The effects of 5-aminolevulinic acid (ALA), a key precursor in the biosynthesis of porphyrins such as chlorophyll and heme, on development and salt tolerance of microtubers of two potato (Solanum tuberosum L.) cultivars Jingshi-2 and Zihuabai were examined under in vitro conditions. ALA at 0.3-3 mg/l promoted microtuber formation by increasing the average number, diameter, and fresh weight of microtubers especially under 0.5% NaCl stress conditions, but further increase in ALA concentration resulted in a reduction of microtuber yield irrespective of NaCl stress. Under 1.0% NaCl stress conditions, microtuberization was seriously repressed and could not be restored by the addition of ALA. The accumulation of malondialdehyde in the microtubers treated with 30 mg/l ALA increased by 22% compared to the controls (no salinity), while only a 7% increase was observed when the microtubers were exposed to 0.5% NaCl, indicating that ALA functions as a protectant against oxidative damages of membranes. Under 0.5% NaCl stress conditions, the highest activities of peroxidase and polyphenoloxidase were detected in microtubers treated with ALA at 0.3 and 3 mg/l, being by 73% and by 28% greater than those in the untreated controls, respectively. These results demonstrate that ALA at lower concentrations of 0.3-3 mg/l promotes development and growth of potato microtubers in vitro and enhances protective functions against oxidative stresses, but ALA at 30 mg/l and higher concentrations seems to induce oxidative damage probably through formation and accumulation of photooxidative porphyrins.

The shoot fresh mass, root length and root numbers of two potato (Solanum tuberosum L.) cultivars Favorita and Helanwuhua were increased significantly by the application of 0.2 -2 mg dm -3 jasmonic acid (JA) in the Murashige and Skoog medium. However, the growth of potato explants was inhibited by JA at high concentrations (20 -50 mg dm -3 ). Chlorophyll content in explant leaves decreased with an increase in the concentration of JA. In leaves treated with 0.2 mg dm -3 JA acid peroxidase activity increased, while in the leaves treated with more than 2 mg dm -3 JA peroxidase activity decreased. Under the dark, the microtuber numbers, fresh mass and percentage of big microtubers of two potato cultivars were not promoted by the application of 0.2 -50 mg dm -3 JA.

Explants obtained from in vitro-propagated plantlets of two potato cultivars, Shepody and Atlantic, were treated with five doses of γ-radiation (0, 2, 4, 6 and 8 Gy) to investigate the stimulating effects of low irradiation on the production and quality of microtubers in vitro. Microtubers of both cultivars treated with γ-radiation initiated 5 d earlier than in the non-irradiated control. The whole period of microtuberization was prolonged by 10 -15 d with 4, 6 and 8 Gy irradiation treatment for cv. Atlantic. Irradiation of the plantlets (4 Gy) led to a significant increase not only in the microtuber number (116.7 and 34.5 % over the control) but also in the fresh mass (77.6 and 23.2 % in Shepody and Atlantic, respectively). Low dose irradiation (2 -4 Gy) increased the starch content of microtubers. High doses (6 -8 Gy) enhanced ascorbic acid and reducing sugar contents. 4 -6 Gy doses also effectively increased the protein contents of microtubers.