The hydrogen isotope composition of nocturnal sucrose does not reflect the 2H-depletion of remobilized leaf starch

The hydrogen isotope composition (δ²H) of cellulose is inherently linked to that of sucrose synthesised in leaves. During the daytime, sucrose is synthesised from photosynthetic products, and at night, from remobilised starch. From theory, the δ²H of starch should be ²H-depleted relative to triose-phosphates generated during photosynthesis. Consequently, sucrose δ²H values should diurnally vary so that on a flux-weighted basis, cellulose δ²H values could provide a sensitive proxy for the partitioning of photoassimilates between sucrose and starch. However, this hypothesis is yet to be tested.

We made diel measurements of sucrose and starch δ²H in three species – Vicia faba (bean), Raphanus sativus (radish), and Helianthus annuus (sunflower) differing in their sucrose/starch dynamics. Plants were grown under controlled environment conditions to minimise variation in leaf water δ²H and physiology, so that changes in sucrose δ²H could be attributed to day/night shifts in photoassmiliates vs transitory starch used for sucrose synthesis. We confirmed that transitory starch in leaves was ²H-depleted compared with sucrose, on average by around 100 ‰. However, whilst there were species-specific trends in daytime sucrose δ²H, surprisingly, there was no significant day-night difference in the three species. Several explanations are discussed for the lack of day/night variation in the δ²H of sucrose, including the signal being subsequently overwritten in the cytosolic processing of sugars or masked by ²H-enrichment at other positions to counteract the ²H-depleted starch signature. We qualify the latter possibility with a simplified steady-state isotopic model.