Leaf isoprene emission declines in Quercus pubescens seedlings experiencing drought – Any implication of soluble sugars and mitochondrial respiration?

Previous studies suggest that the sensitivity of leaf mitochondrial respiration and the pool of soluble sugars to water stress could influence the response of leaf isoprene emission to drought by affecting the availability of extra-chloroplastic carbon for isoprene synthesis. We measured rates of isoprene emission and CO2 exchange, and the concentration of nonstructural carbohydrates in leaves of Quercus pubescens Willd. seedlings subjected to either normal watering (control plants, C) or drought (droughted plants, D). Stopping of watering caused predawn leaf water potential (Ψpd) to decline between −2.3 and −5.1 MPa among D plants, whereas Ψpd remained higher than −0.45 MPa in C plants. Isoprene emission (Is), net CO2 assimilation (An) and dark mitochondrial respiration (Rd) decreased with increasing water deficit, with declines in these variables relative to the respective means of C plants being An > Is > Rd. This resulted in positive pairwise correlations between the three variables. The concentration of nonstructural carbohydrates did not change between treatments, but the concentration of soluble sugars increased and that of starch decreased in D plants as compared with C plants. As a consequence, there was a negative correlation between Is and the concentration of soluble sugars, which supports a limited use of cytosolic sugars in sustaining isoprene synthesis at high to severe water stress. Our data also indicate that competition between Is and Rd for the same carbon substrates had little importance for isoprene emission at high to severe water stress, as compared to the overall constraint on isoprene metabolism probably imposed by the shortage of photosynthetic carbon, energy and reducing equivalents.

► Extra-chloroplastic carbon sources may sustain isoprene emission during drought.
► Isoprene emission, photosynthesis and dark respiration decreased with drought.
► Leaf internal CO2, isoprene and soluble sugar concentrations increased.
► Isoprene formation and dark respiration were not limited by sugar availability.
► Isoprene formation was not affected by dark respiration via substrate competition.