I have read that if to graft a seedling pear tree into the crown of a mature tree, the branch grown from this seedling's graft, will reach the fertile age earlier. And dwarfing rootstocks also force seedlings to reach the fertile age much faster. Does this also apply to citruses?

I am no expert on citrus but everything I have read on the subject says that this is not true for citrus. Maturity comes only from a mature scion and cannot be transferred from the rootstock.

It is possible that grafting onto a mature tree will produce faster growth and that will lead to faster maturity, but it will still have to produce the required node count to reach maturity.

I have read that apple seedlings grafted onto dwarfing rootstock can reach the fertile age in 3...5 years instead of 10...15. Has anyone observed the similar effects also in case of citruses?

There is always that interaction between scion and stock that can hasten the maturity of the seedling scion on mature stock. There are also exceptions to the node count such as ocassionally some citrus branches of a mature tree will revert back to juvenility. I take advantage of such type of branches if I wanted my mature scionwood to grow in size vigorously before it will bloom.

Grafting of plum and peach seedlings unto mature tree I have tried and found production of quality fruits in as little as couple of years but mostly about three years. I haven't tried the citruses yet. Then again citruses from seedlings can have tremendous variations. Most of the seedlings I have started from fortunella and mandarin types have bloomed way too early for me to have a clean unbiased yard trials to test such hypothesis. I wanted to try at least some suspected hybrid new seedlings from either the blood oranges and grapefruit types but haven't found the patience to do it just yet. It is difficult test because I cannot be certain if I would be testing identical plants. But this is one of my to do list. Not really a priority.

While one could always try new varieties on new rootstocks, that research actually was done, back in the '40s, putting orange seedlings on various old, mature trees to see if they could shorten the juvenility period. The only positive result was when they did that AND also girdled the limbs -- in that case they got a small diminution of the juvenility period. Otherwise, no.

Furr, J. R., W. C. Cooper, and P. C. Reese. 1947. Flower formation in citrus trees. Amer. J. Bot. 34:1-8.

It is always good to retry, to revalidate, especially if the research is very old. This time we have bark inversion that we can combine it with instead of girdling which will kill the limb. Oftentimes, the old research miss a detail or more that could dramatically change the results. We have more information and better techniques in research compared to ages ago.

In the Biology of Citrus (p. 195) by Spiegel-Roy and Goldschmidt I found a short note:

Quote:

... However, the use of horticultural techniques such as girdling have not resulted in a significant shortening of the juvenile period. Fruiting is hastened, often by two years,..., entailing budding into older, grafted seedling, training the scion to stakes and bending of shoots. Vardi and Spiegel-Roy (1988) have described the details of a method that has been used very effectively with hybrid progeny of Satsuma mandarin.

Hmm, would be interesting to read this description...

From the same source:
(my summary, not a quotation) Extremely early flowering (in 1.5 years) has been observed in some grapefruit, pummelo and grapefruitXmandarin seedlings if a special temperature regimen was followed: max t up to +20 C and min t below +10 C from November to March.

Wow! I have four grapefruit seedlings at present and they are kept exactly so! Exciting...

Malcolm_Manners wrote:

The only positive result was when they did that AND also girdled the limbs -- in that case they got a small diminution of the juvenility period.

Yes? This seem to indicate that if the downward floem transport is inhibited, the juvenile period may be shorter. And dwarfing rootstocks do exactly this: as a rule, a remarkable bump forms above the graft joint in case of dwarf apple trees. So, if there were true dwarf rootstocks for citruses, would be extremely interesting to try... Has anyone observed bumps above graft joints in case of older citrus trees grafted onto FD? Well, and does the FD forces the varieties to start fruiting earlier than other rootstocks?
Harri.

Thank you Harri for pointing that (temperature regime) out! It seems very interesting technique noteworthy of a retrial. Now I can combine several techniques together. I'll have to keep this in mind to try when I can get a place for a greenhouse that can be fully controlled.

I don't expect to get exactly the same mileage because of genetic variations in the seeds. However, some of the seedlings could potentially have the same reaction to shortening the maturity.

Bark inversion does minimize phloem flow to the roots but without killing the tree unlike complete girdling. It would seem to the tree that the stem is now very long because it takes a lot of time for the phloem materials to get down to the roots. The root system may respond by sending chemical signals back, perhaps riding through the transpirational stream that it is time to bear flowers. The same effect could be at work when using the slow growing rootstocks such as FD.

Nope, I don't intend to seriously breed new citruses, but I just love to push the limits on what can be possible in my backyard tinkering.

One more thought that came into my mind after reading the Biology of Citrus. Some evidences exist, that citrus flowering is triggered by the concentration of GA3: while it's high, only vegetative buds can form and while it's low, the flowerbuds form. GA3 is mainly produced in the tips of actively growing roots and if the conditions are good for roots to grow (it's warm and sufficient water), one can expect no flowers. But lower temperatures or lack of water for a certain time stops root growth ---> GA3 goes down ---> flowerbuds start to differentiate. (This was from the BoC, shortly).
Well, and in citrus seedlings for some reason the concentration of GA3 is too high for forming flowerbuds during all the juvenile period. If there is some substance antagonistic to GA3, maybe it would help to break down the juvenility? Just an uneducated thought, nothing more...
Harri.

Spraying of potassium nitrate have speed up maturity or induced blooming in some fruit trees, especially mangoes. I've had the honor of having the discoverer of this formulation, Dr. Barba visit us and have a pic under my 101-n-1 citrus tree. Will try to find that pic and post it here. Anyway, potassium nitrate may play a role in the interaction with GA but too lazy to research on that right now. It is one of the things that we could try, but of course there are timing issues and concentration of spray. It may not even work on citruses, but could be worth a try.

The extremely early flowering of grapefruit is not a diminution of juvenility. Rather, grapefruit seedlings often flower while extremely young, but then they invariably go into a long-term juvenility after that. those early flowers seldom if ever produce fruit (and you wouldn't want them to -- a full-size grapefruit on a less-than-knee-high tree would be deadly to the tree).

Actually with the ease of grafting, and the overwhelming availability of just about every variety of grafted citrus, I personally don't see much value in expending a lot of time, money and energy in trying to achieve some shorting of time with seedling citrus trees. This end is exactly what grafting does. - Millet (1,090-)

We have different objectives in life, some for pleasure of seeking and validating knowledge, some for profit, some for just the challenge, or various combination depending on our moods and stages in life. What we see as useless to our desires at hand could be a treasure to another.

Why do people climb mountains? Why do we bring grafting to extreme levels? And why this citrus hobby?