The biology of how a product is transported through out the palm:

"It has been determined  that trunk injection will produce uniform foliar levels of OTC; therefore it is interesting that petiole injection is equivalent to trunk injection in therapeutic response. The pattern of distribution of OTC after either trunk or petiole injection is indicative of a xylem pathway for initial translocation. To be effective against a phloem delimited mycoplasma like organism the OTC must get into the phloem. 

The fact that petiole injection is therapeutically effective is evidence that after OTC moves into the leaf and is distributed throughout the apoplast by way of the transpiration stream, it is then transferred, either actively or passively, into the phloem and exported from the leaf in a systemic manner. This transfer must take place  within the tissues of the leaf.

The petiole injection technique was found to be effective therapeutically against lethal yellowing. This technique is significant in that it eliminates the permanent injury associated with trunk injections. (Randolph E. McCoy - "Petiole Injection of Coconut Palm, a Method to Prevent Permanent Trunk Injury During Antibiotic Treatment for Lethal Yellowing.") 

Click this link to read Randloph E. McCoy's Article 
http://www/fshs.org/Proceedings/Password%20Protected/1977%20Vol.%2090/114-117%20(mcCoy).pdf