Photoperiod is one of the environmental factors which affect the tuberization of potatoes. In order to determine the effects of photoperiod on tuberization of two potato cultivars, the effects of long (14 hours) and short (8 hours) days on the growth and production of mini-tubers in two potato cultivars (Agria and Savalan) were evaluated under controlled conditions. The results showed that plants grown under short-day conditions produced more mini-tubers, and these tubers were superior in terms of diameter compared to tubers grown in long days. Agria allocated more assimilates to shoots but lower assimilates to underground in long days which is economically more important. Savalan produced more tubers than Agria in short photoperiods, while, long photoperiod led to increase in shoot and decrease in root weight in this cultivar, still it was significantly higher than Agria. Chlorophyll content of both cultivars decreased under short photoperiod conditions. Irrespective to the cultivar, the number and length of stolon decreased under short photoperiod conditions, but the number of tubers per plant increased under such conditions. Savalan showed lower sensitivity to photoperiod than Agria. Therefore, in the photoperiod sensitive potato cultivars, production of mini-tubers in short days may be recommended, but in the long term, introduction of potato cultivars with lower sensitivity to photoperiod through plant breeding may increase potato yield in short days by maintaining sink and source balance.

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