Summary The effects of inhibitors of putrescine (PUT) biosynthesis, DL-α-difluoromethylornithine (DFMO) and DL-α-difluoromethylarginine (DFMA), chlorogenic acid representing phenolic conjugation partners of PAs, and exogenous putrescine (PUT), on the distribution of free and conjugated PAs were examined in relation to their effects on growth, tuberization and morphology in Solanum tuberosum . Changes in the profile of PAs were consistendy associated with effects on tuberization. DFMO totally prevented, but DFMA only decreased, tuberization. Thus, it is concluded that arginine decarboxylase-activity is low and the PAs required are predominandy generated via the ornithine decarboxylase pathway. DFMO treatment increased the level of free PAs. At 5 mmol/L DFMO the formation of conjugated PAs was strongly reduced, whereas at 1.8 mmol/L conjugated PAs accumulated to a level higher than in untreated controls. Chlorogenic acid promoted tuber formation and tuber growth in a concentration-dependent manner, involving conjugation with PAs. Exogenous PUT, in general, caused improved growth of leafy shoots, rooting and loss of tuber dormancy. Tuberization was either promoted or inhibited, depending on the concentration applied. Added to the tuber-inducing control medium, low concentration (0.2 mmol/L) of PUT accelerated tuberization, establishing the PA-profile typical of tuberization. A high concentration (5 mmol/L) prevented full tuberization and disturbed the typical PA-profile. Neither concentration counterbalanced the impact of DFMO on the distribution of endogenous PAs and tuberization. However, exogenous PUT counteracted the decrease in tuberization due to DFMA treatment and also the delay in tuberization onset caused by chlorogenic acid, which was putatively due to reduced levels of free PAs as a result of conjugation. The results of monitoring the changes in the distribution of endogenous PAs following treatments that perturb tuberization support the conclusion that a specific balance of free and conjugated PAs is required for optimal progress through tuberization.