Dragon's Heart (Pitt Dragon Chronicles, 4) by Jane Yolen, 2009. Published by Houghton Mifflin.
Austar IV isn't the planet it once was, and when Jakkin and Akki finally return to the dragon nursery, their homecoming arouses mixed emotions. Together they've survived the insurmountable, and now they can weather the brutal conditions of Dark After and communicate with the dragons they love. But with this knowledge comes responsibility. What they've learned about survival could transform the planet--or, if entrusted to the wrong hands, bring about its destruction. Akki's insistence that she return to the Rokk to finish her training and begin new experiments drives a chasm between her and Jakkin. Suddenly she finds herself in the midst of a political battle that could claim her life. Only Jakkin can save her. If only he could reach her.