1. Introduction: Potato (Solanum tuberosum L.) is one of the main crops for food security in the world. This plant has a cultivated area of more than 16 million hectares and a production of 360 million tons per year. This plant is propagated sexually (true potato seeds (TPS)), and asexually (tuber formation), but for commercial purposes mainly seed tubers are used. Mini tubers are used to prevent diseases and pests and increase product yield. Recently, microtuber is widely used as starting material for the propagation of potato due to its privileges including germplasm conservation, high storage potency, unseasonal production, decontaminated propagation, easier acclimatization and handling, rapid and economical multiplication procedure, and maximum yield productivity. Several factors affect potato microtuber growth, which can be mentioned as light quality, sucrose, genotype, plant growth regulators, explant type, gelling agent, and nutrition. The positive efficacy of controlled stressful conditions on enhancement efficiency of in vitro geophyte production was reported. According to crucial effect of temperature on tuberization, the purpose of this research is to investigate the role of thermal stresses (heat and cold) on microtuber induction under in vitro circumstances.
2. Materials and Methods: A single node of a greenhouse-grown stem was used as an explant for this experiment. We used ‘Sante’ cultivar as a model plant in this experiment. This research has three phase including explant preparation, plantlet multiplication and temperature stress pretreatments to induce microtuberization. Explants were disinfected with sodium hypochlorite 1% with 1 ml Tween® for 20 min. For multiplication phase, explants were cultured in MS medium contained 30 gr/l sucrose and 7 gr/l agar. Also, plantlets were cultured in MS medium with 80 gr/l sucrose for microtuberization phase. Explants were placed at several cold conditions (1, and 4 ºC) in three duration (4, 8, 12 h) and also hot circumstances (25, 40, 45, and 50 ºC) with different duration (0.5, 1, 2 h) in contrast to control treatment (25 ºC). The explants were placed under long photoperiod (16:8 hours) with fluorescent light (PAR = 750 s-1 m-2). Following 60 days, explant fresh and dry weight, explant diameter, microtuber diameter, microtuber fresh and dry weight, biomass, microtuber fresh weight per glass, microtuber number per glass, tuberization degree, and tuberization percentage were recorded.
3. Results and Discussion: The results illustrated that explant fresh weight and explant diameter increased in response to heat stress compared to control treatment. Based on our results, the highest explant fresh (58.4 gr) and dry weight (33.3 gr) was observed in 50 ºC (0.5 h) and 4 ºC (12 h), respectively. Furthermore, explant grown under 45 ºC for 2 hours had the greatest explant diameter (1.46 mm) in contrast to the rest. The most tuber diameter was observed in explants placed under 4 ºC for 8 h with 3.6 mm. Explants treated with 4 ºC for 8 h had the uppermost fresh weight and dry weight of microtuber with 45.3 gr and 9.5 gr, respectively. In biomass trait, the lowest amount was related to the control treatment, which indicates the significant effect of stress on improving biomass. Among the stress treatments, explant 1 °C for 12 h had the greatest effect in increasing biomass. In microtuber fresh weight per glass, 1 °C for 8 h treatment had the maximum effect with 160 gr. Also, hot treatment had a positive effect on tuberization degree. The most tuberization degree (4) was fond in explants treated with 50 for 2 h. Furthermore, in cold conditions, explants showed the highest number of microtuber induction. Also, the explants under 4 °C for 4 hours significantly increased the number of microtuber compared to the rest. According to results, cold treatment was more effective than heat stress in the tuberization percentage. The explants grown under 4 °C for 8 hours had the most tuberization percentage with 100%.
4. Conclusion: In general, stressors has a key role on microtuberization of potato compared to control condition. Among stress treatments, cold stresses have the most efficacy on in vitro microtuberization (number and fresh weight and dry weight of microtuber) and biomass of potato. In contrast, high temperature ameliorated tuberization degree, tuber diameter and also prevented microtuber growth. Altogether, 4°C for 8 hours as the best treatment can be recommend for industrial purpose.