Time Course of CO₂ Exchange of Potato Cultures In Vitro with Different Sucrose Concentrations in the Culture Medium

Abstract

Short single node cuttings of tissue-cultured potato plantlets (Solanum tuberosum L. cv. Benimaru) were placed onto the culture media with three different sucrose concentrations (5g·l^-1 (S₅), 15g·l^-1(S₁₅) and 25g·l^-1(S₂₅)) to follow the time course of CO₂ exchange of potato cultures for 30 days. Moreover, how the CO₂ balance of the cultures changes during the growth and development phases from short single node cuttings (length: 5mm; dry weight: 1mg) to plantlets (dry weight: >10mg) large enough to transfer to ex vitro conditions was investigated. The CO₂ exchange was calculated from the time course of the difference (ΔC) between CO₂ concentrations of inflow and outflow air (flow rate: 2.1l·h^-1) through the culture vessel.
On day 3 ΔC for all sucrose concentrations showed tendencies to decrease by photosynthesis after the beginning of photoperiod. Cyclic changes in ΔC for all sucrose concentrations according to 8 hours-dark/16 hours-photoperiod cycles were clearly observed after day 5. The difference between maximum and minimum ΔC on the same day in each sucrose concentration increased as the day proceeded, and the difference on day 30 was in the order: S₁₅>S₂₅>S₅. The CO₂ exchange during photoperiod and daily CO₂ exchange for all sucrose concentrations continuously increased as the day proceeded. The daily CO₂ exchange was found to turn to positive values from negative ones around day 16 for S₅, and day 10 for S₁₅ and S₂₅: the cultures turned into positive CO₂ balances. These results indicate that the cultures have grown and developed dominantly depending on sucrose in the culture medium before the CO₂ balance turned to positive, even having contained chlorophyll and been under appropriate CO₂ (880±50μmol·mol^-1 for inflow air) and light (120μmol·m^-2·s^-1 during photoperiod) conditions for photosynthesis. The dry weight increase and net production by photosynthesis for 30 days were in the order: S₁₅>S₂₅>S₅.