Short‐term changes in carbon isotope composition of soluble carbohydrates and starch: from canopy leaves to the root system

Changes in the 13 C discrimination of current leaf photosynthesis might have profound impacts on root respiratory substrates. Therefore, the aim of this study was (1) to refine a method for the isolation of root and leaf starch and soluble sugars (neutral fraction) for stable carbon isotope analysis and (2) to assess the short‐term temporal variability of the C isotope composition ( δ 13 C) of starch and of the neutral fraction of beech roots and leaves at different canopy heights. An existing method for isolating starch for stable C isotope analysis based on enzymatic hydrolysis was modified to account for the low starch content of the samples. This was achieved by removing the enzyme ( α ‐amylase) by ultrafiltration after the hydrolysis, resulting in very low carbon blanks. The neutral fraction was separated from organic acids and cations by ion‐exchange chromatography. An anion‐exchange resin in the [HCO 3 ] − ‐form was chosen that ensured high precision of C blanks. Beech leaves at 5, 10 and 20 m above the forest floor as well as roots were sampled six times during a day/night cycle in July 2003. δ 13 C values of bulk material, starch and the neutral fraction increased from the lower to the higher canopy with mean differences between 5 and 20 m of 3.8, 3.4 and 2.7‰ for the δ 13 C values of starch, neutral fraction and bulk foliage, respectively. The δ 13 C value of foliar starch increased from the morning to the afternoon and decreased during the night, but diurnal differences (up to 3.1‰) were only statistically significant for leaves sampled at 5 and 10 m height. In roots, no diurnal variation in the δ 13 C of starch was observed during the short time frame of one day and the δ 13 C of the neutral fraction did not differ between samples taken at 16:30 and 22:00. Calculated δ 13 C values of starch, which was mobilised during the night, were more positive than the total starch (all sampling times pooled) in leaves. Furthermore, the δ 13 C values of mobilised starch were approximately 5‰ more positive than that of the mobilised neutral fraction. Hence, the δ 13 C of potential sources for export from canopy leaves to roots varied considerably in their C isotope composition. Copyright © 2006 John Wiley & Sons, Ltd.