Hi Giorgos,
Thanks for the file. It would appear to be related to the smoothing groups of the segments. That particular section has the pieces the other way up from the others, and I think the smoothing groups are causing the bottom of the source geometry to shade slightly differently from the top. I found that selecting all the bamboo source objects and adding a Smooth modifier set to smoothing group 1 fixed it.
I hope that helps,
Paul.