We investigated the mechanisms for a decline of putrescine concentration as
sociated with feeding K+ to roots of barley (Hordeum vulgare L. cv. Amagi)
seedlings. The addition of K+ to roots which had fully accumulated putresci
ne resulted in a significant increase of K+ level and a decrease of free pu
trescine. The decrease was not due to conversion of free putrescine to free
spermidine, spermine, or other conjugated forms in the roots. The elevated
K+ supply did not cause transport of free putrescine to shoots or its conv
ersion in the shoots. On the other hand, exogenous K+ did induce efflux of
putrescine from the cytoplasm of the roots into the external medium. Activi
ty of diamine oxidase (DAO; EC 1.4.3.6), which oxidizes putrescine, was mai
ntained by application of K+, while its activity slightly decreased in root
s without K+ application. These results suggest that putrescine efflux is l
argely responsible for the decrease in putrescine level associated with the
supply of exogenous K+ to roots of barley seedlings.