In vitro tuberization responses of eleven clones of potato were evaluated under different photoperiods and light intensities. No growth regulators were added to the medium to avoid the potential modification of the response to the environmental stimuli.Plantlets were grown on MS medium containing 3% sucrose for four weeks under long days (16 h). Tuberization was induced by short days (8 h). Five photoperiodic combinations and three light intensity treatments were applied.Light applied after the tuber induction phase delayed or inhibited tuber initiation (at proper photoperiods both at 111 and 55 ~mol m 2 s 1 light intensities at 24/15 ~ day/night temperature). Darkness following the induction stage accelerated and synchronized tuber initiation after high light intensity. However, these effects of dark treatment depended on light intensity applied in short days before dark treatment. There was an interaction between the duration of darkness applied in photoperiodic treatments and the light intensity applied in short days on tuber initiation. No relationship was observed between the maturity groups of clones tested and their tuber initiation responses. Tight interaction (P<0.01) of treatments and clones was proved by statistical analysis. Reliable tuberization and high rate of tuberization (1.20-1.52) can be reached using these hormon-free systems.

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Microtubers can be stored longer, transported and handled easier than plantlets, but they have some disadvantages related to long dormancy. Dormancy, number of sprouts per tuber and loss of microtubers under different storage temperatures were studied. Microtubers of four genotypes originated from different tuberization treatments (photoperiod combinations) were observed. We found that dormancy depended on cultivar and -in some cases -on the photoperiod treatment applied during tuberization. Generally, the dormacy was long and was greatly elongated by low storage temperature. One of the photoperiod treatments shortened while the other treatment prolonged the rest-period compared to the control, maybe due to a change in temperature accompanying the change in daily light (photoperiod combination). Besides, these treatments affected the number of sprouts per tuber. There was no significant difference in duration of dormancy between different tuber-size groups, but the loss "increased significantly with a decrease in tuber size. Based on this information we can use microtubers more effectively in the seed potato production programme.

The effects of nitrogen supply of in vitro plantlets on subsequent in vitro tuberization were examined in three potato varieties. In vitro plantlets were raised on MS media with varying amounts of ammonium nitrate, resulting in 70, 80, 100, 120 and 160% of the standard N content in MS medium and various nitrate : ammonium ratios. Tuberization was induced by adding an 8% sucrose solution to 4-week old plantlets and keeping them 2 weeks under short or long days (8 and 16 h illumination per day, respectively), followed by darkness for 9 weeks. Under long days, a lower nitrogen content in the medium and higher nitrate : ammonium ratio were favourable for the development of large tubers (>6 mm). Under short days, the total number of tubers increased with an increase in nitrogen content of the medium in cultivars Gülbaba and Désirée but not in cultivar Boró, while the number of large tubers was not significantly influenced by nitrogen supply. The tuber weight per plantlet was strongly influenced by genotype. Nitrogen supply of the in vitro plantlets had a prolonged effect on in vitro tuberization. Because the optimum nitrogen treatment is genotype specific, the quantity of large in vitro tubers could be increased by adjusting the nitrogen content of the medium.

The dormancy of potato microtubers produced under different photoperiodic treatments and light intensities was investigated in the varieties Desiree and Gülbaba. The dormant period was defined as the period between harvest or tuber initiation and the end of dormancy. The effects of environmental factors could be detected due to the use of a hormone-free tuber-producing system. Combined treatments had a slight effect on dormancy, while different light intensities influenced it considerably. The lower the light intensity the longer the dormant period for both cultivars. The effects of light intensities depended on the photoperiodic treatments applied for tuber induction.