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 α- 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 (40-42°C), 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-13C]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.