It seems reducing the juvenility period might be helpful in evaluating new hybrids faster although I am sure it will still take full size trees for a complete evaluation. Still, it may be useful to screen out trees with undesirable fruit.

A good and reliable dwarfing rootstock that brings the fertile age of grafted trees forward, would reduce the time needed to cover expenses made to establish a new plantation. And smaller trees would reduce running expenses per production unit. It was so in apple industry and I'd be extremely surprised if it were not so in citrus industry. But as I've understood, those dwarfing rootstocks for citrus are simply absent at present.
Reducing of the juvenility period of seedlings is a completely different topic and as jrb noted, might be helpful in evaluating new hybrids faster. Smaller trees that start bear fruits earlier would be a remarkable economy of time, of money, of labor... They also would make the life even more enjoyable for a certain kind of wiseacres (like me myself )
Harri.

Harriest, In this case, we're comparing apples to oranges (How often in life does one get to use that idiom and mean it in the very literal sense?! This is the first time in my lifetime!) Apple is rather unusual in that it enters a true, renewed juvenility when budded to a juvenile rootstock. Some varieties are worse than others, but some, like 'Northern Spy', my fail to bloom at all for a decade or more after budding, regardless of how mature the budwood was when budded. Citrus does not do that. We often get flowers and a fruit or two on a citrus tree the first year after budding. Indeed, some varieties load up with too much fruit, too early, and it deleteriously affects the development of the branching architecture. So when we say that a tree comes into commercial production in the 2nd, 3rd, or 4th year after grafting, we mean it is producing enough fruit to be worth harvesting, and/or the fruit is of a quality worthy of use (navels and grapefruit often produce dry, granulated pulp in their early fruits).

Harri:

You said: Did you say here that the time needed for a citrus to reach fertility, is only variety-dependent and is not influenced by the rootstock at all?

True of seedlings, as far as we know. If we're talking grafted/budded mature scions, then no, I would say there is no reaching of fertility that happens -- they are fertile from the moment of graftage. (See the last paragraph here, for a qualification of that statement)

Re. tree branching architecture -- In most cases in commercial citrus grown in the US, Pruning consists of cutting the top center branch out on the day the tree is planted, then absolutely nothing for years after that, until the trees become too large, at which time they may be topped and hedged by large sawing equipment. Yes, one could theoretically do selective pruning of dwarf trees (in a home garden, that would be wise), but commercially, it is not likely to happen, due to labor costs. And I do consider FD to be a "true" dwarfing rootstock. I believe our fully mature orange trees, at 2x2m and our fully mature grapefruit trees at 2x3 m, are smaller than a commercial grower would want, and as small as most home gardeners would want, unless, perhaps, for a tiny urban patio setting.

You said: Yes, and if the plantation would come into commercial production in the 3rd year instead of the 4th, wouldn't that be simply great?

I'd say no, in most cases, in that the architecture of the tree might be compromised. We actually do usually pick a significant crop in year 3, of most varieties. But the production in year 4 is substantially greater.

Certainly there is some rootstock effect on flowering and fruiting. 'Cleopatra' mandarin is notorious for being somewhat slower to start than are other stocks. So I suspect that the overall concept, that dwarfing promotes flowering and fruiting, may be at least somewhat true in citrus (Cleo is fantastically vigorous). But I'm not sure I see a commercial advantage in that fact, and I think the advantage for the hobbyist gardener is exceedingly smaller than it might be for an apple or some other crop.

Thanks, Malcolm, for clearing several things up for me Now I understand much better why citrus growers are not as keen on dwarfing rootstocks than apple growers.

I did not actually expect that commercial plantations, principally, are not pruned... But if to think about, it is quite logic: pruning would be enormously labour-intensive task indeed and if the fruit quality is good enough without pruning...

When I said "the time needed for a citrus tree to reach fertility", I actually kept in mind the age of a tree when it actually starts bearing fruits to harvest and I did not suppose that the graft taken from a mature tree had lost its fruit bearing potential and has to restore it.

Harri.