Micropropagation technology in early phases of commercial seed potato production

<FONT><strong>Key words:</strong>Solanum tuberosum L., in vitro plantlet, in vitro tuberization, microtubers, minitubers, tuber bulking, photoperiod, in vitro storage, jasmonates, micropropagation, seed production. Micropropagation ( in vitro propagation) has been introduced to seed potato production programmes more than two decades ago. The research reported in this thesis studied possibilities of improvements to micropropagation methods commonly used in commercial laboratories. The focus was placed on seed potato growers in Western Canada who either produce the tissue culture plant material themselves and use it for the production of minitubers (nuclear tubers) in their own operations, or acquire it prior to planting. The autotrophic micropropagation (in CO ₂ (1500 ppm) enriched atmosphere, no sucrose in the medium) of Russet Burbank variety showed its usefulness for the small commercial laboratories where often the full sterility is difficult to maintain. Autotrophically grown cultures produced stems very similar to conventionally grown in vitro cultures (no significant differences in length, number of nodes and dry weight). Also, the conventionally propagated cultures benefited from CO ₂ enrichment during the 4 week growing period by a 20% increase in the number of nodes per stem and a 50% increase in stem dry weight (doubled stem length).The use of continuous low red light (690 nm) at 3<font>μm</font><sup><FONT FACE="Symbol" SIZE=3>-2</SUP></font>s <sup><FONT FACE="Symbol" SIZE=3>-</font><font size="3">1</SUP></font><font>PPFD and 30 g l</font><sup><FONT FACE="Symbol" SIZE=3>-</font><font size="3">1</SUP></font>sucrose in the medium during the in vitro low temperature (4°C) storage of potato cultures were beneficial for maintenance of vigorous, high quality cultures with a high re-grow capacity. In small tissue culture laboratories, installation of a simple low light device in the regular refrigerator would improve maintenance of the high quality of the stored cultures. Significantly better production of microtubers (number of tubers and weight) was observed on solid (agar) than on liquid media, and under the 8 h photoperiod (SD) compared to no light. Microtubers derived from SD were greenish and seemed less juvenile than the tubers from 0 h light. Such microtubers performed better in the field or the greenhouse than microtubers produced in darkness. The 16 h photoperiod was inhibitory to the production of microtubers. The production of microtubers in all six commercial varieties tested in the studies benefited from SD. Tuber bulking rates were lower in the darkness than under the SD. Independently of the variety, fewer microtubers per explant with significantly lower weights, were produced under 0 h photoperiod than in SD. Production of microtubers in all three Russet varieties and in Sangre was superior to this of Shepody and Atlantic. Effects of jasmonic acid (JA) on in vitro tuberization of potato were also variety specific. Varieties Amisk, Russet Burbank, Sangre and Umatilla Russet produced the highest number of microtubers per nodal cutting (1.0 - 1.7) and their tubers were also the largest with 70 - 75% of microtubers in the&gt; 0.1 g size category. Results with Shepody were inconsistent and Atlantic produced the lowest number of tubers per nodal cutting. Amisk, Atlantic and Umatilla Russet benefited (significantly higher total tuber weight, percentage and weight of microtubers &gt; 0.1 g) from JA supplement to the tuberization medium (JAMed), but not from the pre-conditioning treatment (JAPret). However, none of these varieties clearly benefited from any JA treatments under light. Russet Burbank and Sangre also benefited from the JA supplement in the dark, although stock plantlet conditioning (JAPret) was more effective than the JA supplement to the tuberization medium (JAMed). Shepody did not show any benefits of JA on microtuberization, neither under dark or light conditions. The double treatment with JA (JAPret and JAMed) in light reduced production of microtubers comparing to the control and the individual JA treatments. The greenhouse/field performance of microtubers was highly dependent on JA conditioning of plantlets prior to in vitro tuberization, presence of JA in tuberization media, the photoperiod during tuberization and the dormancy release treatment. Although plantlets of all five tested cultivars performed well in the greenhouse, the microtuber performance was variety dependent, so were the responses to JA. JA conditioning of stock plants prior to taking explants for tuberization was beneficial for minituber production and it can be proposed as a treatment enhancing the quality of microtubers and their performance in the greenhouse production of minitubers. In the field, the results with JA were inconclusive; stock plantlets pre-treated with JA (JAPret) enhanced the Pre-elite tuber production in Russet Burbank by approximately 40% but significantly lowered it in Shepody by 17%. In the greenhouse, the three Russet varieties responded best to JA treatments producing results comparable to in vitro plantlets in production of minitubers whereas microtubers of Shepody produced inconclusive results and variety Atlantic performance was poor. The varietal responses were similar in the field studies, however, yield and the number of Pre-elite tubers produced from microtubers were less than 50% of these from plantlets. SD during in vitro tuberization was an important factor (irrespective of other treatments) in producing microtubers which then performed well in the greenhouse and in the field. Microtubers produced in dark performed poorly. Although Rindite proved to be very effective in microtuber dormancy release in greenhouse conditions, more studies are required to provide evidence that the product is safe to use with microtubers. No severe damage to microtubers treated with Rindite was observed when microtubers were from 8 h light tuberization treatment (minimal damage to tubers from the dark treatment). In the field situation, GA gave better results in dormancy release. In all three Russet varieties, the highest number and yield of Pre-elite tubers were obtained when the microtubers were soaked in 100 ppm solution of GA prior to field planting. Optimalization of commercial production of disease/virus-free propagules in the seed potato system encompasses various aspects of micropropagation technology. These include improvements in mass multiplication phase, storage of cultures and the use of microtubers in the system. Microtubers of the Russet varieties can be successfully used to speed up the multiplication in the seed potato system, in the greenhouse production of minitubers.