Stem Shading Promotes Mannitol Accumulation in the Bark of Fraxinus ornus and Prevents Sucrose Transport in Roots Under Drought.

Drought induces the accumulation of osmolytes, including soluble non-structural carbohydrates (NSC), to support osmotic adjustment and hydraulic recovery. Bark has been proposed as a major site of sugar storage, with stem photosynthesis potentially contributing to NSC production and drought responses. Some species also accumulate polyols, such as mannitol, often linked to membrane protection. However, the allocation of carbon compounds among plant organs during drought and recovery remains unclear. In this study, Fraxinus ornus saplings were exposed to stem shading, followed by drought and recovery to investigate the single and combined effects of these factors on NSC allocation in bark, wood, and roots. Hydraulic parameters were measured alongside concentrations of glucose, fructose, sucrose, starch, and mannitol under well-watered, drought, and recovery conditions. Stem shading increased xylem vulnerability to embolism and reduced glucose concentration in stems and roots, while other sugars and mannitol were unaffected. Drought triggered starch degradation and increased hydraulic conductance loss, regardless of light treatment. Sucrose concentration increased in bark and roots, especially in non-shaded plants, whereas mannitol increased mainly under combined drought and shading. During recovery, sucrose declined, whereas mannitol remained elevated. Our results indicate that carbon partitioning in F. ornus is strongly affected by drought and influenced by stem shading. Root sucrose appears central to whole-plant osmotic adjustment but is sensitive to shading. Mannitol, likely sustained by starch degradation, may instead support osmotic adjustment during recovery, representing a more carbon-efficient osmolyte.