Putrescine, spermidine, spermine have been tested for their ability to substitute for inorganic nitrogen for the growth of explants from dormant tubers of Helianthus tuberosus. Explants grew as well on putrescine + indole acetic acid (IAA) as on nitrate + IAA or gluta-mine + IAA but no growth occurred with spermine + IAA and an intermediate growth effect was observed in explants grown on spermidine + IAA, although this last result was not confirmed in tubers of different years. Growth increments have been related to pH variation in culture medium. Radioactive polyamines were transformed by the tuber explants to arginine and glutamine.
The presence of radioactive putrescine after [1,4-14C] spermidine feeding to explants grown on spermidine + IAA, as well as the presence of large amounts of spermidine in explants grown on spermine + IAA, indicate a reversal of the pathways of spermidine and spermine biosynthesis. We suggest that putrescine arises by such reversal. This hypothesis does not exclude processes of direct transamination from polyamines to α-ketoacids.