Is there any ¹² C/¹³C fractionation during starch remobilisation and sucrose export in potato tubers?

The δ¹³C (carbon isotope composition) variations in respired CO₂, total organic matter, proteins, sucrose and starch have been measured during tuber sprouting of potato (Solatium tuberosum) in darkness. Measurements were carried out both on tubers and on their growing sprouts for 23 days after the start of sprout development. Sucrose was slightly ¹³C-depleted compared with starch in tubers, suggesting that starch breakdown was associated with a small isotope fractionation. In sprouts, all biochemical fractions including sucrose were ¹³C-enriched compared with source tuber-sucrose, suggesting that sucrose translocation from tuber to sprouts fractionated against ¹²C. However, both apparent fractionations were explained by the consumption of ¹³C-depleted carbon for respiration or growth that enriched in the ¹³C sucrose molecules left behind. In addition, whole tuber sucrose is constantly composed of recent sucrose from starch breakdown and old sucrose associated with an inherited, slightly ¹³C-depleted pool. We therefore conclude that any fractionation at either the starch breakdown or the sucrose translocation level is unlikely under our conditions.