Hi Akram,
this may be the explanation your looking for ,
its in the third post from Tapla.

Here is a link that might be useful: http://forums.gardenweb.com/forums/load/fig/msg070007054857.html

Yes, thanks Martin
I had started that thread before my current observations. I had thought of that explanation by Tapla but that explanation intuitively seem falling short (in my opinion only) when I think of (say) a huge tree
and air-layer a branch where the huge mother tree will continue producing. It is possible that tapla explanation may apply.
My question (in the subject above) can be put another way as to why the current year wood growth on the mother tree trunk can't bear fruit? It did not do it in the cases of three mother trees mentioned in the post above and all three had new year wood and the trees used to fruit before.