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

This is the authors' abstract. We don't add key points for this paper.

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.