Xylem-transported glucose as an additional carbon source for leaf isoprene formation in Quercus robur

In order to test whether xylem-transported carbohydrates are a potential source for isoprene biosynthesis, [U-(13)C]-labelled alpha-D-glucose was fed via cut ends of stems into the xylem of Quercus robur seedlings and the incorporation of (13)C into isoprene emitted was studied. Emission of (13)C- labelled isoprene was monitored in real time by proton -transfer-reaction mass spectrometry (PTR-MS). A rapid incorporation of (13)C from xylem-fed glucose into single (mass 70) and double (mass 71) (13)C-labelled isoprene molecules was observed after a lag phase of approx. 5-10 min. This incorporation was temperature dependent and was highest (up to 13% (13)C of total carbon emitted as isoprene) at the temperature optimum of isoprene emission (40-42degreesC), when net assimilation was strongly reduced. Fast dark-to-light transitions led to a strong single or double (13)C-labelling of isoprene from xylem-fed [U-(13)C]glucose. During a period of 10-15 min up to 86% of all isoprene molecules became single or double (13)C-labelled, resulting in a (13)C- portion of up to 27% of total carbon emitted as isoprene. The results provide evidence that xylem-transported glucose or its degradation products can potentially be used as additional precursors for isoprene biosynthesis and that this carbon source becomes more important under conditions of limited photosynthesis.