In article , wrote...

A true graft hybrid is a plant that has genetic material from both
parents in all cells and which has arisen from cells right at the
graft union. True graft hybrids are very rare, because you have to get
actual mixing of cell contents and then development of a shoot from
these mixed cells.

Ummm, I don't think that's accurate. As I recall it, "graft hybrids"
are just chimeras, with tissues composed of a mix of cells of both the
stock and scion of the original graft union. The cells don't actually
fuse, but both cell types take part in the same growing tips that
produce the shoots, leaves, flowers, etc. of the "hybrid".

Such periclinal chimeras in angiosperms can be quite stable [forming
shoots with outer tissues from one "parent" over inner tissues of the
other], and graft hybrids will sometimes arise as adventitious buds
formed from the graft union tissues. It can be possible to destroy all
the lateral buds on a grafted plant, cut it off through the graft union,
and get "graft hybrid" shoots regenerating from callus formed from the
graft union region.

Hybrids formed by somatic cell fusions such as you describe are also
possible [especially in laboratory cell cultures], but they're
a separate thing from "graft hybrids".

http://www.universaldbase.com/cshs/a...posium_S20.pdf

I don't know if lemons and oranges would do this.
IF it happened, the resulting hybrid would bear something intermediate
between an orange or a lemon, not oranges AND lemons.

A true "graft hybrid" here might have epidermal tissues of one species
over internal tissue of the other. [But would the rind and the pulp
vesicles both be of epidermal origin?]

BUT, citrus varieties and species are often widely compatible as
grafting scions and stocks,

Not only that, but they're also graft-compatible with species of
related genera, such as kumquats [Fortunella] and trifoliate orange
[Poncirus].

so it would be possible to have a whole
tree full of, say, different orange varieties. Not sure if oranges
and lemons are compatible as scion and stock, but I'd bet both are
graftable to trifoliate orange, which is widely used as a stock for
citrus. If they are, then you could have a tree that bears both
oranges and lemons.

I've little doubt one might also graft limes and grapefruits and
mandarins onto a single tree along with the oranges and lemons.

For that matter, Citrus and kin are also widely hybridizable in the
usual sexual way [including with Poncirus and Fortunella]. Some
intergeneric hybrids are being used as grafting stocks.

http://www.lal.ufl.edu/castle/Abstracts.html
http://www.lal.ufl.edu/castle/citation.html
http://www.fcprac.ifas.ufl.edu/citru...rootstocks.htm
http://edis.ifas.ufl.edu/XC004
http://www-horticulture.tamu.edu/citrus/l2304.htm

[snip]

cheers