Developmental changes in carbohydrate content and sucrose degrading enzymes in tuberising stolons of potato (Solanum tuberosum)

Tuberising stolon tips of potato ( Solanum tuberosum L. cv. Record) accumulate starch and sucrose but the hexose content, particularly fructose, declines rapidly. Similar changes occur in the region 2 cm behind the swelling apex but the decline in glucose is far more pronounced than in the developing tuber. Tuberisation is characterised by an apparent switch from an invertase-dominated sucrolytic system (both acid and alkaline invertases [EC 3.2.1.26] are present) to one dominated by sucrose synthase (EC 2.4.1.13). Sucrose synthase and fructokinase (EC 2.7.1.4) activities were, at a maximum, ca 10- and 5-fold higher, respectively in the swelling stolon tip compared with the non-tuberising region. At the highest starch contents attained, the starch level in the young developing tuber was approximately double that in the adjacent non-tuberising stolon region. Immunoblots revealed that developmental changes in sucrose synthase. fructokinase and alkaline invertase polypeptides corresponded with enzyme activities. Antibodies raised against the N-terminal amino acid sequence of a soluble invertase purified from mature tubers did not detect significant quantities of a polypeptide in stolons and young, developing tubers. Antibodies raised against an in vitro expression product of an apoplastic invertase cloned from a leaf cDNA library detected a polypeptide in developing tubers but not in mature ones. However, expression of the protein did not correlate well with acid invertase activity during early tuber formation.     

Effects of illumination source, culture ventilation and sucrose on potato (Solanum tuberosum) microtuber production under short days

The culture yield of a simple method of microtuber production of potato was increased by assessing the interactions of illumination source (Thorn Lighting (Philips) “Colour 84” lamps (TL‐84) or Grolux lamps (Sylvania) in a conventional growth room or natural light in a glasshouse cabinet), type of vessel closure (unventilated or ventilated) and sucrose concentration (1%, 2%, 4% or 8%). Microtuber initiation and growth in unventilated cultures was 100% at 8% sucrose falling to 40–50% at 4% sucrose and was absent at 1% or 2%. With ventilation, rapid tuberisation (90–100%) occurred at initial sucrose concentrations of 2–8%, but only when the medium was allowed to dry before transfer of cultures to short days. Water supplementation of cultures at day 28 prevented tuberisation at 1–4% sucrose up to day 56. The fresh weight and dry weight of microtubers per plant increased significantly with sucrose concentration, with ventilation of cultures and under natural light. In ventilated cultures, the mean number of usable microtubers (± 0.1 g weight) increased from between 1.0–1.4 per plant at 8% sucrose to between 1.6–2.6 per plant at 4% sucrose, with the highest numbers (1.8–2.6 per plant) produced under natural light for the cvs Desirée and King Edward. The mean % dry matter content of microtubers was reduced to 11.3% at 4% sucrose compared with 17.3% at 8% sucrose, but the survival rate of microtubers after 6 months storage was unaffected. Microtuber production under short days was improved at a higher intensity of natural light with culture ventilation in a partially‐shaded glasshouse cabinet, whilst using reduced inputs (lower sucrose supply and no lamps).     

Changes in cytokinins during initiation and development of potato tubers

Cytokinin-like activity was assayed in stolons and tubers of Solanum tuberosum L. ssp. andigena (Juz. et Buk.) Hawkes cv. 165 grown in pots under controlled environment conditions. The plants were allowed to tuberise without the application of environmental or other external stimuli. The soluble sugar and starch contents of stolon tips and tubers were measured. Starch accumulation was a precise indicator of tuber initiation. Cytokinin-like activity began to increase in tubers with a diameter greater than 7.5 mm and, as assessed on a per tuber basis, was greatest in the largest size-category analysed. However, expressed as a function of fresh and dry weight, activity was greatest in tubers of 15–20 mm in diameter. Increases in cytokinin-like activity occurred subsequent to tuber formation, indicating that the tuberisation stimulus is unlikely to be cytokinin-like in nature.     

Are cuttings suitable for assessing maturity type in potato (Solanum tuberosum)?

Two experiments were carried out to evaluate the potential of single‐node cuttings of potato (Solanum tuberosum) as a tool to assess genotypic differences in maturity type. Plants were exposed to different photoperiodic treatments (different photoperiods, different numbers of photoperiodic cycles), and cuttings were taken at different plant ages. Cuttings from early (and to a lesser extent also late) maturing varieties exposed to short photoperiods showed strong induction to tuberise, irrespective of plant age; the induction increased with an increase in the number of short photoperiodic cycles. The response of cuttings taken from early‐maturing varieties exposed to long photoperiods depended on plant age: cuttings showed stronger induction when mother plants were older; cuttings from late‐maturing varieties hardly tuberised after exposure to long photoperiods. The tuberisation of the cuttings did not depend on the length of the long photoperiod (18 or 24 h) or on the number of cycles of a photoperiod of 18 h. Tuberisation on cuttings did not properly reflect the tuber formation on the mother plants, although within varieties, significant correlations between tuberisation on cuttings and tuber yield per plant 9 weeks after planting were found with different numbers of photoperiodic cycles of 12 h. Our experiments show that the cutting technique cannot be used on older plants to assess the maturity type of potato varieties, as there are interactions between photoperiod, genotype, plant age and number of photoperiodic cycles, in the reflection of the degree of induction to tuberise on single‐node cuttings.