Comparison of isoprene emission, intercellular isoprene concentration and photosynthetic performance in water-limited oak (Quercus pubescens Willd. and Quercus robur L.) Saplings

The influence of prolonged water limitation on leaf gas exchange, isoprene emission, isoprene synthase activities and intercellular isoprene concentrations was investigated under standard conditions (30degreesC leaf temperature and 1000 mumol photons m(-2) s(-1) PPFD) in greenhouse experiments with five-year-old pubescent oak (Quercus pubescens Willd.) and four-year-old pedunculate oak (Quercus robur L.) saplings. Net assimilation rates proved to be highly sensitive to moderate drought in both oak species, and were virtually zero at water potentials (Psi(pd)) below -1.3 MPa in Q. robur and below -2.5 MPa in Q. pubescens. The response of stomatal conductance to water stress was slightly less distinct. Isoprene emission was much more resistant to drought and declined significantly only at Psi(pd) below -2 MPa in Q. robur and below -3.5 MPa in Q. pubescens. Even during the most severe water stress, isoprene emission of drought-stressed saplings was still approximately one-third of the control in Q. robur and one-fifth in Q. pubescens. Isoprene synthase activities were virtually unaffected by drought stress. Re-watering led to partial recovery of leaf gas exchange and isoprene emission. Intercellular isoprene concentrations were remarkably enhanced in water-limited saplings of both oak species during the first half of the respective drought periods with maximum mean values up to ca. 16 mul l(-1) isoprene for Q. pubescens and ca. 11 mul l(-1) isoprene for pedunculate oak, supporting the hypothesis that isoprene serves as a short-term thermoprotective agent in isoprene-emitting plant species.