Oxygen isotope enrichment of organic matter in Ricinus communis during the diel course and as affected by assimilate transport

The oxygen isotope composition in leaf water and organic compounds in different plant tissues is useful for assessing the physiological performance of plants in their environment, but more information is needed on Δ¹⁸O variation during a diel course. Here, we assessed Δ¹⁸O of the organic matter in leaves, phloem and xylem in stem segments, and fine roots of Ricinus communis during a full diel cycle. Enrichment of newly assimilated organic matter in equilibrium with leaf water was calculated by applying a nonsteady-state evaporative enrichment model. During the light period, Δ¹⁸O of the water soluble organic matter pool in leaves and phloem could be explained by a 27‰ enrichment compared with leaf water enrichment. Leaf water enrichment influenced Δ¹⁸O of phloem organic matter during the night via daytime starch synthesis and night-time starch remobilization. Phloem transport did not affect Δ¹⁸O of phloem organic matter. Diel variation in Δ¹⁸O in organic matter pools can be modeled, and oxygen isotopic information is not biased during transport through the plant. These findings will aid field studies that characterize environmental influences on plant water balance using Δ¹⁸O in phloem organic matter or tree rings.