So, has anyone done this type of grafting and what were your results or anyone have thoughts on whether this will work and how far can I push the chill hour difference, ie: will a 1,000 chill hour limb produce fruit on a low chill (200 hours) Dorset tree in the high desert and if it will produce fruit will it be the same as if receiving 1,000 chill hours?

http://www.newcrops.uq.edu.au/acotanc/papers/campbel1.htm

Found the following line (but no followup):
A technique of top grafting has been performed in Taiwan for temperate pears on to tropical pears, allowing production of temperate pears in tropical lowlands of Taiwan, and earlier production.

at:

http://www.agrsci.unibo.it/wchr/wc2/ganry1.html

Found this abstract which says temperate species are being grown in Peru (tropical), but you'd probably have to buy the paper to find out what they're doing:

http://www.actahort.org/books/199/199_8.htm

And here's a slew of papers on temperate fruit in tropical areas:

http://www.actahort.org/books/199/

But it looks like you may have to do your own research.

Good luck!

Chris

Most likely I will need to do my own research, perhaps I can talk my boss into springing for a trip to these sites to first hand research, LOL, but did find a few interesting tidbits:

The Australia article contained some nice tidbits:

"The chilling requirement of a rootstock can have an effect on the chilling requirement of the variety."

"Where a rootstock is high chill (eg. MM.111 1100 hours, Young 1992), the effect on a low chill scion variety may be less than the other way about as with pears, as the buds of the scion have had enough chill."

It is going to take awhile to go through this great site: http://www.actahort.org/books/199/

Thanks again!

Don't know exactly what they're doing in the Peru article mentioned, but I know they can get high-chill blackberries to grow in the tropics by periodically defoliating them.