| Summary: | The study aimed to observe the influence of day-length on tuberization of three potato varieties (Trezor, Christian and Cumidava) in geographical conditions with long photoperiod. Varieties belonged to three different maturation groups: early, semi-early and semi-late. Potatoes were planted at three different dates and data was collected at 20, 25 and 45 days from emergence. The number of stolons formed at 20 days from emergence was greatly influenced by the number of day-light hours, the influence being greatest in the case of early and semi-early varieties. At 25 and 45 days from emergence varieties had similar reactions to photoperiod with differences being negatively highly significant when having 15.0 and 15.5 hours of day-light in comparison to 14.5 hours of day-light. Great differences were observed under different photoperiod conditions in the case of tuber development. In long photoperiod conditions, like the case of Romania, plating should take place so that tuberization occurs at the beginning of May when 14.5 hours of light per day are available. Getting over this photoperiod threshold determines great reduction in tuber number.
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