might i suggest the information at this link...

http://www.patentstorm.us/patents/7308775/claims.html

it is the details of the "RPM" patented process used at Forrest Keeling Nursery (the place that now represents Peterson paw paws) to grow paw paws, among other things. they use shallow pots with open bottoms - the soil is held in by mesh trays that the pots sit in. when the roots reach the bottom, they hit air (pots have been elevated to allow air to move freely underneath) and they "air prune," which means they gently stop themselves from growing out any further and instead send out multiple secondary roots further back along the root. this differs from the shock of trimming roots off.

apparently the more root tips a tree has, the healthier it is, the easier it is to transplant, and the earlier it fruits. they get oak trees to produce acorns in their 3rd year, rather than the average 20 or 25 years. not bad. anyhow, i am learning from their process and have started a bunch of seeds in a similar manner to theirs. i believe a deep pot allows a long taproot, however, from the research i have seen, long tap roots seem to be much more fragile when transplanting. hope this helps!

Here is a link that might be useful: RPM method