Cryopreservation has been recognized as a practical and efficient tool for long-term storage of vegetatively propagated plants. This study was conducted to investigate effects of modified vitrification techniques on cryopreservation of potato. In vitro plants of potato cultivars "Superior" and "Atlantic" were cold acclimated, and axillary buds were precultured, osmoprotected, exposed to PVS-2 solution, plunged into liquid nitrogen, thawed, and finally planted in the regeneration medium. In the modified vitrification technique an ice-blocking agent, Supercool X1000, was added with PVS-2 solution. Cold acclimation affected survival of cryopreserved shoot tips, and the highest survival (46.7%) was obtained after 3 weeks of acclimation at 10 degrees C. Shoot tips exposed to 2M glycerol plus 0.6M sucrose for 40 min gave 51.5% and 11.7% survival in "Atlantic" and "Superior" at 10 degrees C, respectively. Cold acclimated and osmoprotected shoot tips were dehydrated with PVS-2 containing different concentrations of Supercool X1000 prior to a plunge into liquid nitrogen. Treatments with 0.1% and 1% of Supercool X1000 significantly improved survival by 55% in "Superior" and 71.3% in "Atlantic", respectively. After cryopreservation, vitrified shoot tips resumed growth within a week in a medium (1 mg l(-1) GA(3), 0.5 mg l(-1) zeatin, and 0.1 mg l(-1) IAA) with a low level of Pluronic F-68 (0.005%) and survival was 33.7% higher in "Atlantic" and 14.7% higher in "Superior" than the control (without Pluronic F-68).

Characterization of strains of Ralstonia solanacearum, the causal agent of potato bacterial wilt disease from Nepal and Thailand was performed based on pathogenicity, biochemical/physiological and serological tests. Fifteen R. solanacearum strains isolated from wilt infected potato plants and tubers grown in Nepal were characterized as race 3, biovar II based on the pathogenicity on different host plants, hypersensitive reaction on tobacco leaf and utilization of some sugars. Results of pathogenicity test show that all strains from Nepal had limited host range. Degree of virulence of all strains varied from high to medium in potato and tomato and medium to low in eggplant. They did not cause wilting in tobacco, pepper and peanut plants. Six strains from Thailand were characterized as biovar II and III. Additionally, comparisons on the physiological, biological and serological characters of seven strains from Nepal and six from Thailand revealed similar characters. Race 3 and biovar II of the pathogen was widely spread over potato growing areas of mid and high hills of Nepal. Both biovars II and III were prevalent in the potato growing areas of Thailand but biovar III was the most dominating one.Key words: Bacterial wilt; Potato; Pseudomonas solanacearum; Ralstonia solanacearumDOI: http://dx.doi.org/10.3126/narj.v4i0.4868Nepal Agriculture Research Journal Vol. 4&5, 2001/2002Page: 42-47Uploaded date: 9 June, 2011

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Response of widely grown potato cv. Superior and newly developed cvs. Gui valley and Bora valley to plant growth regulators (PGRs) for direct plant regeneration from internode, leaf blade and petiole explants were investigated. The explants were cultured on a MS solid medium supplemented with different concentrations and combinations of 6-benzylaminopurine (BAP), 1-naphthaleneacetic acid (NAA), zeatin, indole-3-acetic acid (IAA) and gibberellic acid (GA 3 ). Potato cv. Superior, regenerated direct shoot without callus and root formation on MS solid medium supplemented with BAP or zeatin, proliferous roots were produced on NAA or IAA supplemented medium and only some calli were produced on GA 3 supplemented medium. The regeneration response varied with different concentrations of PGRs, singly and also in combinations. In the case of combined application of PGRs, the highest shoot regeneration (75.3%) and number of shoot per explant (11.5) and number of roots per explant (7.0) were obtained from the MS solid medium supplemented with zeatin (2 mg l -1 ), NAA (0.1 mg l -1 ) and GA 3 0.1 mg l -1 ). Among the three types of explants evaluated, internodes produced the highest number of shoots and roots for both potato cvs. Gui valley and Bora valley, and petiole produced the least number of shoots and roots. The regenerated shoots were rooted in PGRs-free MS solid medium and successfully established under glasshouse condition. Leaf, flower, and tuber morphology were identical to in vitro control and mother plants in the same conditions. This optimized regeneration system can be used for rapid shoot proliferation and also for gene transformation.