rootstock that sprouted and immediately flowered. It might be one or two years old when grafted. Maybe macrophylla flowers very early, I don't know.

Sylvain, interesting about your macrophylla. I wonder if the macrophylla's early flowering, is the same response as happens rarely with a seedling grapefruit. However, the early flowering on a seedling grapefruit (as early as maybe the 6-to 7 node), never produces a fruit, and further, the grapefruit never flowers again for many years, until the tree matures. - Millet

> I wonder if the macrophylla's early flowering, is the same response as happens rarely with a seedling grapefruit.
I don't think so, because it is not an accidental flowering (one or two flowers). It is now the second flush of flowers, two flowers at each leave base! And one fruit of the first flush is growing.

Maybe it could be a grounded cutting but it would be very odd because I think Macrophilla is propagated by seeds.

Genetic engineering does provide a method for removing the juvenile phase in citrus. You just have to insert a few genes from Arabidopsis!
See
http://www.nature.com/nbt/journal/v19/n3/abs/nbt0301_263.html
By the way, it is possible to produce a fruit from the single terminal flower of a one or two year old grapefruit or pummelo. I did this with a Goliath pummelo seedling. It required a complicated system of nets to support the developing fruit on top of a quarter inch stem. It grew to about the size of a small orange before dropping.

citrange, how did you get the pummelo to flower in the first place?

Further, although Grapefruit can produce a bloom as a fairly new juvenile tree, it is rare. Personally, I have never seen, nor heard, of an early Grapefruit bloom ever actually producing a fruit. Citrange, did you photo the process of your fruit, and is it on your web site? - Millet

It was nothing special I did. It is a well-recorded fact that grapefruits & pummelos do occassionally produce a single terminal bud in their first or second year.
Yes, I clearly remember photographing it, but it was before my website really got going and before digital cameras. So it will be in my very disorganised drawers and plastic bags full of old photos. I will start rummaging!