it is suggested that ethrel promote  sucrose metabolism through  ethylene biosynthesis pathway and by which stimulate the  latex yield.

good paper to read Plant Signal Behav. 2009 Nov; 4(11): 1072–1074.

ETHREL IS  A PLANT GROWTH REGULATOR WHICH RELEASES THE NATURAL PLANT HORMONE ETHYLENE INTO PLANT TISSUES AND HENCE IT HAS ALL THE PHYSIOLOGICAL EFFECTS OF ETHYLENE.  ETHYLENE BRINGS ABOUT LATEX FLOW BY DELAYING  THE PLUGGING PROCESSES OF LATEX VESSELS. ETHREL  WHICH IS A LIQUID,IS APPLIED TO SCRAPED AREA OF THE BARK OR TAPPING GROOVE. THE USE OF ERTHREL OR ETHEPHON HAS NOW BECOME A PRACTICE IN MOST RUBBER PRODUCING COUNTRIES.

Thank you SIr Saman Seneweera and Muayed Fadhil Abbas for your answers....

Etherel is a trade name. active ingrediant is ethephon (2chloro ethyl phosphonic acid). when we applied ethephon on the bark it   penetrates into the plant tissues, and is translocated and progressively decomposed to ethylene which is a kind of plant hormone that affects the biochemical and physiological status of the tree resulting high yields. With the presence of water, Ethephon decomposes in the tissues releasing ethylene which acts as the yield stimulant.

Ethylene is thought to enhance latex production by;

delaying plugging of latex vessels

increasing Both initial flow rate, duration of flow and drainage area 

increase in the anabolic activities in the cells and specially biosynthesis of rubber and of proteins involved in this renewal  

rise in latex pH after stimulation results in an increase in invertase activity which, intensify sucrose mobilization and transported towards vessels where regeneration of latex and metabolism activated. 

Increased latex pH and other contents due to Ethephon stimulation affect directly the osmolarity of cytosol of laticifers and enhance ingress of water and ultimately latex flow

 Stimulation with Ethephon greatly reduces plugging index by increasing the stability of lutoids by and thereby prolonging the latex flow 

Thank you K V V Sagari Kudaligama for your elaborate answer...

Ethephon alter the latex water relations thus results in lutoid stability and prolonged latex flow