The effects of physiological age and nutritional status of Solanum potato mother plants on the vitro growth and morphogenesis of shoots and micro tubers

In vitro plantlets and micro tubers can be induced form disinfested sprouts excised from potato tubers or nodal cuttings derived from green house potted plants. Studies were conducted to evaluate the performance of explants (sprouts tips) derived from potato tubers at different physiological ages (young= 100 day -degrees, apical- 750; multiple=1200; and senile= 2500 sprouts), and greenhouse-grown plants that received different combinations of nitrogen, phosphorus, and potassium pre-treatments (NPK kg ha-1: 0:0:0, 150:0:0, 0:100:0, 0:0:80, 150:100:80, 150:100:80). In each case explants were cultured on modified Murashige and Skoog’s (MS) salts, (1962), supplemented with appropriate vitamins and growth regulators, for shoot or microtuber induction. Microtubers were induced directly form explants (sprout tips and nodal cutting) inoculated on tuberisation medium or defoliated in vitro shoots previously grown on shoot propagation medium containing Ms salts supplements with vitamins, benzyl aminopurine (1 mg/1), sucrose (30 g/1), and agar (6 g/1) and then transferred onto tuberculosis medium. The results obtained revealed a significant (P<0.05) effect of physiological age as well as genotype on percent regeneration of sprouts, shoot growth and vitro micro tuberisation. There was progressive decline in regeneration with advance in sprout physiological age. The rate of decline was most pronounced in sangema followed by Kisoro and then Victoria respectively. Young and multiple sprouts significantly (P<0.05) produced more shoots and nodes per explant than apical and senile sprouts, although the latter produced significantly (P<0.05) taller shots. There was a significant (P<0.05) inverse and genotype dependent relationship between time to micro tuber and advance in physiological age in both cases (sprout tips and defoliated shoots). Sangema produced micro tubers earlier than Victoria and Kisoro respectively. Compared with other physiological ages, the apical sprouts significantly (P, 0.05) promoted low numbers of microtuber induction per explant. Microtuber frequency significantly (P<0.05) declined within increase in physiological age, when sprout tips were used as source of explants. The response was genotype dependent, with Sangema having the poorest tuberisation (P<0.05) smaller microtubers in contrast with apical dominant sprout tips which were characterized by microtubers bearing significantly (P<0.05) larger diameters and fresh weight (P<0.05). Sangema showed very poor microtuber formation and a constracting trend relative to Victoria and Kisoro. Victoria was associated with microtubers bearing secondary growth. Similar trends in time to microtuber induction and size were observed in the defoliated in vitro shoots. However, higher microtuber frequencies were obtained as the cumulated day degrees increased from 100 through 1200 to 2500, except for the apical dominant sprout-derived shoots at 750 day degrees, which had the least microtuber frequency. Overall, higher microtuber frequencies were obtained when sprout tips were first grown into plantlets prior to microtuber induction. This study has provided some indication on how to enhance the frequency and size of microtubers in a seed multiplication scheme. With mother plant pre-treatment, different mineral combinations influenced the growth and morphogenic responses (Shoot growth and microtuber yield components) of the two potato genotypes (Victoria and Kisoro). In vitro shoot performance (shoot length and node number) was significantly (P<0.05) influenced by N-P-K fertilization of mother plants, with sole N (150:0:0) application having the best growth, and sole P, (0:100:0), K (0:0:80) with the poorest growth. Mother plant fertilization with N along with P or K (150:0:80, 150:100:0, 150:100:80) showed intermediate effects, but improved shoot growth. Single N application was inimical to microtuber formation, whereas P (0:100:0) and K (0:0:80) promoted early tuberisation, characterized with significantly (P<0.05) large diameters and fresh weights. Victoria was associated with bigger and fewer microtubers than Kisoro in all cases. Increased number of microtubers was achieved by combined application of NPK (150:0:80, 150:100:0, 150:100:80). Microtubers derived from micro propagated plantlets did not show significant (P>0.05) difference in size (diameter and fresh weight) due to mother plant treatment. However, there was a significant (P<0.05) difference in number of microtubers per explant, this could be explained by the difference in the initial shoot numbers. The six week-old nodal cuttings bearly regenerated (5-15%0 on tuber induction medium, and had 30-70% regeneration on shoot induction medium. Better regeneration (60-70%) was observed with treated mother plants compared with P and K treatments (30-50%). This is possibly due to the senescence delaying effect of N.Microtubers derived from micropropagated plantlets, irrespective of genotype and mineral treatment either did not tuberize, ot produced very few, tiny microtubers which were desiccated during incubation for dormancy break. Therefore no yield advantage was realized. No variation was observed between the conventional mother clones and the in vitro cultures and microtubers on the morphological characteristics evaluated, indicating preserved clonal fidelity after in vitro passage. The purple pigmentation on some of the few shoots which shortly disappeared in the screen house may be attributed to epigenetic change influenced by the environment.